摘要:

GENERAL DISCUSSIONS OF THE FARADAY SOCIETY Date 1962 1962 1963 1963 1964 1964 1965 1965 1966 1966 1967 1967 1968 1968 I969 1969 1970 1970 1971 1971 1972 1972 1973 1973 1974 1974 1975 1975 1976 1977 1977 Subject Inelastic Collisions of Atoms and Simple Molecules High Resolution Nuclear Magnetic Resonance The Structure of Electronically-Excited Species in the Gas-Phase Fundamental Processes in Radiation Chemistry Chemical Reactions in the Atmosphere Dislocations in Solids The Kinetics of Proton Transfer Processes Intermolecular Forces The Role of the Adsorbed State in Heterogeneous Catalysis Colloid Stability in Aqueous and Non-Aqueous Media The Structure and Properties of Liquids Molecular Dynamics of the Chemical Reactions of Gases Electrode Reactions of Organic Compounds Homogeneous Catalysis with Special Reference to Hydrogenation and Bonding in Metallo-Organic Compounds Motions in Molecular Crystals Polymer Solutions The Vitreous State Electrical Conduction in Organic Solids Surface Chemistry of Oxides Reactions of Small Molecules in Excited States The Photoelectron Spectroscopy of Molecules Molecular Beam Scattering Intermediates in Electrochemical Reactions Gels and Gelling Processes Photo-effects in Adsorbed Species Physical Adsorption in Condensed Phases Electron Spectroscopy of Solids and Surfaces Precipitation Potential Energy Surfaces Radiation Effects in Liquids and Solids Oxidation For current availability of Discussion volumes, see back cover. Volume 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63GENERAL DISCUSSIONS OF THE FARADAY SOCIETY Date 1962 1962 1963 1963 1964 1964 1965 1965 1966 1966 1967 1967 1968 1968 I969 1969 1970 1970 1971 1971 1972 1972 1973 1973 1974 1974 1975 1975 1976 1977 1977 Subject Inelastic Collisions of Atoms and Simple Molecules High Resolution Nuclear Magnetic Resonance The Structure of Electronically-Excited Species in the Gas-Phase Fundamental Processes in Radiation Chemistry Chemical Reactions in the Atmosphere Dislocations in Solids The Kinetics of Proton Transfer Processes Intermolecular Forces The Role of the Adsorbed State in Heterogeneous Catalysis Colloid Stability in Aqueous and Non-Aqueous Media The Structure and Properties of Liquids Molecular Dynamics of the Chemical Reactions of Gases Electrode Reactions of Organic Compounds Homogeneous Catalysis with Special Reference to Hydrogenation and Bonding in Metallo-Organic Compounds Motions in Molecular Crystals Polymer Solutions The Vitreous State Electrical Conduction in Organic Solids Surface Chemistry of Oxides Reactions of Small Molecules in Excited States The Photoelectron Spectroscopy of Molecules Molecular Beam Scattering Intermediates in Electrochemical Reactions Gels and Gelling Processes Photo-effects in Adsorbed Species Physical Adsorption in Condensed Phases Electron Spectroscopy of Solids and Surfaces Precipitation Potential Energy Surfaces Radiation Effects in Liquids and Solids Oxidation For current availability of Discussion volumes, see back cover.Volume 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63

ISSN:0301-7249    

DOI:10.1039/DC97763FX001

出版商:RSC    

年代:1977

数据来源: RSC

摘要:

GENERAL DISCUSSIONS OF THE FARADAY SOCIETY Date Subject 1907 1907 1910 191 1 1912 1913 1913 1913 1914 1914 1915 1916 1916 1917 1917 1917 1918 1918 1918 1918 1919 1919 1920 1920 1920 1920 1921 1921 1921 1921 1922 1922 1923 1923 1923 1923 1923 1924 1924 1924 1924 1924 1925 1925 1926 1926 1927 1927 1927 1928 1929 1929 1929 1930 Osmotic Pressure Hydrates in Solution The Constitution of Water High Temperature Work Magnetic Properties of Alloys Colloids and their Viscosity The Corrosion of Iron and Steel The Passivity of Metals Optical Rotatory Power The Hardening of Metals The Transformation of Pure Iron Methods and Appliances for the Attainment of High Temperatures in a Laboratory Refractory Materials Training and Work of the Chemical Engineer Osmotic Pressure Pyrometers and Pyrometry The Setting of Cements and Plasters Electrical Furnaces Co-ordination of Scientific Publication The Occlusion of Gases by Metals The Present Position of the Theory of Ionization The Examination of Materials by X-Rays The Microscope: Its Design, Construction and Applications Basic Slags : Their Production and Utilization in Agriculture Physics and Chemistry of Colloids Electrodeposition and Electroplating Capillarity The Failure of Metals under Internal and Prolonged Stress Physico-Chemical Problems Relating to the Soil Catalysis with special reference to Newer Theories of Chemical Action Some Properties of Powders with special reference to Grading by The Generation and Utilization of Cold Alloys Resistant to Corrosion The Physical Chemistry of the Photographic Process The Electronic Theory of Valency Electrode Reactions and Equilibria Atmospheric Corrosion.First Report Investigation on Oppau Ammonium Sulphate-Nitrate Fluxes and Slags in Metal Melting and Working Physical and Physico-Chemical Problems relating to Textile Fibres The Physical Chemistry of Igneous Rock Formation Base Exchange in Soils The Physical Chemistry of Steel-Making Processes Photochemical Reactions in Liquids and Gases Explosive Reactions in Gaseous Media Physical Phenomena at Interfaces, with special reference to Molecular Atmospheric Corrosion. Second Report The Theory of Strong Electrolytes Cohesion and Related Problems Homogeneous Catalysis Crystal Structure and Chemical Constitution Atmospheric Corrosion of Metals. Third Report Molecular Spectra and Molecular Structure Colloid Science Applied to Biology Elutriation Orientation Volume Trans.3 3 6 7 9 9 9 10 10 11 12 12 13 13 13 14 14 14 14 15 15 16 16 16 16 17 17 17 17 18 18 19 19 19 19 19 20 20 20 20 20 21 21 22 22 23 23 24 24 25 25 26 26 aGENERAL DISCUSSIONS OF THE FARADAY SOCIETY Date Subject 1931 1932 1932 1933 1933 1934 1934 1935 1935 1936 1936 1937 1937 1938 1938 1939 1939 1940 1941 1941 1942 1943 1944 1945 1945 1946 1946 1947 1947 1947 1947 1948 1948 1949 1949 1949 1950 1950 1950 1950 1951 1951 1952 1952 1952 1953 1953 1954 1954 1955 1955 1956 1956 1957 1958 1957 1958 1959 1959 1960 1960 1961 1961 Photochemical Processes The Adsorption of Gases by Solids The Colloid Aspect of Textile Materials Liquid Crystals and Anisotropic Melts Free Radicals Dipole Moments Colloidal Electrolytes The Structure of Metallic Coatings, Films and Surfaces The Phenomena of Polymerization and Condensation Disperse Systems in Gases: Dust, Smoke and.Fog Structure and Molecular Forces in (a) Pure Liquids, and (6) Solutions The Properties and Functions of Membranes, Natural and Artificial Reaction Kinetics Chemical Reactions Involving Solids Luminescence Hydrocarbon Chemistry The Electrical Double Layer (owing to the outbreak of war the meeting The Hydrogen Bond The Oil-Water Interface The Mechanism and Chemical Kinetics of Organic Reactions in Liquid The Structure and Reactions of Rubber Modes of Drug Action Molecular Weight and Molecular Weight Distribution in High Polymers.(Joint Meeting with the Plastics Group, Society of Chemical Industry) The Application of Infra-red Spectra to Chemical Problems Oxidation Dielectrics Swelling and Shrinking Electrode Processes The Labile Molecule Surface Chemistry.(Jointly with the SociCte de Chimie Physique at Colloidal Electrolytes and Solutions The Interaction of Water and Porous Materials The Physical Chemistry of Process Metallurgy Crystal Growth Lipo-Pro teins Chromatographic Analysis Heterogeneous Catalysis Physico-chemical Properties and Behaviour of Nuclear Acids Spectroscopy and Molecular Structure and Optical Methods of Investi- gating Cell Structure Electrical Double Layer Hydrocarbons The size and shape Factor in Colloidal Systems Radiation Chemistry The Physical Chemistry of Proteins The Reactivity of Free Radicals The Equilibrium Properties of Solutions on Non-Electrolytes The Physical Chemistry of Dyeing and Tanning The Study of Fast Reactions Coagulation and Flocculation Microwave and Radio-Frequency Spectroscopy Physical Chemistry of Enzymes Membrane Phenomena Physical Chemistry of Processes at High Pressures Molecular Mechanism of Rate Processes in Solids Interactions in Ionic Solutions Configurations and Interactions of Macromolecules and Liquid Crystals Ions of the Transition Elements Energy Transfer with special reference to Biological Systems Crystal Imperfections and the Chemical Reactivity of Solids Oxidation-Reduction Reactions in Ionizing Solvents The Physical Chemistry of Aerosols Radiation Effects in Inorganic Solids The Structure and Properties of Ionic Melts was abandoned, but the papers were printed in the Transactions) Systems Bordeaux.) Published by Butterworths Scientific Publications, Ltd.Volume 27 28 29 29 30 30 31 31 32 32 33 33 34 34 35 35 35 36 37 37 38 39 40 41 42 42 A 42 B Disc. 1 2 Trans. 43 Disc. 3 4 5 6 7 8 Trans. 46 Disc. 9 Trans. 47 Disc. 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32GENERAL DISCUSSIONS OF THE FARADAY SOCIETY Date 1962 1962 1963 1963 1964 1964 1965 1965 1966 1966 1967 1967 1968 1968 I969 1969 1970 1970 1971 1971 1972 1972 1973 1973 1974 1974 1975 1975 1976 1977 1977 Subject Inelastic Collisions of Atoms and Simple Molecules High Resolution Nuclear Magnetic Resonance The Structure of Electronically-Excited Species in the Gas-Phase Fundamental Processes in Radiation Chemistry Chemical Reactions in the Atmosphere Dislocations in Solids The Kinetics of Proton Transfer Processes Intermolecular Forces The Role of the Adsorbed State in Heterogeneous Catalysis Colloid Stability in Aqueous and Non-Aqueous Media The Structure and Properties of Liquids Molecular Dynamics of the Chemical Reactions of Gases Electrode Reactions of Organic Compounds Homogeneous Catalysis with Special Reference to Hydrogenation and Bonding in Metallo-Organic Compounds Motions in Molecular Crystals Polymer Solutions The Vitreous State Electrical Conduction in Organic Solids Surface Chemistry of Oxides Reactions of Small Molecules in Excited States The Photoelectron Spectroscopy of Molecules Molecular Beam Scattering Intermediates in Electrochemical Reactions Gels and Gelling Processes Photo-effects in Adsorbed Species Physical Adsorption in Condensed Phases Electron Spectroscopy of Solids and Surfaces Precipitation Potential Energy Surfaces Radiation Effects in Liquids and Solids Oxidation For current availability of Discussion volumes, see back cover.Volume 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63

ISSN:0301-7249    

DOI:10.1039/DC977630X003

出版商:RSC    

年代:1977

数据来源: RSC

摘要:

GENERAL DISCUSSIONS OF THE FARADAY SOCIETY Date 1962 1962 1963 1963 1964 1964 1965 1965 1966 1966 1967 1967 1968 1968 I969 1969 1970 1970 1971 1971 1972 1972 1973 1973 1974 1974 1975 1975 1976 1977 1977 Subject Inelastic Collisions of Atoms and Simple Molecules High Resolution Nuclear Magnetic Resonance The Structure of Electronically-Excited Species in the Gas-Phase Fundamental Processes in Radiation Chemistry Chemical Reactions in the Atmosphere Dislocations in Solids The Kinetics of Proton Transfer Processes Intermolecular Forces The Role of the Adsorbed State in Heterogeneous Catalysis Colloid Stability in Aqueous and Non-Aqueous Media The Structure and Properties of Liquids Molecular Dynamics of the Chemical Reactions of Gases Electrode Reactions of Organic Compounds Homogeneous Catalysis with Special Reference to Hydrogenation and Bonding in Metallo-Organic Compounds Motions in Molecular Crystals Polymer Solutions The Vitreous State Electrical Conduction in Organic Solids Surface Chemistry of Oxides Reactions of Small Molecules in Excited States The Photoelectron Spectroscopy of Molecules Molecular Beam Scattering Intermediates in Electrochemical Reactions Gels and Gelling Processes Photo-effects in Adsorbed Species Physical Adsorption in Condensed Phases Electron Spectroscopy of Solids and Surfaces Precipitation Potential Energy Surfaces Radiation Effects in Liquids and Solids Oxidation For current availability of Discussion volumes, see back cover. Volume 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63GENERAL DISCUSSIONS OF THE FARADAY SOCIETY Date 1962 1962 1963 1963 1964 1964 1965 1965 1966 1966 1967 1967 1968 1968 I969 1969 1970 1970 1971 1971 1972 1972 1973 1973 1974 1974 1975 1975 1976 1977 1977 Subject Inelastic Collisions of Atoms and Simple Molecules High Resolution Nuclear Magnetic Resonance The Structure of Electronically-Excited Species in the Gas-Phase Fundamental Processes in Radiation Chemistry Chemical Reactions in the Atmosphere Dislocations in Solids The Kinetics of Proton Transfer Processes Intermolecular Forces The Role of the Adsorbed State in Heterogeneous Catalysis Colloid Stability in Aqueous and Non-Aqueous Media The Structure and Properties of Liquids Molecular Dynamics of the Chemical Reactions of Gases Electrode Reactions of Organic Compounds Homogeneous Catalysis with Special Reference to Hydrogenation and Bonding in Metallo-Organic Compounds Motions in Molecular Crystals Polymer Solutions The Vitreous State Electrical Conduction in Organic Solids Surface Chemistry of Oxides Reactions of Small Molecules in Excited States The Photoelectron Spectroscopy of Molecules Molecular Beam Scattering Intermediates in Electrochemical Reactions Gels and Gelling Processes Photo-effects in Adsorbed Species Physical Adsorption in Condensed Phases Electron Spectroscopy of Solids and Surfaces Precipitation Potential Energy Surfaces Radiation Effects in Liquids and Solids Oxidation For current availability of Discussion volumes, see back cover.Volume 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63

ISSN:0301-7249    

DOI:10.1039/DC97763BX006

出版商:RSC    

年代:1977

数据来源: RSC

摘要:

Electron Spin-Lattice Relaxation Mechanisms of Radiation Produced Trapped Electrons and Hydrogen Atoms in Aqueous and Organic Glassy Matrices : Modulation of Electron Nuclear Dipolar Interaction by Tunnelling Modes in a Glassy Matrix B Y MICHAEL K. BOWMAN* AND LARRY KEVAN Department of Chemistry, Wayne State University, Detroit, Michigan 48202, U.S.A. Received 6th December, 1976 The spin lattice relaxation of trapped electrons in aqueous and organic glasses and trapped hydrogen atoms in phosphoric acid glass has been directly studied as a function of temperature by the saturation recovery method. Below 50 to 100 K, the major spin lattice relaxation mechanism involves modulation of the electron nuclear dipolar(END) interaction with nuclei in the radical's environment by tunnelling of those nuclei between two or more positions.This relaxation mechanism occurs with high efficiency and has a characteristic linear temperature dependence. The tunnelling nuclei around trapped electrons do not seem to involve the nearest neighbour nuclei which are oriented by the electron in the process of solvation. Instead the tunnelling nuclei typically appear to be next nearest neighbours to the trapped electron. The identities of the tunnelling nuclei have been deduced by isotopic substitution and are attributed to: Na in 10 mol dm-3 NaOH aqueous glass, ethyl protons in ethanol glass, methyl protons in methanol glass and methyl protons in MTHF glass. For trapped hydrogen atoms in phosphoric acid, the phosphorus nuclei appear to be the effective tuiinelling nuclei.Below -10 K the spin lattice relaxation is dominated by a temperature independent cross relaxation term for H atoms in phosphoric acid glass and for electrons in 10 mol dn1-3 NaOH aqueous glass, but not for electrons in organic glasses. This is compared with recent electron-electron double resonance studies of cross relaxation in these glasses. The spin lattice relaxation of 0- formed in 10 mol dmP3 NaOH aqueous glass was also studied and found to be mainly dominated by a Raman process with an effective Debye temperature of about 100 K. Electron paramagnetic resonance (e.p.r.1 spectroscopy has proved to be a most powerful tool in the study of trapped radicals in y-irradiated solids. E.p.r. techniques have been very successful in the identification and quantitative studies of radical kinetics and mechanisms in radiation chemistry.Two of the most studied radicals have been trapped electrons and trapped hydrogen atoms in frozen glasses. E.p.r. studies of the static properties of trapped electrons in a series of matrices of differing polarity have given a rather detailed picture of the trapped electron in aqueous glasses like 10 mol dm-3 NaOH glassy ice,' ethanol,2 methyltetrahydrofuran and 3-methyl- pentane4 glasses. One rather neglected area of e.p.r. research in radiation chemistry has been the study of the dynamic properties of radicals and their environment. This can be done most incisively by the study of spin lattice relaxation of the radicals as a function of temperature. Electron spin lattice relaxation is the process by which a collection of radical spins attain thermal equilibrium.In solids, the thermal motions of both the lattice * Present address : Argonne National Laboratory, Chemistry Division, Argonne, Illinois.8 ELECTRON SPIN-LATTICE RELAXATION MECHANISMS OF RADIATION itself and the radical in the lattice are capable of causing a time dependent perturbation of the electron spin of the radical. Possible spin lattice interactions are the Kronig- Van Vleck mechanism involving the spin-orbit orbit-lattice interaction or the electron nuclear dipolar (END) intera~tion.~ The temperature dependence of the spin lattice relaxation rate gives information about the dynamics of the modulating interaction, while the magnitude of the relaxation rate is determined by the magnitude of the perturbation.We report here on the results of a spin lattice relaxation study of trapped electrons and trapped hydrogen atoms in a number of y-irradiated frozen glasses. In the analysis of the temperature dependence of the electron spin lattice relaxation rates we shall make use of a new relaxation mechanism which we have recently described and which seems particularly important in glassy matrices. This relaxation mechanism is based on the modulation of the END interaction by tunnelling of nuclei or molecular groups in the environment of the radical. The general form of the temperature dependence of the relaxation rate (Tl-I) in the glassy systems studied is given by eqn (1) Ti-' = C2 + D2T + O2 exp(-AE/kT). (1) The C2 term is due to cross relaxation between the radical under study and another much faster relaxing radical in the sample.The second term, D2T, is the temperature dependence expected for the relaxation process involving the tunnelling modes. D2 is a measure both of the strength of the time dependent modulation of the END interaction and of the characteristic correlation time of that modulation. The final term, O2 exp(-AE/kT) where k is Boltzmann's constant, is merely a functional form which we have found useful in describing the spin lattice relaxation rate at high temperatures. At temperatures above -loOK, other spin lattice relaxation mechan- isms can become much more eEective than the relaxation due to the tunnelling modes. In addition, the temperature dependence of the tunnelling mode relaxation mechanism itself may also deviate from T1-' cc Tat these temperatures.6 As a result, the last term in eqn (1) is included for its utility in describing the data and not from any understanding of the high temperature relaxation mechanisms.We shall, therefore, confine our interpretation to the first two terms in eqn (1). The effect of deuteration of the glass on the relaxation rate due to tunnelling modes (D2T) has been calculatedS6 Three limiting cases were treated. For conditions typical of the frozen glasses used in this study: TiH/TiD = 1.17 when deuteration does not affect the mass of the tunnelling particle or the END interaction between the trapped radical and the tunnelling particle ; when the END interaction does involve a proton but deuteration does not change the mass of the tunnelling particle; and TiH/TiD = 0.030 when the tunnelling particle is a hydrogen nucleus.Thus the effect of isotopic substitution on the magnitude of the spin lattice relaxation rate due to tunnelling particles (D2) can be used to identify the nuclei involved in the END interaction being modulated by tunnelling and also to identify the tunnelling particle.MICHAEL K. BOWMAN A N D LARRY KEVAN 9 EXPERIMENTAL All H20 used was triply distilled in Pyrex. The NaOH was obtained from B & A Speciality Chemical Division of Allied Chemical. Stohler Isotope Chemicals was the source of D20, 40 weight % NaOD/D20, 85 weight % D3P04/D20, CH30D, CD30H, CD30D and C2D50H. The 85% H3P04 and CH30H were obtained from Mallinckrodt.The C2H50H was Rossville Gold Shield Alcohol and Matheson, Coleman and Bell was the source of 2-methyltetrahydrofuran (MTHF). The MTHF was purified by passing it over activated silica gel, dried by repeated vacuum distillations into flasks containing Na + K alloy and stored under vacuum over a potassium mirror until vacuum distilled into a sample tube and sealed. All of the methanol samples had 5 % water added to assure formation of a good glass. H20 was added to the -OH samples and D20 to the -OD samples. The deuterated or partially deuterated samples were placed in 3 mm 0.d. Spectrosil quartz tubes, evacuated and sealed. Nondeuterated samples were placed in similar tubes but were unsealed. Sample tubes were 4-5 cm in length and contained 2.0-2.5 cm of sample solution.Samples were frozen in as nearly a reproducible manner as possible by slowly lowering them into liquid nitrogen at a rate which minimized bubbling of liquid nitrogen around the sample. All samples were clear glasses with a minimum of cracks. Samples were irradiated in liquid nitrogen in a U.S. Nuclear Corp. GR-9 Co-60 y-irradiator at a dose rate of approximately 0.16 Mrad h-l. After irradiation, samples were stored in liquid nitrogen in darkness until measurements were made. The time domain spectrometer used to measure spin lattice relaxation rates is similar to the saturation recovery spectrometer described by Brown and Sloop7 and has been described in detail.* Great care was taken to ensure that the saturating pulses were several times longer than the observed recovery time and that the magnitude of the microwave power used to observe the recovery of the e.p.r.signal after saturation was far below a level which would perturb the populations of the spin levels. The control and measurement of the sample temperature has been described.6 Estimations of the coefficients in eqn (1) were made by an iterative least-squares fit’ to the data. The F test (variance ratio test statistic) the R test (deviation ratio test statistic) or the t-test (Student’s t ) were used to test hypotheses about the various coefficients in eqn (1)’ with the significance criterion beingp = 0.05 or better. RESULTS TRAPPED ELECTRONS IN 10 mol dmV3 SODIUM HYDROXIDE GLASSY ICE The spin lattice relaxation rate as a function of temperature was studied most completely for trapped electrons in 10 mol dm-3 sodium hydroxide glassy ice.Fig. 1 shows representative saturation recovery curves for trapped electrons. It was noticed quickly that the spin lattice relaxation rate was strongly dependent on the freezing conditions for the sample. As a consequence, great care was taken in this study to ensure that the samples were frozen as reproducibly as possible. Some of the different samples of 10 mol dm-3 NaOH glassy ice measured were; three samples of 10 rnol dm-3 NaOH irradiated to 0.42 Mrad, one sample each of 10 mol dm-3 NaOH irradiated to 4.0 Mrad, 10 mol dm-3 NaOD irradiated to 0.42 Mrad and 9 mol dm-3 NaOH doped with 20 atom % 1 7 0 irradiated to 1.0 Mrad. Radio- lysis produces trapped electrons and 0- ions in this matrix.Least squares fits were made to eqn (1) by varying all parameters, with C2 set equal to zero and with AE/k set equal to 400 K. Statistical analyses (R-test) of the results of these fits showed that C2 was non-zero for every sample, indicating a significant amount of cross relaxation between the trapped electrons and another fast relaxing radical in the matrix. The10 ELECTRON SPIN-LATTICE RELAXATION MECHANISMS OF RADIATION tl s FIG. 1 .-Representative saturation recovery signals plotted as the difference between the signal at infinite time after a pulse and the signal at time t after a saturating pulse. The lower left signal is the signal on a linear scale, the sample is trapped electrons in 10 mol dm-3 NaOH at 62.4 K. The observing power is 40 nW and the pump pulses are 20 mW.The spin-lattice relaxation time TI is 8.2 ms and the zero point is not indicated on the graph. The upper right corner shows the logarithm of the recovery of a sample of trapped electrons in 10 mol dm-3 NaOH at 10.1 K. The observing power is 1 nW and the pump power is 1 mW. TI is 175 ms. radical 0- is the other major radical in this matrix and, since it saturates less easily than the trapped electrons, it is a prime candidate for a cross relaxation partner. The statistically best fits (R-test) for these samples are given in table 1. Experi- mental rates as a function of temperature are plotted in fig. 2 for trapped electrons in 9 mol dm-3 Na170H with the best least-squares fit. TABLE l.-cOEFFICIENTS OF EQN (1) FOR TRAPPED ELECTRONS IN 10 MOL DM-3 SODIUM HYDROXIDE GLASSY ICE matrix dose/Mrad C2/s - D2/s - lK - 021s-1 AEk - l/KU NaOHb 0.42 2.25 1.81 1.40 x 104 (400) NaOHb 0.42 7.92 1.08 1.80 x 104 (400) NaOHb 0.42 3.86 1.62 5.25 x 103 (400) NaOHb 4.0 27.4 2.25 6.90 x 103 (400) NaODb 0.42 11.6 2.22 6.65 x 103 3 14 Na”0H“ 1 .o 19.7 2.27 9.09 x 104 577 ~ ~~~~ ~~ ~ ~~~~~~ Values in parentheses were coiistrained to the values shown.* Concentration 10 mol dm-3. Concentration 9 rnol dm-3. The cross relaxation rate (C2) is larger for trapped electrons in 10 mol dm-3 NaOH irradiated to 4.0 Mrad than for samples irradiated to 0.42 Mrad (P = 0.01, t-test), which is to be expected since 0: radicals are closer on the average to trapped electrons in samples irradiated to a higher dose. D2 is not significantly different for samples of 10 mol dm-3 NaOH irradiated to different doses, nor is it significantly different for the 10 mol dm-3 NaOH, 10 mol dm-3 NaOD or 9 mol dm” Na170H samples.The relaxation rate was also found to be independent of which part of the e.p.r. line was being observed and is a property of the entire e.p.r. line.MICHAEL K. BOWMAN AND LARRY KEVAN 11 The trapped electrons in the sample of 10 mol dmW3 NaOH irradiated to 4.0 Mrad were progressively optically bleached by 50% and the spin lattice relaxation rate was remeasured as a function of temperature. Further bleaching was then done until only 12.5% of the original trapped electrons remained. Neither C2 nor D2 showed any statistically significant changes due to this bleaching.1 I 1 1 10 25 50 100 TI K FIG. 2.-The spin lattice relaxation rate of trapped electrons in 20 atom % oxygen-17 doped 9 mol dmP3 NaOH irradiated to a dose of 1 .O Mrad. The solid line is the least-squares fit described in the text. The spin lattice relaxation rate of the trapped QT radical was measured at the g1 feature of its e.p.r. spectrum. The results are plotted in fig. 3. The solid line is given by Ti1 = lS(T/8)’J8(0/T) s-l with 8 equal to 100 K; J8 is the eighth transport integral. This functional form is that of the well-known Raman process for the Kronig-Van Vleck relaxation mechanism in a matrix with a Debye temperature of 100 K5 Below 10 K the temperature dependence of the relaxation rate is much weaker, characteristic of a direct process. TRAPPED ELECTRONS I N ETHANOL GLASS Detailed experimental results for trapped electrons in glassy C2H50H, C2D50H The best fits and C,H,QD irradiated to 1 Mrad have been presented of eqn (1) to the ethanol data are given in table 2.TRAPPED ELECTRONS I N MTHF GLASS The best fit of eqn (1) to the experimental spin lattice relaxation rates for trapped electrons in MTHF glass is T-l = 38.8(s-IK-l)T + 2.69 x 104(s-l) exp[-128(K)/T]. The coefficient of the temperature independent term is not statistically different from zero, but note the large coefficient for the term linear in temperature.12 ELECTRON SPIN-LATTICE RELAXATION MECHANISMS OF RADIATION 5 10 25 50 T I K FIG. 3.-The spin lattice relaxation rate of oxygen radicaI anions, OT in 10 mol dm-3 NaOH as a function of temperature.The solid line is a plot of T1-l = 15 (T/lOO K)9 J8 (100 K/T). TABLE 2.-cOEFFICIENTS OF EQN (1) FOR TRAPPED ELECTRONS IN ETHANOL IRRADIATED TO 1 MRAD matrix C"S - la D2/s-lK- 0 " s - AEk - l/K C2H50H (0) 18.9 4.74 x 103 84 C2H50D (0) 7.30 4.71 x 103 92 C2D5OH (0) 3.22 1.69 x 104 148 Values in parentheses were constrained to the value shown. TRAPPED ELECTRONS I N METHANOL GLASS Samples of trapped electrons in CH30H, CH30D, CD30H and CD30D showed very peculiar behaviour. At low temperatures, the recovery of the e.p.r. signal from saturation depended strongly on experimental conditions, particularly on the micro- wave powers used for both observing and saturating the signal. These effects were strongest in CD30D and data for this sample are shown in fig. 4.Below 10 K there is a cluster of points with Ti1 E 50 S" which were measured using a very low saturating pulse power and low pulse repetition rate. Between 8 and 30 K there are two sets of data where Ti1 actually increases with decreasing temperature. These two sets of data were measured under similar conditions but with different microwave powers. Also, it was found that at 6 K, a 10 min, 100 mW microwave pulse reduced the magnitude of the e.p.r. signal by nearly 90%. The e.p.r. signal took several minutes to return to its original intensity. These very peculiar observations have been rather simply explained by Clough and Hill.lo If the methyl groups in the methanol are undergoing tunnelling rotation at a frequency equal to the e.p.r. frequency, the electron spins are strongly coupled toMICHAEL K .BOWMAN A N D LARRY KEVAN 13 100 c 50 1 + +++ + I- I I I t 10 25 50 100 T I K FIG. 4.-The saturation recovery rate as a function of temperature of trapped electrons in [2H4]methanol. The saturating pulses had a 50% duty cycle. The points denoted by circles were measured using 4 nW observing power and 40 mW, 50 ms pulses; the squares with 4 nW observing power and 1 mW, 50 ms pulses; the triangles with 1 nW observing power and 1 mW, 50 m pulses and the crosses with 1 nW observing power and 400 JAW, 860 ms pulses. the rotation. The assumptions used in deriving eqn (1) are then violated and eqn (1) cannot be expected to be applicable. At low temperatures, the rotation of the methyl groups is only weakly affected by the lattice.The methyl groups form a large thermal reservoir in good thermal contact with the electron spin system but isolated from the lattice. During the saturating pulses, the electron spin system is “ heated ” to a high temperature (the spin level population difference defines a Boltzmann spin temperature). Some of this “heat” flows from the spin system into rotational motion of the methyl groups to heat them up. After the pulse, the spin system “ cools ” (the population difference and hence the e.p.r. signal increases) until it reaches the temperature of the collection of methyl groups. The spin system then follows the temperature of the methyl groups as they “ cool ’’ down to the temperature of the lattice. It is expected that at higher methyl group temperatures, the approach of the spin temperature to the methyl group temperature is more rapid.Thus, the decrease plotted in fig. 4 shows the approach of the spin temperature to the methyl group temperature, the latter being determined by the average microwave power. The several minute recovery following the 10 min pulse is the cooling of the methyl group system. These observations are perhaps the clearest evidence for the existence of the tunnelling mode relaxation mechanism for an electron spin system in a glass. TRAPPED HYDROGEN ATOMS IN PHOSPHORIC ACID GLASS The spin lattice relaxation rate of both the high field hydrogen atom e.p.r. line and om of its spin flip satellites in 85 weight % M3P04 + H,O irradiated to a dose of 0.50 Mrad is described by T,-l = 22.6(s-l) + 0.206(~-~K-~)T + 7.41 x 102(s-l) exp[-195(K)/q.14 ELECTRON SPIN-LATTICE RELAXATION MECHANISMS OF RADIATION A I I I I 10 25 50 100 10 TI K FIG.5.-The spin lattice relaxation rate as a function of temperature of the low field (inverted, v), middle field (0) and high field (upright, A) deuterium atom e.p.r. lines in D3P04 + DzO glass. The solid lines are the minimum parameter least-squares fit for all three lines as described in the text. All three deuterium atom lines were studied in 8576 D3P04/D20. The data are shown in fig. 5; the solid curves are the best fits of eqn (1) to the data from each e.p.r. line using the smallest number of different coefficients. The coefficients are given in table 3. TABLE 3.-cOEFFICIENTS OF EQN (1) FOR EACH E.P.R. LINE OF TRAPPED DEUTERIUM ATOMS IN 85% D3P04 + D20.IRRADIATED TO 0.5 MRAD e.p.r. line C'/S - 1 D2/s-lK-l" 02/s - 1 AEk - I/Ka high field 1 5 . 2 0.243 0.429 x lo3 128 middle field 97.8 0.243 1.03 x 103 1 2 8 low field 42.3 0.243 1.05 x 103 1 2 8 ~~~ (I Coefficients in this column were constrained to be the same for all lines. DISCUSSION CROSS RELAXATION We have interpreted the temperature independent term in the relaxation rate as due to cross relaxation between the saturated spin system (electrons in our case) and another faster relaxing species in the matrix. If present, this relaxation mechanism dominates at sufficiently low temperature since all other mechanisms are temperature dependent. This can be seen most clearly in fig. 5 for trapped D atoms in D3P04 glass. The cross relaxation rate depends upon the matrix elements of the dipolar inter- action between the two spin systems and on the cross relaxation line-shape function.llMICHAEL K .BOWMAN A N D LARRY KEVAN 15 In concentrated spin systems the lineshape function depends on the overlap of the e.p.r. spectral lines of the cross relaxing species, but in magnetically dilute systems, such as we have, has shown that the second moment of the cross relaxation lineshape is much greater than the sum of the second moments of the individual lines. Thus cross relaxation can still be quite significant when the two spectral lines are separated by more than 20 times their linewidth. By saturation recovery methods cross relaxation is typically only observed when the relaxation times of the two species are quite different ; otherwise, the cross relaxation rate from the saturated spin system does not become dominant.The largest cross relaxation coefficients among the systems studied are found for D atoms in D3P04 glass (see table 3). In this system the other main radical produced by radiolysis is PO2, with a g-value of ~ 2 . 0 1 , ~ ~ above the free electron value. Cross relaxation of the D atoms corresponding to the centre D atom e.p.r. line nearg = 2.002 with PO2: is expected to be more efficient than cross relaxation to the other D atom e.p.r. lines due to the spectral overlap. Similarly, the D atoms corresponding to the low field D atom e.p.r. line should interact with some- what less efficiency, while the D atoms corresponding to the high field D atom e.p.r.line should interact least efficiently. The saturation recovery studies over the temperature I ange investigated do not indicate significant cross relaxation from trapped electrons in organic glasses (MTHF, ethanols, methanols) to the organic radicals in these glasses. The organic radicals formed by radiolysis do have shorter relaxation times than the trapped electrons but the difference is not too large.14 Furthermore, the D2 coefficient for the tunnelling relaxation mechanism is much larger for trapped electrons in organic glasses (MTHF, C2H50H) than in aqueous glasses (table 1) or for D atoms in acid glasses (table 3). These two factors suppress any observable contribution of cross relaxation from electrons to radicals. However, electron-electron double resonance (ELDOR) studies do demonstrate cross relaxation from radicals to electrons in organic glas~es.'~ In ELDOR two microwave frequencies are used, termed pumpitzg and detecting frequencies.Cross relaxation is revealed by saturation transfer from the pumped radical spins to the detected electron spins. For this observation the pumped spins are partially saturated but must be less easily saturated than the detected spins. Thus, ELDOR studies do not show saturation transfer when the electron spins are pumped and the radical spins are detected. In contrast to organic glasses, the trapped electrons in 10 mol dm-3 NaOH aqueous glass do show detectable cross relaxation from the saturation recovery data (table 1). It is postulated that cross relaxation occurs to the other major radical in this glass which is 0- with g, = 2.07.Fig. 3 shows that 0- has a much larger spin-lattice relaxation rate than e,. Cross relaxation of e; to 0- is supported by the radiation dose effect on the cross relaxation coefficient, C2. At 4.0 Mrad the average e;, 0- distance is less than at 0.4 Mrad so that cross relaxation rate is larger as observed. Optical bleaching of e; also destroys 0- because most of the mobilized electrons react with 0- radicals.16 The lack of an effect on the cross relaxation by the bleaching can be explained if correlated pairs of e;- and 0- react by bleaching so that the average distance between remaining e;- and 0- pairs remains the same. ELDOR studies have also been done on e, in 10 mol dm-3 NaOH ice.17 Evidence for cross relaxation from 0- to e; could not be observed, apparently because the 0- relaxation rate is too fast to allow enough saturation.It appears that ELDOR and saturation recovery experiments give complementary information on cross relaxation. This is what is observed. These predictions are borne out by table 3.16 ELECTRON SPIN-LATTICE RELAXATION MECHANISMS OF RADIATION NATURE OF TUNNELLING NUCLEI For trapped electrons in glasses the spin lattice relaxation due to tunnelling modes, (Ti1 = D2T) is much more efficient than in single crystals (e.g., F-centres in KC1) where there are no tunnelling modes. The value of D2 in the glasses studied here ranged from 1 to 40 s-lK-l while for F-centres in single crystals 18*19 the coefficient of the low temperature relaxation process linear in temperature is 5 x s-lK-l! This shows that the relaxation of trapped electrons in glasses is quite different from that in crystalline solids.We now try to deduce the details of this tunnelling relaxation model by evaluating isotopic results to determine the nature of the tunnell- ing nuclei. For electrons in 10 mol dm-3 NaOH aqueous glass the coefficient of the tunnelling mode relaxation term, D2, is independent, (within the accuracy of this study) of deuteration, doping with "0, irradiation dose and optical bleaching. The dose independence implies that D2 reflects properties of isolated trapped electrons rather than of interacting clusters. The independence of 1 7 0 and D substitution indicates that the tunnelling particle does not contain oxygen or deuterium atoms and that the END interaction responsible for the tunnelling relaxation does not involve oxygen atoms or protons.This severely limits the possibilities and brings us to the con- clusion that the trapped electron in 10 mol dm-3 NaOH aqueous glass has an END interaction with nearby sodium ions which is modulated by tunnelling of the Na+ ions between two potential minima. Recent work has elucidated the structure of the first solvation shell around e; in this it contains only H20 molecules. The Na+ nearest to e; must be at least 5 A away. It thus appears that the tunnelling nuclei must not be too tightly coupled to the electric field of the trapped electron in order to undergo a tunnelling modulation. This appears to be supported by the trapped electrons in other matrices, which will now be discussed.In ethanol, deuteration studies show that the ethyl protons affect the tunnelling relaxation rate coefficient more than do the hydroxyl protons even though the strongest magnetic interaction is with the hydroxyl protons.21 This is consistent with the picture that the nearest neighbour nuclei to a trapped electron are immcbilized by the potential field of the electron and do not undergo tunnelling motions while the more distant nuclei like the ethyl protons are able to undergo tunnelling motion. Similarly, it is the motion of the methyl groups in methanol which dominates. The identity of the tunnelling group in MTHF cannot be conclusively determined from the data presented. However, if it is assumed that the MTHF ring is im- mobilized in the trapped electron's first solvation shell, than the most likely candidate is the tunnelling rotation of the 2-methyl group which points away from the trapped ele~tron.~ The contribution of the methyl groups in MTHF to the second moment of the trapped electron3 is 0.6 G2.If this 0.6 G2 is assumed to be the magnitude of the modulation of the END interaction, the characteristic correlation time for tunnelling of the methyl groups can be calculated from eqn (1) in ref. (6). The calculated correlation time is 2 ns K/T i.e., 200 ps at 10 K and 20 ps at 100 K. This is slower than the characteristic correlation time of the methyl groups in methanol, which is the order of the e.p.r. frequency, and is long enough such that the approxi- mations in ref.(6) are valid. For trapped H atoms in phosphoric acid glass the tunnelling mode relaxation rate is independent of deuteration within experimental error and also independent of the nuclear quantum number of the trapped deuterium atom. This suggests that the END interaction is modulated by tunnelling motion of the phosphorus nuclei in the matrix. The detailed structure of the matrix molecules around trappedMICHAEL K. BOWMAN AND LARRY KEVAN 17 H atoms is not yet known. But perhaps in the acid glasses hydrogen bonded net- works prevent tunnelling motions of the protons. The picture of the trapped electron that has evolved from this study is that the electron is trapped in a very stable arrangement in which the nearest nuclei out to several Angstroms are constrained from undergoing much motion.The -OH groups in alcohols, the first solvation shell water molecules in aqueous glasses and the MTHF ring protons, which are all oriented by the potential field of the trapped electron are all fairly well immobilized compared with the more distant methyl or ethyl groups in MTHF and alcohols and the sodium ion in the aqueous glass. There is evidence22 that during the process of electron solvation, the charge of the electron helps to orient the nearest nuclei around it into an almost crystal-like structure. However, nuclei not immediately adjacent to the trapped electron are less perturbed by it and retain random, glass-like characteristics including the ability to tunnel between potential minima intrinsic to the glassy state.This research was supported by the U.S. Energy Research and Development Administration. M. Bowman is grateful to the National Science Foundation and to Wayne State University for Graduate Fellowships. We thank Dr. S. Schlick for a sample of I7O enriched water. P. A. Narayana, M. K. Bowman, L. Kevan, V. F. Yudanov and Yu. D. Tsvetkov, J. Chem. Phys., 1975, 63, 3365. R. N. Schwartz, M. K. Bowman and L. Kevan, J. Chem. Phys., 1974, 60,1690. L. Kevan, M. K. Bowman, P. A. Narayana, R. K. Boeckman, V. F. Yudanov and Yu. D. Tsvetkov, J. Chem. Phys., 1975, 63,409. P. A. Narayana and L. Kevan, J. Chem. Phys., 1976,65, 3379. Spin-Lattice Relaxation In Ionic Solids, ed. A. A. Manenkov and R. Orbach (Harper and Row, New York, 1966). M. K. Bowman and L. Kevan, J. Phys. Chem., 1977, 81, in press. M. K. Bowman, Ph.D. Dissertation (Wayne State University, 1975). W. C. Hamilton, Statistics in Physical Science (The Ronald Press, New York, 1964), sections 5-4 and 5-6. ’ I. M. Brown and D. J. Sloop, Rev. Sci. Instr., 1970, 41, 1774. lo S. Clough and J. R. Hill, J. Phys. C: Solid State Phys., 1975, 8, 2274. l1 N. Bloembergen, S. Shapiro, P. S . Pershan and J. 0. Artman, Phys. Rev., 1959,114,495. l2 A. Kiel, Phys. Rev., 1960, 120, 137. l3 L. Kevan, Actiones Chim. Biolog. Radiations, 1969,13, 57. l4 D. P. Lin, P. Hamlet and L. Kevan, J. Phys. Chem., 1972,76, 1226. l5 H. Yoshida, D. F. Feng and L. Kevan, J. Chem. Phys., 1973,58,4924; D. P. Lin, D. F. Feng, F. Q. H. Ngo and L. Kevan, J. Chem. Phys., 1976,65, 3994. l6 P. Hamlet and L. Kevan, J. Amer. Chem. SOC., 1971,93, 1102. l7 H. Yoshida, D. F. Feng and L. Kevan, J. Chem. Phys., 1973,58, 3411. l 8 D. W. Feldman, R. W. Warren and J. G. Castle, Phys. Rev., 1964,135, A470. l9 H. Panepucci and L. F. Mollenaurer, Phys. Rev., 1969, 178, 589. 2o S. Schlick, P. A. Narayana and L. Kevan, J. Chem. Phys., 1976, 64, 3153. 21 B. L. Bales and L. Kevan, J. Chem. Phys., 1974, 60, 710. 22 L. Kevan, Adu. Radiation Chem., 1974,4, 181.

ISSN:0301-7249    

DOI:10.1039/DC9776300007

出版商:RSC    

年代:1977

数据来源: RSC

摘要:

Optical Spectra and Yields of Solvated Electrons and Metal-Electron Species in Alkali Metal Systems BY J. W. FLETCHER AND W. A. SEDDON Physical Chemistry Branch, Chalk River Nuclear Laboratories, AECL, Chalk River, Ontario KOJ IJO, Canada Received 2nd December, 1976 The effect of solvent on the optical spectra of solvated electrons, e; , cation-electron aggregates, (M+, e;), and alkali metal anions, M-, is discussed for a variety of alkali metal systems. With increasing solvent polarity the optical band maxima shift toward the blue with the solvent dependence decreasing significantly in the order (M+, e;)> e; > M-. The relative differences are interpreted in terms of ion-pairing and cation solvation. In ethylamine + ammonia mixtures only the e; band exhibits a pronounced dependence on solvent composition consistent with preferential solvation by NH3 molecules.Initial yields of e; measured by pulse radiolysis in methylamine (MA), ethylamine (EA) and isopropylamine (IPA) correspond to G(e;) N 2, whereas the fraction of electrons escaping the spur decreases markedly in the order MA > EA > IPA. The results indicate that the decay processes for e; in MA (and NH3) are slow compared with the rate of diffusion. In EA and IPA the decay is markedly temperature dependent and at temperatures approaching the freezing point is comparable with that in MA. This leads to an escaped yield approaching that observed for MA. The chemistry of alkali metal solutions has been studied extensively and excellent background information is contained within the proceedings of the Colloque Weyl I-IV.1-4 Pulse radiolysis of alkali metal salts in amine and ether solutions has substantiated that solvated electrons, e ; , cation-electron aggregates of stoichiometry M, and alkali metal anions M- coexist in equilibri~rn.~ e; + M+ + M M + M + M- -+ M+.(1) M + e ; + M - (2) (3) The precise nature of the species M is still controversial '-lo but for the purposes of this paper will be considered as an ion-pair (M+, e;). The optical band maxima for each species are solvent dependent with, in general, the most pronounced shifts with polarity being in the order (M+, e;) > e; > M-. This paper discusses first, the role of the solvent on the various optical spectra and its significance with respect to the structure of e;, (M+, e;) and M- and secondly, the radiolytic yield of e; in the solvent series ammonia, methylamine (MA), ethyl- amine (EA) and isopropylamine (IPA) and its relation to other solvent systems.EXPERIMENTAL Details of the experimental facilities and preparative techniques have been described previously."J2 In brief, solutions were irradiated with 0.3 pus (width at half-height) electron pulses from a 2.5 meV Van de Graaff accelerator or with 20 ns pulses from a Linac.13 Optical absorptions were monitored in quartz cells with an optical path length of 0.5 or 1 .O cm.J . W. FLETCHER AND W. A. SEDDON 19 RESULTS AND DISCUSSION OPTICAL SPECTRA Fig. 1 shows the effect of solvent composition on the relative shifts of the optical band maximum, vmax cm-l for e; in a wide variety of solvent mixture^.^^-^^ The data are plotted as a function of the electron fraction of the more polar molecule to correct for the relative change in energy absorbed by the two components.This scale 0 10 20 30 40 50 60 70 80 90 100 electron fraction % of p o l a r component FIG. 1 .-Relative shift of the optical band maxima, vmax cn1-l for e; in mixed solvent systems. The more polar component-corresponding to larger vmax, is quoted last. A, n-hexane + ethanol and 3-methylpentane + ethanol; By dioxan + H,O; C, tetrahydrofuran (THF) + H20; D, dimethyl- sulphoxide (DMSO) + H20 ; E, hexamethylphosphoramide (HMPA) + H20 ; F, ethylenediamine (EDA) + HzO; G, 2-methyltetrahydrofuran (MTHF) + EDA; H, diethylether (DEE) + EDA; I, THF + EDA; J, HMPA + EDA; K, NH3 + H20 and ND3 + D20; Z, EDA + ethanol.is approximately proportional to the volume fraction.23 Fig. 2 shows a comparable plot for e; in C2H5ND, + ND3 and EA + N2H4 mixtures and M in C2H5ND2 + ND3 and THF + EA solutions. In this case M refers to the (Na+, e;) ion-pair produced in solutions containing the solvent bases NaND2 and NaBD, respectively. It is evident from fig. 1 that in many cases there is a wide departure from ideal behaviour. The effect is particularly pronounced in polar + non-polar mixtures such as alcohol + alkanes 14*15926-29 or water + h y d r o c a r b ~ n l ~ . ~ ~ solutions where the e; spectrum closely resembles that of the polar component, even at very low concentra- tions of the polar molecule. This has been interpreted as being due to presence of small clusters or aggregates of the polar molecules which act as pre-existing trap^.^^,^'*^* As the differences in polarity become less the effect is diminished, indicating that the solvation sphere of e; is more typical of the average composition of the liquid.At the other extreme in NH3 + H20 mixtures some other effect, possibly the formation of NH40H, favours the localisation of e; towards NH3 molecules.20 OPTICAL SPECTRA AND YIELDS OF SOLVATED ELECTRONS Referring now to fig. 2, it is clear that the effect observed for e; in CH3CH2NH2 + NH3 mixtures is analogous to that in alcohol + alkane solutions, even though differences in solvent polarity are by no means as pronounced. We suggest the explanation is similar and relates to the existence of NH3 clusters acting as preferential traps for e;.This also seems to be the case for EA + NzH4 mixtures with, in this case, NzH4 30 forming preferred clusters but with much less specificity. 0 10 20 30 40 50 60 70 80 90 100 e l e c t r o n f r a c t i o n % of polar component FIG. 2.-Relative shift of the optical band maxima, v,,, crn-l, for e; in A, CzH5ND2 3- ND3. B, EA + NzH4 and C,D, for (Na+, e;) in C2H5NDz + ND3 and THF + EA mixtures, respectively; In A and B the shift with increasing NH3 or NzH4 is toward higher energies whereas for C and D the band shifts to lower energies with increasing ND3 or EA. For the (Na+, e;) species in partially deuterated EA + NH, mixtures the deviation from ideality is less pronounced than for e; indicating that the ion-pair has a solvation shell which is more representative of the bulk composition.This is perhaps not too surprising since the spectrum of the aggregate involves the solvation sphere of both the cation and e;. The red shift in v,,, with increasing NH3 con- centration also correlates with the decrease in atomic character observed by electron spin resonance, again implying a looser, well solvated Considering now the M- species, Lok et aZ.32 have shown an excellent correlation between the shift to lower energies in v,,, for Na‘ and K- with a wide variety of solvents. We have utilised this correlation to establish an empirical measure of increasing solvent polarity in the order of di-isopropylether (DIPE) < di-ethylether (DEE) < hexamethylphosphoramide (HMPA) < tetrahydrofuran (THF) < di- methoxyethane (DME) < diglyme < EA < 1,2 propanediamine (PDA) < ethylene- diamine (EDA).Fig. 3 illustrates the absolute shifts of the M’ band maxima5*22g31*32 as a function of solvent polarity for the series Na-, K-, Rb- and Cs-. Corre- sponding values are also shown for e; 5*32 and the charge transfer to solvent (CTTS) band for the iodide ion.32*33 In the latter case the dotted line shown for I- corre-J . W . FLETCHER AND W. A . SEDDON 21 sponds to the relative solvent dependence of 1.65 for e; to I- obtained in more polar solvents.33 In each case a linear correlation with solvating power is obtained. An increase in the slope therefore corresponds to an increasing participation of the solvent in the overall structure of e; or M-.The scatter exhibited by the CTTS iodide band probably reflects the complications due to ion-pairing effects in solvents of low p ~ l a r i t y . ~ ~ * ~ ~ On this basis it is interesting to note that no such deviation D ---n--o------~--- l o t a b c d e f g h i j increase inempirical so [vent polarity FIG. 3.-Absolute shift of the optical band maximum, ymax cm-’, for M-, e; and I- as a function of increasing solvent polarity. A, Na-; B, K-; C, Rb-; D, Cs-; E, e;; F, I-; a, DIPE; b, DEE; c, HMPA; d, THF; e, DME; f, diethylamine (DEA); g, diglyme; h, EA; i, 1,2- propanediamine (PDA); j, EDA. The slope shown for F corresponds to 0.6 that of E observed in more polar solvents (see text). The symbol, +, represents I- data for which v,,, is suspect due to ion-pairing.Similarly, 7, refers to a value for vmax e; which is probably too high because of experimental difficulties due to solvent absorption. For clarity the ordinate for E is displaced towards higher energies by 7000 cm-l and that for F towards lower energies by 27 000 cm-l. occurs for the M- species and furthermore, to our knowledge, no evidence has been obtained which would indicate that the M- band maxima are dependent on either Mf or M- concentration. It would appear therefore that in solution any association with M+ is very loose and solvent ~ e p a r a t e d , ~ ~ ’ ~ ~ a feature which is also consistent with M- being a centrosymmetric species.35 M f M- + M-, M+ (loose). The relatively smaller effect observed for Cs” is consistent with a significant lack of solvation for the Cs+ cation relative to Na+ or K+.34*37 A further point can be derived from fig.3 in that if the correlation extends to22 OPTICAL SPECTRA AND YIELDS OF SOLVATED ELECTRONS NH3 for which vmax (e;) = 5570 cm-1 at 25"C,12 then the Na- peak position should be observed at or about the same wavelength (690 nm) as in EA. Earlier results3' on the optical spectra of Na- observed in EA + NH3 alkali metal solutions containing <7% NH3, extrapolate instead to a band maximum of (1300-1600 nm) for the Na- species in pure NH3. In the EA + NH, mixtures referred to in fig. 2 we observe no shift in v, (Na-) with increasing NH3 concentration but rather a decrease in the equilibrium concentration of Na-. Our observation is in agreement with the results of H o h l ~ t e i n ~ ~ * ~ ~ and Dalton,41 which show no significant shift in vmaX for Na-, but instead a decreasing concentration of Na- in alkali metal solutions containing <40 mol % NH3 in EA.This leads us to the conclusion that the metal-independent band maxima observed in alkali metal ammonia solutions42 do not involve the formation of a species structurally equivalent to M-, but instead represent the formation of a different spin-paired species which absorbs in the infrared. In concluding this section it should be noted that in all cases reported to date the optical spectrum of (M+, e;) lies intermediate between those of e; and M- observed in the same solvent. The band maximum is very solvent dependent with the magnitude of the blue shift from the e; band being associated with a decrease in atomic ~haracter.~~ In other words a decreasing polarity of the solvent increases the interaction of e; with the cation and is reflected by an increasing blue shift of the (M+, e;) absorption band.ELECTRON YIELDS The concepts of various types of electron states ranging from quasi-free (e;)qf or dry (e-)dry to partially solvated or weakly trapped (e;),,, or (e;)damg to deeply trapped or fully solvated (e;) are currently in v o g ~ e . ~ ~ * ~ ~ In water and methanol the yield of solvated electrons corresponds to G(e;) = 4.6 (100 ps) and 2.3 (30 ps) molecules per 100 eV, r e s p e ~ t i v e l y . ~ ~ ~ ~ ~ Since G (total ionisation) is approximately 5, the fraction of electrons which become solvated and escape the spur decreases significantly with decreasing solvent polarity.Consequently we have examined the effect of polarity on the yield and escape probability of e; in the series NH3, MA, EA and IPA. Picosecond flash photolysis of these solvents at temperatures between -40 and -80°C indicates the solvation process is complete in - 6 ps.47-49 This time scale is much faster than that observed in alcohols at similar low temperature~,~~*~~ but comparable to that observed in water. In ammonia, G(e;) = 3.3 & 0.3 independent of temperature from -75 to 23°C and from 3 to 500 ns.12352953 The yields of e; in MA, EA and IPA, expressed as the product of G(e;) and the extinction coefficient E e s at the absorption maximum, are shown in fig. 4-7 as a function of dose and temperature immediately following a 20 ns or 300 ns pulse.In MA the yields are independent of temperature from 20 to -89°C (m.p. -94°C) and with decreasing dose extrapolate back to a common value of GE = 6.5 & 0.3 x lo4 (fig. 4). In EA and IPA, with decreasing temperature two time domains can be resolved; a fast initial process, independent of dose attributed to reactions in the spur, and a slower second order component due to decay processes in the bulk solution. At low doses (G0.5 x 1019 eV dm-3) or when the initial decay is 290% complete the bulk processes become more pseudo first order presumably due to the presence of impurities or a significant difference in the ratio of remaining e; to oxidising component. The fraction of e; escaping the spur processes increases with decreasing temperature such that in EA (m.p.-84°C) the value for GE in the bulk solution ranges from -2 x lo4 at 20°C to -4.5 x lo4 at -80°C (fig. 5 and 6). Distinct differences are evident from MA to EA and IPA.J . W. FLETCHER AND W. A. SEDDON 23 Corresponding values for IPA (m.p. --10loC) extend from -1 x lo4 at 20°C to 4.5 x lo4 at -97°C (fig. 7). In both EA and IPA the extrapolated value at the end of a 20 ns pulse approaches 5.5 st 0.5 x lo4. At low temperatures the optical band maxima for e; can be clearly resolved with relatively minor differences in band shape between the different amine~.’~ No evidence for spectral shifts with time were observed after a 300 ns pulse. Assuming then that E e s is independent of temperature the observed differences in GE can then be taken to reflect changes in G(e;).1 I 6 5 4 U I E a 3 X Q 2 1 t i m e after p u l s e / s FIG. 4.-The effect of dose per pulse on the yield of e; in methylamine at 20°C; A, 0.3; B, 0.5; C , 1.2; D, 3.6; E, 6.4; F, 9.0; G, 17.6; H, 23.0 x eV dmA3, respectively. Values for G(e;) can be estimated on the basis of the solute and temperature independence of the yields in MA. Assuming G(e;) = 2,” then E ~ ; - 3 x lo4 for each amine. These values are summarized in table 1 along with the corresponding yields in H20, alcohols, N2H4 and NH, for comparison. Within the ammonia-amine series the results exhibit a gradual decrease in G(e;) with decreasing solvent polarity and static dielectric constant (0,). However the effect of increasing D, within the series is more pronounced than observed, for example, in polar organic liquids including the alcohol^.^^^^^ Comparing the amine and liquid NH3 data one must infer that there is very little spur decay of e, in NH,.This is probably due to a combination of the relatively high mobility of the ammoniated electron58 (approximately ten times greater than H,O) and its slow rate of reaction with the positive i o 1 P This would also seem to apply to MA where the diffusion rate is apparently faster than the competing rate processes for spur decay. In EA and IPA the corresponding spur processes appear to be much faster than in MA. However, with decreasing temperature these rates are slowed down, thereby enhancing the escape probability, giving an observed G(e;) comparable with MA.The rela- tively high values obtained for G(e;) in the lower dielectric amines, as compared with the alcohols, presumably reflects the slower decay within the spur for the amine series. Table 2 summarizes the measured second order rate constants for the decay24 OPTICAL SPECTRA AND YIELDS OF SOLVATED ELECTRONS U I 52 x w c;l 10-8 10-7 10-6 10-5 lo-" time a f t e r p u l s e / s FIG. 5.-The effect of dose per pulse on the yield of e; in ethylamine at 20 and -80°C. A-D, -80°C at 1.6, 5.6, 9.6 and 23.0 x eV dm-3, respectively. E-J, 20°C at 0.5, 1.3, 3.2, 6.8, 8.9 and 23.0 x 10'' eV dm-3, respectively. 1 1 I I U 1 s1 x w u 1 I 10-8 10-6 10-L t i m e after p u l s e / s FIG, 6.-The effect of temperature at doses G0.5 X 1019 eV dm-3 on the yield of e; in methylamine and ethylamine.A and B, methylamine at -89 and 20°C, respectively. C-G, ethylamine at -80, -70, -50, -30 and 20°C, respectively.J . W. FLETCHER AND W. A . SEDDON 25 t i m e after pulse / s FIG. 7.-The effect of temperature and dose per pulse on the yield of e; in isopropylamine. A-E, -93°C at 0.1,0.25,0.8,1.5 and 23 x lo1' eV dm-3; F-H, -70°C at 0.7,2.2 and 23 x 10'' eV dm-3; I-K, -33°C at 0.8,4.4 and 23 x 10'' eV dm-3; L, -14°C at 23 x 10'' eV dm-3; M-0,20°C at 2.4, 6.6 and 23 x 10'' eV dm-3. TABLE ~.-COMPAFUSON OF THE INITIAL SOLVATED ELECTRON YIELD, G(e;) AND THE YIELDS ESCAPING SPUR PROCESSES, G(e;) ESCAPED, IN VARIOUS SOLVENTS. IN MA, EA AND PA WE ASSUME AN EXTINCTION COEFFICIENT Ees- = 3 x lo4 dm3 mol-' crn-l. SOURCES OF THE OTHER DATA ARE REFERRED TO IN THE TEXT temp ./"C G (total ionisation) G(G-) G(e ;) escaped 20 - 15 -30 - 60 - 80 H2O N2H4 CH30H NH3 MA EA IPA 5.4 4.6 4.8 4.8 4.8 4.7 3.4" 2.3 3.3 2.2 1.8 1.8 2.7 2.6 1.2 3.3 2.2 0.66 0.33 3.3 2.2 3.3 2.2 0.9 0.66 3.3 2.2 1.26 1.03 2.2 1.53 1.43 a Ref.(61). TABLE 2.-sECOND ORDER RATE CONSTANTS FOR THE NON-SPUR DECAY PROCESSES OF e, IN NH3 AND AMINES AS A FUNCTION OF TEMPERATURE. EXPERIMENTAL VALUES OF k/c ARE ALL CORRECTED WITH RESPECT TO cmax = 3 x lo4 dm3 mol-1 cm-l rate constant, k x 10-1°/dm3 mol-' s-l temp./"C NH3 MA EA IPA 20 2 10 150 300 - 30 8 60 70 -60 6 10 20 -80 4 5 726 OPTICAL SPECTRA AND YIELDS OF SOLVATED ELECTRONS of e; in the bulk solution. If these values are considered to be representative of the spur processes then the relative loss of e; in the spur increases significantly in the order of IPA > EA > MA.Interestingly, these rate constants are all comparable at -80°C. It is also significant that while G(e,) is -2 in the pure amines the formation of Na- and biphenyl anion observed in basic s o l u t i o n ~ ~ ~ ~ ~ corresponds to a yield of e; of 4.8. Thus there appears to be either an initial yield of " dry " electrons, G(e-d,y) -2.8, which in the absence of base do not become solvated, or alternatively, the additional source of e; results from the reaction of the solvent base with radicals,59 or possibly H atoms. Further studies are in progress on this aspect but at the present time the question remains unresolved. On the basis of the results in H20, alcohols and the amines it would seem reasonable to expect G (total ionisation) in NH3 of -5. To-date we have no experimental evidence for such a yield.This merits further study on a picosecond time scale. Finally it is worth reporting that preliminary results in EA + NH3 mixtures give G(e;) E ~ ; - 2 x lo4 at concentrations of NH3 <2% electron fraction (\(5 mol dm-3). However, over this same concentration range the optical band has clearly shifted toward that of e; in NH3 (fig. 2). In pure ammonia,55 GE = 1.5 x lo5 so it appears, as in the alcohol + alkane mixtures,27 that the yield does not change in concert with the optical spectrum. At this ammonia concentration the preferential solvation process should be complete on a picosecond time scale27*60 indicating that the escape probability of e; is not related to the trap depth as monitored by vmaX but rather by the bulk dielectric properties.The authors acknowledge the cooperation and assistance of Dr. H. A. Gillis in Thanks are also due to Mr. F. C. Sopchyshyn the use of the NRC linear accelerator. and Mr. J. J. Jevcak for their technical assistance. Metal-Ammonia Solutions, Colloque Weyl I, ed. G. Lepoutre and M. J. Sienko (Benjamin, New York, N.Y., 1964). Metal-Ammonia Solutions, Colloque Wevl 11, ed. J. J. Lagowski and M. J. Sienko (I.U.P.A.C.) (Butterworths, London, 1970). Electrons in Fluids, Colloque Weyl 111, ed. J. Jortner and N. R. Kestner (Springer-Verlag, Berlin, 1973). Electrons iri Fluids-the Nature of Metal-Ammonia Solutions, Colloque Weyl IV, ed. B. Craw ford, J.Phys. Chem., 1975, 79, 2789. J. W. Fletcher and W. A. Seddon, J. Phys. Chenz., 1975, 79, 3055. K. Bar-Eli and T. R. Tuttle, Jr., J. Clzem. Phys., 1964, 40, 2508. J. L. Dye and L. R. Dalton, J. Phys. Chem., 1967,71, 184. R. Catterall and P. P. Edwards, J. Phys. Chem., 1975, 79, 3010. Wiley and Sons, New York, 1974), vol. 2, p. 133. * T. R. Tuttle, Jr., J. Phys. Chenz., 1975, 79, 3071. lo M. Szwarc and J. J. Grodzinski, Ions and Ion-Pairs in Organic Reactions, ed. M. Szwarc (J. l1 J. W. Fletcher, W. A. Seddon and F. C. Sopchyshyn, Canad. J. Chenz., 1973, 51, 2975. l2 W. A. Seddon, J. W. Fletcher, J. Jevcak and F. C . Sopchyshyn, Canad. J. Chem., 1973, 51, 3653. l3 N. V. Klassen, H. A. Gillis and 6. G. Teather, J. Plzys. Chern., 1972, 76, 3847. l4 R. R. Hentz and G .A. Kenney-Wallace, J. Phys. Chem., 1972, 76, 2931. l5 R. R. Hentz and G. A. Kenney-Wallace, J. Phys. Chem., 1974, 78, 514. l6 T. J. Kemp, G. A. Salmon and P. Wardman, Pulse Radiolysis, ed. M. Ebert, J. P. Keene, A. J. Swallow and J. H. Baxendale (Academic Press, London, 1965), p. 247. l7 J. H. Baxendale and M. A. J. Rodgers, J. Phys. Chem., 1968,72, 3849. Is L. M. Dorfman, F. Y. Jou and R. Wageman, Beu. Bunsenges. Phys. Chem., 1971,75, 681. l9 F. Y. Jou and L. M. Dorfman, J. Chem. Phys., 1973,58,4715. 2 o J. L. Dye, M. G. DeBacker and L. M. Dorfman, J. Chem. Phys., 1970,52, 6251.J . W . FLETCHER AND W. A . SEDDON 27 'l E. A. Shaede, L. M. Dorfnian, G. T. Glynn and D. C. Walker, Canad. J. Cliem., 1973, 51, 22 P. S. Childs and R. R. Dewald, J. Pliys. Chem., 1975, 79, 58.23 E. I. Mal'tsev, A. M. Koulkes-Pujo, A. V. Vannikov and N. A. Bakh, High Energy Chem., 24 R. Olinger and U. Schindewolf, Ber. Bunsenges. Phys. Chem., 1971, 75, 693. " T. K. Cooper, D. C. Walker, H. A. Gillis and N. V. Klassen, Canad. J. Chem., 1973, 51,2195. 26 J. R. Brandon and R. F. Firestone, J. Phys. Chem., 1974,78, 792. "J. H. Baxendale and E. J. Rasburn, J.C.S. Faraduy I, 1974, 70, 705. 28 J. H. Baxendale, J. P. Keene and E. J. Rasburn, J.C.S. Faraday I , 1974, 70, 718. 29 J. H. Baxendale and P. H. G. Sharpe, Chem. Phys. Letters, 1976, 41, 440. 30 W. A. Seddon, J. W. Fletcher and F. C. Sopchyshyn, Canad. J. Chem., 1976,54, 2807. 31 W. A. Seddon, J. W. Fletcher and R. Catterall, Canad. J. Chem., 1977, 55, 2017. 32 M. T. Lok, F. J. Tehan and J.L. Dye, J. Phys. Chem., 1972,76,2975. 33 M. F. Fox and E. Hayon, Chem. Phys. Letters, 1974, 25, 511. 34 J. Smid, Ions and Ion-Pairs in Organic Reactions, ed. M. Szwarc (Wiley Interscience, New York, 35 J. L. Dye, C. W. Andrews and J. M. Ceraso, J. Phys. Chem., 1975,79, 3076. 36 M. Szwarc, Ions and Ion-Pairs in Organic Reactions, ed. M. Szwarc (Wiley Interscience, New 37 D. N. Bhattacharyya, C. L. Lee, J. Sniid and M. Szwarc, J. Phys. Chem., 1965, 69, 608. 38 S. Matalon, S. Golden and M. Ottolenghi, J. Phys. Chem., 1969, 73, 3098. 39 von G. Hohlstein and U. Wannagat, Z. Anorg. Chem., 1956, 284, 191. 40 von G. Hohlstein and U. Wannagat, Z. Anorg. Chem., 1956, 288, 193. 41 L. R. Dalton, J. D. Rynbrandt, E. M. Hansen and J. L. Dye, J. Chem. Phys., 1966,44, 3969. 42 J. L. Dye, Metal-Ammonia Solutions, ed. J. J. Lagowski and M. J. Sienko, Colloque Weyl 11, 43 R. K. Wolff, J. E. Aldrich, T. L. Penner and J. W. Hunt, J. Phys. Chem., 1975,79,210. 44 W. J. Chase and J. W. Hunt, J. Phys. Chem., 1975,79,2835. 45 C. D. Jonah, M. S. Matheson, J. R. Miller and E. J. Hart, J. Phys. Chem., 1976,80, 1267. 46 D. W. Johnson and G. A. Salmon, Canad. J. Chem., 1977, 55,2030. 47 D. Huppert and P. M. Rentzepis, J. Chem. Phys., 1976, 64, 191. 48 D. Huppert, P. M. Rentzepis and W. S. Struve, J. Phys. Chem., 1975,79,2850. 49 D. Huppert, W. S. Struve, P. M. Rentzepis and J. Jortner, J. Chem. Phys., 1975, 63, 1205. J. H. Baxendale and P. Wardman, Chem. Comm., 1971,429. 51 J. H. Baxendale and P. Wardman, J.C.S. Faraday I, 1973,69, 584. 52 Farhataziz and L. M. Perkey, J. Phys. Chem., 1975, 79, 1651. 53 J. Belloni, P. Cordier and J. Delaire, Chem. Phys. Letters, 1974, 27, 241. 55 W. A. Seddon, J. W. Fletcher, F. C. Sopchyshyn and J. Jevcak, Canad. J. Chem., 1974, 52, 56 E. Hayon, J. Chem. Phys., 1970,53,2353. 57 G. R. Freeman and J. M. Fayadh, J. Chem. Phys., 1965,43, 86. st? J. Jortner, Ber. Bunsenges. Phys. Chem., 1971, 75, 696. 59 J. L. Dye, M. G . DeBacker, J. A. Eyre and L. M. Dorfman, J. Phys. Chem., 1972,76,839. 6o A. Mozumder, J. Phys. Chem., 1972,76, 3824. 3905. (Eng. Trans.), 1975, 9, 168. 1972), vol. 1, p. 86. York, 1972), vol. 1, p. 1. (I.U.P.A.C.) (Butterworth, London, 1970), p. 1. W. A. Seddon and J. W. Fletcher, in preparation. 3269. J. Delaire, P. Cordier, J. Belloni, F. Billiau and M. 0. Delcourt, J. Phys. Chem., 1976, 80, 1687.

ISSN:0301-7249    

DOI:10.1039/DC9776300018

出版商:RSC    

年代:1977

数据来源: RSC

摘要:

Reactions in Spurs : Mechanisms of Hydrated Electron Formation in Radiolysis of Water BY AJIT SINGH Medical Biophysics Branch, Whiteshell Nuclear Research Establishment, Atomic Energy of Canada Limited, Pinawa, Manitoba, Canada ROE 1LO AND W. JOHN CHASE AND JOHN W. HUNT Ontario Cancer Institute and Department of Medical Biophysics, University of Toronto, Toronto, Ontario, Canada M4X 1K9 Received 7th December, 1976 Hydrated electron and solute anion formation in H20 and D20 has been investigated with the picosecond pulse radiolysis technique. The spectrum of e& in D20 is slightly shifted towards the blue as compared with that in H20. The yields of erq in liquid H20 and D20 are larger than those of e- in the corresponding vapours. Use of salicylate anion (Sal,,) as an electron scavenger shows that the yields of its electron adduct (Sal;) are considerably larger than those of the e,.The importance of electronically excited water molecules in the formation of these extra yields is indicated. Three possible mechanisms to explain the extra yields are considered: (i) solvent assisted e;, formation from excited water; (ii) energy transfer from excited water to OH, leading to e; formation; the OH:.js postulated to result from ionic dissociation of vibrationally excited water molecules in spurs ; and (111) interaction of Sal,, and excited water to produce Sal,. Early processes in radiolysis of water have been the subject of numerous theoretical and experimental investigations.' The development of the pulse radio- lysis te~hnique,l-~ has been particularly helpful in investigating the formation and reactions of erq. Here we present results obtained recently with the picosecond pulse radiolysis system4 and correlate them with the theoretical predicti~ns.~-l~ Picosecond pulse radiolysis studiesl1-l5 indicate that the yields of eFq and the yields of solute electron adducts are coiisiderably greater than the yields of e- in water vapour.16 This investigation is designed to check the yields in D20, both in pure water and in concentrated solutions, correlate these results with the theoretical predictions5-lo and finally to try to show whether the participation of excited water molecules can explain these high yields.Role of excited molecules in steady state radiolysis of liquid water and aqueous solutions has been discussed by many worker^.'^-^^ The bulk of the energy deposited in a chemical system by high energy radiations, per energy deposition The transient species so formed in gases are homogeneously distributed.However, in the condensed phases they are heterogeneously distributed and are localized in small regions25 called " spurs ". Despite this difference, the spectrum of primary activations should be the same in the condensed phases as in the gaseous However, phase does influence radiolysis of a substance. This is mainly due to the differences in the secondary reactions of the primary transient species formed, since in spurs the concentration of the transients is several orders of magnitude greater, e.g., > mol dm-3 in spurs6v9 in contrast with values < mol dmq3 in a gaseous system (at a dose of 1 krad in is 20-60 eV.A .SINGH, W. J . CHASE AND J . W. HUNT 29 1 11s). This difference in the transient concentration, increased probability of col- lisional deactivation and the occurrence of the cage effect2' in the liquid phase are the three main reasons for the differences in the radiolysis of a substance in the two phases. K ~ p p e r r n a n n , ~ ~ ~ Schwartz and others lo have developed diffusion kinetic models on the basis of which yields of erq have been predicted. In essence, these involve extrapolation of the microsecond yield of e& to early times s) on the basis of the known rate constants and other relevant parameters of the primary species formed in the spurs. Schwartz' has predicted G(erq)initial = 4.8 and G(eTq) ns = 3.5.More recent calculations by Kuppermann6 assume a bigger spur than that assumed by Schwartz9 (radius 67.5 compared with 23 A) and larger energy per spur (172 as compared with 62.5 eV). This gives a better fit to the observed data6 with G(eLq)initial = 4.35 and G(eFq)' ns = 4.04. The observed values of G(e&) at 30 psll and 100 ps12 are 4.0 and 4.6, respectively and the observed decay12 of eYq in 1 ns is -10%. Thus the gap between the observed re~u1tsll-l~ and the predictions of the diffusion models 6*9 has narrowed over the years though some differences still remain. Santar and BedniE have extended the calculations of Platzman and ~ o w o r k e r s ~ * ~ ~ * ~ ~ on the yields of ionization and excitation in water. Their estimate of the primary yields in water is: G(ion) = 3.3 and G(exc) = 3.5 - 4.5.There is also a discrepancy between these theoretical predictions and the observed yields of products expected from primary ionization and excitation of liquid water. The sum of G(e-) + G(H) + G(H2) can be taken as an approximate measure of G(ion) + G(exc), on the basis of the known mechanisms of radiolysis of water.1°*16 The observed total primary yield, G(e-) + G(H) + G(H2)- 7.5 in water vapour,16 is in good agreement with the theoretically predicteds sum, G(ion) + G(exc) - 7.3 & 0.5. However, G(e-) + G(H) + G(H2) in liquid water9 is only -5.7. The fate of the missing G(ion) + G(exc) - 1.6 is not known. Further, G(eTq) in liquid water 9~11~12 ( -4) is greater than G(e-) in water vapour l6 (3.3) and G(H) in liquid water (-0.6) is much less than in water vapour16 (-4).The mechanistic reasons for these differences are not known. It has been suggestedgJoJ6 that G(H) is lower in liquid water due to the cage effect. However, high y(H) in photolysis of liquid water3' argues against this. that in liquid water most of these discrepancies involve previously disregarded roles of electronically and vibrationally excited water molecules (H20* and H20*"). Possible involvement of H20* in producing eG has been mentioned by many a ~ t h o r s . ~ ~ ~ ~ ? ~ ~ Pnvolvenient of electronically excited states in producing ions has recently been suggested34 to explain the observed ion yields obtained on radiolysis of liquefied rare gases (Xe, Kr, Ar). Its ionization potential is to that of H20 [12.619 & 0.006 eV (H20) and 12.636 5 0.006 eV (D20)].The yield of G(e-) in D20 is reported'' to be about 2.7 as com- pared to G(eTq) = 4.5 (this work). The value of G(ion) + G(exc), given by G(e-) + G(D) + G(D2) is -8 in D20 vapour,16 whereas it is ~ 5 . 6 ~ ' in liquid D20, [taking G(e;J = 4.5 from this work] suggesting a similarity of discrepancies in the radiolysis of D20 and H20. It has been The W value of D20 does not seem to have been measured. EXPERIMENTAL The picosecond pulse radiolysis system has already been d e ~ c r i b e d . ~ * ~ ~ - ~ ~ A 1 A, 6 ns beam of 45 MeV electrons was used in these experiments; the beam consisted of fine structure pulses, 35 ps apart. The dose was 3-5 krad/6 ns pulse. Two picosecond absorption traces in HzO and D20 are shown in fig. 1, in which the " steps " are the absorption signals from30 REACTIONS I N SPURS: MECHANISMS OF HYDRATED ELECTRON FORMATION FIG.1.-Typical absorption traces at 700 nm. (a) HzO, (b) DzO. the fast formation of e&, As explained in detail in the earlier the observed signal is the summation of all of the analysis Cerenkov flashes. When the signals do not decay to the baseline in 350 ps, the absorption signals build up during the 6 ns electron pulse. For example, the average intensity of the analysis light beam is reduced by -50% due to absorption and the resulting signals are expanded electronically to fill the particular field. The yields of the various transient species have been obtained by comparison with the yield of e 4 in HzO [G(e&)30p, = 4.01.l' Recently, Jonah et a1.12 have reported G(eJloo ps = 4.6, in HzO.However, this value is not consistent with the extensive scavenger studies reported earlier by Wolff et al.l5 If the higher valuelZ of G(e,) is later found to be the correct one, all the yields reported here will have to be increased accordingly (see table 1). All work was done at room temperature (-21°C). RESULTS The decay curves of el;, in H20 and D20 are shown in fig. 1. There does not seem to be any difference in the eTq decay rates at 700 nm, in 350 ps. The spectra of eTq in D20, obtained at 30 ps and 6 ns are shown in fig. 2. In an earlier run a small shift in the spectrum was observed (A,,,, 30 ps = 710 nm; A, 6 ns = 685 nm). But we have been unable to confirm it in subsequent runs.The A, in the 30 ps I I 400 500 600 700 800 900 1000 wavelength / n m FIG. 2.-Absorption spectra of e l . (a) HzO, at 30 ps and 6 ns; (6) DzO, - 6 ns; c1 and 0 one run, 0 another run, 30 ps; 30 ps data are normalized to the corresponding 6 ns data at the A,,,.A . SINGH, W. J . CHASE AND J . W. HUNT 31 and the 6 11s spectra appear to be the same, at 705 nm (fig. 2). A careful comparison of the decay rates of eFq at 600 nm and at 850 nm shows that the latter is slightly faster in D20 while it is the same in H,O; it would seem, therefore, that the resulting shift in the spectrum is too small to be measured accurately by our equipment. The spectrum of eLq in H20 which is the same at 30 ps and 6 ns, is also shown in fig.2, for comparison. The eFQAmax in D20 is at shorter wavelength than in H20, as is shown clearly in fig. 3, which is consistent with the behaviour of ions with charge I 1 I I I I I I 6ot .n 301 L n 30- s 20- .n ---A- __L L - i 650 700 750 800 850 900 950 w a v e l e n g t h / n m FIG. 3.-Absorption spectra of e, at 6 ns (a) - - - - - y H20, (b) - Y D20. transfer type of transition^.^^-^' This spectrum of eTQ in D20 is similar to that observed by other worker^.^^,^^ A comparison of the transient spectra obtained on pulse radiolysis of Na, K and Li salicylates is shown in fig. 4. The spectra obtained in the solutions of Na and Li salicylates are quite similar. However, the main peak of the spectrum in the solution of K salicylate is shifted slightly towards the red and there are minor differences also in the 400 nm and the 600 nm regions. This suggests a role of the cation in the spectra of transients that are formed by the addition of eYq to the salicylate anion.As observed b e f ~ r e , ~ ~ , ~ ' * ' ~ the initial yield of eFq at 30 ps is reduced by electron w a v e l e n g t h / n m FIG. 4.-Absorption spectra of Sal, at 6 ns in 1 mol dm-3 solutions in H,O. - Li salicylate; Na salicylate; ----- K salicylate. ----32 REACTIONS IN SPURS: MECHANISMS OF HYDRATED ELECTRON FORMATION scavengers at high concentrations. The solute concentration to reduce the initial e,, yield to 37% is called the C37 value.36 The C3, values obtained in H,O and D,Q are similar for NaNO, (0.45 * 0.05 and 0.38 5 0.05 mol dm-3) and sodium salicylate (0.70 & 0.10 and 0.76 rf 0.10 mol dm-,).There is similar fast formation of the solute electron adducts in H,O and D20, as shown in fig. 5. The traces are the absorption signals from a 2 mol dm-, solution of sodium salicylate at 460 and 600 nm. These signals can be explained by the competing reactions e- ‘Ferq with -8% of the absorption at 460 nm being due to e;, followed by the normal reaction Sal,, + e;, - Sal;,. (2) k- 1Olo mol-1 dm* s-l / FIG. 5.-Typical absorption traces of Sal, and e; in 2 mol dm-3 solutions of Na salicylate. (i) HzO (a) 460 nm; (b) 600 nm. (ii) DzO (a) 460 nm; (b) 600 nm.A. SINGH, W. J . CHASE AND J . W . HUNT 33 The fast formation (30 ps) of the signal at 460 nm is mainly due to reaction (1); the signal due to the formation of SalLq by reaction (2) (30-350 ps) is almost exactly balanced by the decay of egq.Thus, there is a small additional formation of Sal; in the 30-350 ps region. In the assessment of the total yield of Salrq at 30 ps, cor- rections for electron densities of the solutions and for the eFq present were made. It has been assumed that the radiation yield of electrons from sodium salicylate in solution is similar to that from water, except by the mechanisms discussed later. The spectra of Salrq in D20 and H20, at 30 ps and 6 ns, are shown in fig. 6. The w a v e l e n g t h I n m FIF. 6.-Absorption spectra of Sal, in 2 mol dm-3 solutions of Na salicylate. (a) ___ , H20, and (6) - - - - -, D20; 6 ns; 0 and @ 30 ps. 30 ps spectra are corrected for the yield of e, at 30 ps and normalized to the corresponding 6 ns spectra at the Am,,.spectrum of SalFq is the same at the two times and in the two solvents (fig. 6). It is slightly different than that reported by Amphlett et aZ.46 in the 440-460 nm region. However, it agrees with the recent microsecond pulse radiolysis measurements by Hunt and Fielden and Hart4' measured E(e;q)D20 = 2.02 x lo4 rno1-I dm3 cm-'. This was based on G(e;q)free ion in D,O = 2.9,48 for which higher values have more recently been reported (3.06 & 0.0549 and 3.1 & 0.1).50 Taking the latter value is equal to 1.91 x lo4 mo1-' dm3 cm-l, which we have used in our calculations. The value in H20 is the same, within experimental error (1.85 x lo4 mol-' dm3 cm-l). We have assumed E(Sal;q)D20 = c(Sal;q)H20, on the basis of the similarity of the spectra of SalLq in H 2 0 and D 2 0 (fig.6) and on the basis of similar E of eZq and some other ions in the two solvents, e.g., (i) the e (cobaltous chloride) at (490 nm is the same 51 in D20 and H20, and (ii) Fielden and Hart4' found only small differences in E(MNOI) and E(MnO:-) in D,O as compared with H20 solutions. The value of E(SalTq) reported by Amplilett et aZ.46 was 4000 mo1-I dm3 cm-l at 440 nm. More precise measurements by Hunt and coworkers 47 have found the value to be 4500 mol-l dm3 cm-l at 456 nm. The measured G values of elq and SalTq are given in table 1. The rate constant, k[(e,g)D20 + (D30+),,], has been found to be equal to 0.8 &- 0.2 x 1Olo mol-1 dm3 s-'. This compares with k[(e;q)H20 + (H30+),,] = 1.5 & 0.2 x 1Olo ino1-I dm3 s-l. Addition of NaOH to H20 and NaOD to D20 increased the yield of eLq at 30 ps (-1076 with 1.5 mol dm-3 added alkali).34 REACTIONS I N SPURS: MECHANISMS OF HYDRATED ELECTRON FORMATION TABLE YI YIELDS OF eLq AND Sal, species A,,,/nm &max/mOl-l dm3 cm-l G (a, c) G (4 ea\ (H2O) 71 5 1.85 x 104 4.0 f 0.3 4.6 f 0.2b e, (D20) 705 1.91 x 104 4.5 f 0.3 5.1 f 0.3 Sal, (H20)" 458 4.5 x 104 5.2 f.0.4 6.0 f 0.4 Sali @B>" 458 4.5 x 104 5.4 rt: 0.4 6.2 & 0.4 at 100 ps; Jonah et d . 1 2 G(eJ = 4.6 (Jonah et UZ.),'~ at 30 ps. Based on G(eJ = 4.0 (Wolff et uZ.).ll Based on in 2 mol dm-3 solutions. DISCUSSION (a) MECHANISMS OF eFa FORMATION The yields of e; in the liquid phase are higher than those in the vapour phase, in both D20 and H20.On examining the mechanisms by which these are produced we have the following comments on the differences in the total yield of e- in the two phases. (i) DIRECT IONIZATION ionization yield by the following reaction Since the primary activation steps do not depend on the p h a ~ e ~ * ~ ~ * ~ ~ the maximum Water water ,--) water+ + e- 4 e; (3) should be only G = 3.3 in liquid H20 and G = 2.7 in liquid D20. (ii) IONIZATION FROM EXCITED STATES The effective ionization potential of compounds is lowered in solution in polar liquids since the solvation energy of the resulting ions becomes available for ioniza- t i ~ n . ~ ' The threshold for the following reaction was found to be 6.5 eV by Boyle et ~ l . , ' ~ H O H20* H20& + e i . (4) However, all the excited states of water with energy >6.5 eV are not likely to The quantum ionize with unit efficiency [if they did, G(ei) would have been 271.yields3' for e i and H atom formation in liquid water are given in table 2. TABLE 2.-PHOTOLYSIS OF WATER 31 exciting 1 Inm lev &GI> P(H) 184.9 6.7 0.03 rt 0.01 0.33 f 0.01 147.0 8.4 0.06 f 0.02 0.72 L- 0.02 123.6 10.0 0.09 f 0.03 1.03 f 0.02 Similar values of q(e;) at 184.9 nmS4 and q(H) at 123.6 nmS5 have been reported by other workers. The possibility of the observed q ( e ~ ) ~ l being low due to geminate recombination can be ruled out since q(ea,) + y(H) z 1 at 123.6 nm (table 2). The yields of excited water are quite high in H20 (G - 4) and D20 (G - 5) (see Introduction). However, in view of the reported31 values of q(ez) (table 2), it is unlikely that in radiolysis, the efficiency of e& formation from H20* would be >O.1. We estimate that G(eJ - 0.4 would be expected from reaction (4). Assuming similar photolysis results for D20, G(eJ from D20* would be -0.5.A. SINGH, W . J . CHASE AND J . W . HUNT 35 The total G(e;)H20 and G(e&,,o by the two mechanisms thus come to 3.7 and 3.2, respectively. The higher observed values (table 1) suggest that additional mechanism(s) also contribute to e, formation. The following one is postulated as an important additional mechanism. (iii) e 4 FROM OH,-, Ionic dissociation of water is end other mi^^^ by 0.58 (H20) and 0.62 eV (D20). Before electron thermalization takes place in spurs, vibrational excitation of the substrate by subexcitation electrons (eTe) is quite l i k e l ~ . ~ ~ , ~ ~ The average vibrational excitation energy imparted to water m 0 1 e c u l e s ~ ~ ~ ~ ~ would be -1 eV for H20 and -0.8 eV for D20; this should be enough in both cases to overcome the endo- thermicity ; ionic dissociation should, therefore, proceed efficiently in spurs.Ionic dissociation of vibrationally excited water has been reported by Goodall and Green- From their5' results, yl(0H;) % 1 from H20*" can be calculated. The yield of OH4 in spurs is likely to be 32 2 0.2 mol d111-~, based on G(H20*") z 9, [G(e,) z 1.5, E(e,) z 6.6 eV] and y(0H;) z 1. Therefore e; can be produced in spurs by the following reactions in H20, H20 -3 H20+ +- eTe + . . . etc. (5) (6) (7) H O H20 + eye --+ H20*" H& + OH& H20* + OH; -+ H20 + OH + e; and by analogous reactions in D20.The energy required for solvent assisted e& formation60 from OH; is 23.3 eV. Photoionization of OH, has been observed61962 to occur on irradiation at 6.7 eV. The quantum yield of e, formation61*62 is 0.11 to 0.15. Since the electronic excitation energy63-65 of H20* is >3.3 eV, (>6.6 eV, except for 3H20* which is at -4.5 eV) e i formation in spurs by this mechanism is possible. We suggest that this mechanism contributes significantly to the difference between the observed e; values (table 1) and those from mechanisms (i) and (ii) above, in H20 and D20. The small increase in the yield of e; on the addition of sodium hydroxide to water is consistent with this mechanism. (b) POSSIBLE MECHANISMS OF FORMATION The yields of SalG in both H20 and D20 are larger than those of eG (table 1).Two causes for this have been mentioned before : (i) Sal; formation from electrons that recombine during the resolution time of the picosecond pulse radiolysis Most probably, H20* is also an important precursor of the extra yield of Sal;, by the following reaction and (ii) reaction of Sal,, with dry e l e c t r o n ~ ~ * ~ ~ ? ~ ~ to produce Sal;. H20 *+ Sal,, --f H20& + Salz. (8) Since the energy of most of the electronically excited states of water 6365 is > 6.6 eV, and the threshold of photoionization of liquid water53 is 6.5 eV, reaction (8) should proceed from all these excited states. Whether reaction (8) can also proceed from triplet excited water, whose energy63 is -4.5 eV, depends on the electron affinity of Sal,,.During the interaction of H20* with Sal,, energy transfer is also possible to give Sal',,, H20* -1- Sal,, -+ H20 + Sal',, (9)36 REACTIONS I N SPURS: MECHANISMS OF HYDRATED ELECTRON FORMATION which, in turn, can show fluorescence and yield e;. The energy requirement or the efficiency of reaction (11) are not known. The photoionization of phenol takes place6’ at 8.5 eV in aqueous solutions. Therefore, reaction (1 1) is most likely to require < 8.5 eV of energy. Fluorescence of salicylates has been reported. 68 Recently, evidence for reaction (9) has been ~btained;~’ radiolysis of solutions of Na salicylate in water showed fluorescence due to Sal’,,. The fluorescence intensity increased as a function of the concentration of Sal,, in a manner characteristic of solvent to solute energy trans- fer.57p70 Similar results were reported by Stein and Tomkiewicz. 71 Continued formation of SalG over the 30-350 ps period following the electron pulse (fig.5) is also consistent with the implication of H20-* in the extra G(Sa1i) as compared to G(ei), by reactions (8), (9) and (1 1). The help of M. A. O’Laughlin and A. Worthington in performing many lengthy experiments and of the staff of the University of Toronto Linac Laboratory for providing good electron beam parameters, is gratefully acknowledged. We wish also to thank Dr. A. Mozumder for helpful discussions, and Dr. R. S. Dixon and Dr. G. R. Freeman for communicating their results to us prior to publication.This work was financially supported by the National Research Council, the Medical Research Council and the National Cancer Institute. J. W. Hunt, Advances in Radiation Chemistry, ed. M. Burton and J. L. Magee (John Wiley, New York, 1976), vol. 5, chap. 3, p. 186. Pulse Radiolysis, ed. M. Ebert, J. P. Keene, A. J. Swallow and J. H. Baxendale (Academic Press, London, 1965). M. S. Matheson and L. M. Dorfman, Pulse Radiolysis (M.I.T. Press, Cambridge, Mass., 1969). M. J. Bronskill, W. B. Taylor, R. K. Wolff and J. W. Hunt, Rev. Sci. Instr., 1970, 41, 333. A. Kuppermann, Radiation Research, ed. G. Silini (North-Holland, Amsterdam, 1967), p. 212. A. Kuppermann, Physical Mechanisms in Radiation Biology, ed. R. D. Cooper and R. W. Wood (USAEC, Division of Biomedical and Environmental Research, Washington, 1974), p.155 (Conf. 721001). R. L. Platzman, Radiation Research, ed. G. Silini (North-Holland Amsterdam, 1967), p. 20. I. Santar and J. BednBf, The Chemistry of Ionization and Excitation, ed. G. R. A. Johnson and G. Scholes (Taylor and Francis, London, 1967), chap. 5, p. 217. H. A. Schwarz, J. Phys. Chem., 1969,73, 1928. lo I. G. DraganiC and Z . D. DraganiC, The Radiation Chemistry of Water (Academic Press, New York, 1971). l1 R. K. Wolff, M. J. Bronskill, J. E. Aldrich and J. W. Hunt, J. Phys. Chem., 1973, 77, 1350. I t C. D. Jonah, M. S. Matheson, J. R. Miller and E. J. Hart, J. Phys. Chem., 1976,80, 1267. l3 J. W. Hunt, R. K. Wolff, M. J. Bronskill, C. D. Jonah, E. J. Hart and M. S . Matheson, J. Phys. l4 C. D.Jonah, E. J. Hart and M. S. Matheson, J. Phys. Chem., 1973, 77, 1838. l6 R. S. Dixon, Radiation Res. Rev., 1970, 2, 237. Chem., 1973, 77,425. R. K. Wolff, J. E. Aldrich, T. L. Penner and J. W. Hunt, J. Phys. Chem., 1975,79, 210. T. J. Sworsky, Solvated Electron (Advances in Chemistry, Series 50, American Chemical Society, Washington, 1965), p. 263. H. A. Mahlman and T. J. Sworsky, The Chemistry of Ionization and Excitation, ed. G. R. A. Johnson and G. Scholes (Taylor and Francis, London, 1967), p. 259. l9 G. Stein, The Chemistry of Ionization and Excitation, ed. G. R. A. Johnson and G. Scholes (Taylor and Francis, London, 1967), p. 25. R. W. Matthews, J.C.S. Faraday I, 1974, 70, 1384.A . SINGH, W . J . CHASE AND J . W. HUNT 37 21 A. M. Rauth and F. Hutchinson, in Biological Effects of Ionizing Radiation at the Molecular 22 A.M. Rauth and J. A. Simpson, Radiation Res., 1964,22, 643. "A. Mozumder, Advances in Radiation Chemistry, ed. M. Burton and J. L. Magee (Wiley, 24 I. Santar and J. Bednit?, Int. J. Radiation Phys. Chem., 1969, 1, 133. 25 A. H. Samuel and J. L. Magee, J. Chem. Phys., 1953, 21, 1080. 26 D. M. Brown and F. S. Dainton, Radiation Res. Rev., 1968, 1, 241. 27 M. J. van der Weil, Radiation Research: Biomedical, Chemical and Physical Perspectives. Proc. 5th Cong. Radiation Res., ed. 0. F. Nygaard, H. I. Adler and W. K. Sinclair (Academic Press, New York, 1975), p. 205; and his discussion remarks (Seattle, 1974). Leuel (International Atomic Energy Agency, Vienna, 1962), p. 25. New York, 1969), vol. 1, chap.1, p. 1. " J. Franck and E. Rabinowitch, Trans. Faraday SOC., 1934,30, 120. 29 J. Dump and R. L. Platzman, Disc. Faraday SOC., 1961, 31, 156. 30 R. L. Platzman, Int. J. Appl. Radiation Isotopes, 1961, 10, 116. 31 N. Getoff and G. 0. Schenck, Photochem. Photobiol., 1968, 8, 167. 32 A. Singh, Reactions in Spurs. Role of Excited States in Hydrated Electron Formation in Radiolysis ofwater, a paper presented at the E. J. Hart International Conference on Radiation Chemistry, Argonne, 1975. 33 S. J. Rzad and R. H. Schuler, J. Phys. Chem., 1973, 77, 1926. 34 S. S.-S. Huang and G. R. Freeman, Canad. J. Chem., 1977, in press. 35 D. H. Katayama, R. E. Huffman and C. L. O'Bryan, J. Chem. Phys., 1973,59,4309. 36 J. E. Aldrich, M. J. Bronskill, R. K. Wolff and J. W. Hunt, J.Chem. Phys., 1971,55, 530. 37 M. J. Bronskill, R. K. Wolf€ and J. W. Hunt, J. Chem. Phys., 1970, 53, 4201. 38 J. E. Aldrich, P. Foldvary, J. W. Hunt, W. B. Taylor and R. K. Wolff, Rev. Sci. Instr., 1972, 39 M. F. Fox and E. Hayon, Chem. Phys. Letters, 1974,25, 51 1. 40 M. Halmann and I. Platzner, Proc. Chem. Soc., 1964, 261. 41 H. Benderly and M. Halmann, J. Phys. Chem., 1967,71, 1053. 42 D. M. Brown, F. S. Dainton, J. P. Keene and D. C. Walker, Proc. Chem. Soc., 1964,266. '' V. Lopata and R. S. Dixon, personal communication, 1976. 44 R. K. Wolff, M. J. Bronskill and J. W. Hunt, J. Chem. Phys., 1970, 53, 4211. 45 K. Y . Lam and J. W. Hunt, Int. J. Radiation Phys. Chem., 1975, 7, 317. 46 C. B. Amphlett, G. E. Adams and B. D. Michael, Radiation Chemistry (Advances in Chemistry, 47 J.W. Hunt and coworkers, unpublished results, 1974-76. 4 8 E. M. Fielden and E. J. Hart, Radiation Res., 1968, 33, 426. 49 K.-D. Asmus and J. H. Fendler, J. Phys. Chem., 1969,73, 1583. 51 D. C. Martin, Nature, 1938, 142, 756; Chem. Abs., 1939, 33, 922-2. 52 M. Kasha, Comparative Efects of Radiation, ed. M. Burton, J. S. Kirby-Smith and J. L. Magee 53 J. W. Boyle, J. A. Ghormley, C. J. Hochanadel and J. F. Riley, J. Phys. Chem., 1969, 73, 2886. 54 U. Sokolov and G. Stein, J. Chem. Phys., 1966, 44, 2189. 55 W. Zich and N. Getoff, Monatsh. Chem., 1970, 101, 1583; Chem. Abs., 1971, 74, 26605d. 56 L. G. Hepler and E. M. Woolley, Water, A Comprehensive Treatise, ed. F. Franks (Plenum Press, New York), Vol. 3, pp. 149, 157. 57 A. Singh, Radiation Res. Rev., 1972, 4, 1. 58 A. Mozumder, personal communication, 1974. 59 D. M. Goodall and R. C. Greenhow, Chem. Phys. Letters, 1971,9, 583. 6o P. B. Merkel and W. H. Hamill, J. Chem. Phys., 1971,55,2174. 61 J. Jortner, M. Ottolenghi and G. Stein, J. Phys. Chem., 1964, 68, 247. F. S. Dainton and P. Fowles, Proc. Roy. SOC. A , 1965,287, 312. 63 F. W. E. Knoop, H. H. Brongersma and L. J. Oosterhoff, Chem. Phys. Letters, 1972, 13, 20. 64 W. J. Hunt and W. A. Goddard 111, Chem. Phys. Letters, 1969, 3, 414. 65 A. Chutjian, R. I. Hall and S. Trajmar, J. Chem. Phys., 1975, 63, 892. 66 W. H. Hamill, J. Phys. Chem., 1969, 73, 1341. 67 L. I. Grossweiner and H. I. Joschek, Soluated Electron (Advances in Chemistry, Series 50, 68 G. Stein and M. Tomkiewicz, Trans. Faraday SOC., 1971, 67, 1009. 69 A. Singh and G. W. Koroll, unpublished results, 1976. 'O G. A. Salmon, Int. J. Radiation Phys. Chem., 1976, 8, 13. 71 G. Stein and M. Tomkiewicz, Trans. Faraday Soc., 1971, 67, 1678. 43, 991. Ser. 81, American Chemical Society, Washington, 1968), p. 231. K. N. Jha, T. G. Ryan and G. R. Freeman, J. Phys. Chem., 1975,79,868. (John Wiley, New York, 1969), p. 72. American Chemical Society, Washington, 1965), p. 279.

ISSN:0301-7249    

DOI:10.1039/DC9776300028

出版商:RSC    

年代:1977

数据来源: RSC

摘要:

Numerical Studies of Electron Tunnelling in Liquids BY P. ROBIN BUTLER AND MICHAEL J. PILLING Physical Chemistry Laboratory, South Parks Road, Oxford, OX1 342 Received 13th December, 1976 The diffusion equation, derived from Fick‘s second law, with an added exponential sink term to simulate electron tunnelling, is integrated numerically to determine the rate of electron decay at times greater than 1 ps. The effect of a coulomb interaction with a charged scavenger is examined and the steady-state rate constant shown to approximate closely to that obtained by combining the separate effects of tunnelling and charge-affected diffusion, which can be expressed analytically. Diffusion in the presence of a charge-induced dipole interaction is investigated for the case of scavenging of localised electrons in alkanes.The rate constant is shown to be dominated by random diffusion and tunnelling and the bias induced by the interaction is of little consequence. The sensivity of the rate constant to changes in the pre-exponential factor in the sink term is shown to be most favourable at short times. The experimental evidence for tunnelling of electrons to scavengers following irradiation of molecular and aqueous ionic glasses is now well estab1ished.lp2 Theoretical models have been proposed which describe the time dependence of the electron decay sati~factorily.~*~ Tunnelling has also been implicated in electron scavenging reactions in liquids and an analytical steady-state treatment has been presented which agrees well with conventional room temperature experiment^.^ Numerical studies in the time-dependent zone have also found agreement with experimental data obtained on a picosecond time scale at room temperature7 and at longer times at reduced temperatures.* This paper investigates numerically the effect of scavenger charge and polarizability on the reaction rate. It also examines the sensitivity of the rate constant to changes in the pre-exponential factor, a’, in the electron tunnelling term; a’ depends upon the electronic transition moment and the Franck-Condon factors for the reaction and is thus scavenger dependent. A. DIFFUSION EQUATION Pilling and Rice5 proposed that the influence of tunnelling on a reaction of a solvated electron may be determined by including an exponential sink term in the equation derived from Fick’s second law.The equation must be further modified when there are additional long range interactions between the reactants which affect their motion : where S(r, t ) is the ensemble averaged concentration of scavengers around electrons, r is the electron scavenger distance, D the diffusion coefficient and u the inter-reactant potential energy. (2) Zs(r) is the sink term: &) = a’ expi-& - R))P. ROBIN BUTLER AND MICHAEL J . PILLING 39 where a' is a frequency factor and p is a parameter related to the trap depth [fig. l(a)] : p == 2(2172 "( V, - E)>*/tZ (3) where m* is the effective mass of the electron and R is the true encounter distance. Since the electron is produced instantaneously, the initial distribution is assumed random, i.e., the inter-reactant potential is assumed not to influence the electron trapping distribution.The Smoluchowski boundary conditions may consequently be applied: S(r, t ) = 0 r < R, t 2 0 S(r, t ) = So r > R , t = O S(r, t) -+ s o r + co, t > 0. (6) Spur effects, non-random scavenger-scavenger distances and the effects of other ions on u are all neglected. These will all have only small effects at low scavenger concentrations and at the long times associated with steady state reactions (t > 10-los in water at 298 K). The approximations will become less good at the high scavenger concentrations required on picosecond time scales. As will be shown, however, charge effects have only a very small influence on the reaction rate at very short times (< s).B. COULOMB INTERACTION The coulomb potential energy is u = -Ze2/(4m,~r) ( 7 4 where Ze is the charge on the scavenger, e the charge on the electron, E, the vacuum permittivity and E the relative permittivity of the solvent. Expressing eqn (1) in terms of the reduced variables X = pr/2, Y = XS/S,, L2 = 4a' exp(pR)/p2D, z = P2Dt/4, and RT = IZle2/(4ne,~kT). ( 9 4 R, is the distance at which the coulomb interaction energy is equal to kT. tunnels9 and eqn (2) and (3) become inappropriate. r' from the electron [fig. l(b)] is now given by: The coulomb term also modifies the potential barrier through which the electron The barrier height at a distance V(r') = V, - E + Ze2{(r - re)-' - (Y - r ' ) - 1 ) / 4 x ~ o ~ . Thus Zs(r) = a' exp[-p (1 + b/(r - re) - b/(r - r')} dr'] (10) where b = Ze2/{4n~,~(V0 - E)).The integral in eqn (10) was determined analytically and included in eqn (1) and40 NUMERICAL STUDIES OF ELECTRON TUNNELLING I N LIQUIDS 1b) FIG. l.-(a) Schematic diagram of an electron trap, radius Y, and a scavenger trap, radius rS, with their centres separated by a distance Y. (b) The distortion of the potential energy profile for a charged scavenger. (7b). Eqn (7b) was expressed in incremental form and solved numerically, using the modified Crank-Nicolson method.'' to obtain the time-dependent scavenger profile, The time-dependent rate constant [k(t)] was found by summing the diffusive S(r, 9. [k,(t)] and tunnelling [kT(t)] rate constants: 5 9 6 k,(t) = 4nR2D(ap(R, t)/ar) kT(t) = co 4zr2p(r, t ) I, ( r ) dr R where p(r, t> = S(r, t)/So. A difficulty arises in the choice of the value of the relative permittivity. The aupr term requires the static value whilst it could be argued that the barrier per- meability term requires a high frequency value.Table 1 shows the percentage change (related to the E = co solution) in the rate constant for various values of e. Only the dependence of the barrier permeability on the coulombic interaction has been TABLE ~.-MoDIFJIcATION OF T m TIME DEPENDENT RATE CONSTANT BY A COULOMBIC PER- TURBATION OF THE POTENTIAL ENERGY BARRIER. (a' = s-l, j3 = 1Olo m-l, D = lo-* mz S-l, R = 0.5 ~ n , 2 = - 1) - log(t/s) k,-co/m3 molecule-1 s-' {(k,=m - k,)/k,=,) x 100 e = 80 E = 40 & = 8 12 1.54 x 10-15 2.0 2.5 10.9 10 2.17 x 10-l6 0.9 1.8 7.6 8 1.42 x 10-l6 0.7 1.3 5.9 6 1.36 x 10-l6 0.7 1.3 5.7 4 1.36 x 10-l6 0.7 1.3 5.7P.ROBIN BUTLER AND MICHAEL J . PILLING 41 included i n the calculation; the term in au/ar was excluded. The results show that, even for E = 8, the change in the rate constant is small. Thus %he inclusion of this term is of only minor importance and the exact choice of E is I:ot critical. This conclusion substantiates the qualitative argument proposed by Pilling and Rice.g Eqn (10) was included in the remaining calculations, but the static dielectric constant was employed. Fig. 2 shows the time dependence of the rate constant for a range of values of r -'I -171 ' I I I I J -12 -10 -8 -6 -4 log ( t l s ) FIG. 2.-Time dependence of the scavenging rate constant, k: A, Z/E = 0; X , Z/E = -1.25 x +, Z/E = 1.25 x My Z/E = 2.5 x 0, Z/E = 2.5 x e, Z/E = 5 x a' = 1014 s-l, p = 1O1O m-l, D = m2 s-', R = 0.5 nm.(Trap depth = 1 eV.) Z/e. The charge effect is small at short times (< the steady state. state rate constant is given by: where s) and is most pronounced in For uncharged reactants, Pilling and Rice showed that the steady k = 4nReffD (1 1) and I.V: = 4a'/P2D, y is Euler's constant (0.57721 . . .) and Io(wo) and K,(w,) are the modified first and second kind Bessel functions of zero order and argument w,. The rate constant for a difftision-controlled reaction between charged reactants, exclusive of any tunnelling contribution, is given by;'' where k = 4nRD6/(es - 1) 6 = ZAZ,e"/(4~EoERkT) and ZAe and 2,e are the reactant charges.Fig. 3 shows the percentage difference (based on the analytic value) between the computed steady rate constant (k,) and that obtained from eqn (13) with R replaced by Reff(ka). The analytic expression under- estimates the rate constant for oppositely charged species and overestimates it for42 NUMERICAL STUDIES OF ELECTRON TUNNELLING I N LIQUIDS I I 1-80 - 2.5 0 2.5 102z/c FIG. 3.-Percentage difference ({(k, - k,)/k,) x 100) between the computed steady state rate constant, k,, and the analytic rate constant, k,, based on eqn (12) and (13), as a function of Z/E (a). Percentage difference ({(kEZm - k,)/k,=,) x 100) between the rate constant with Z/E = 0 and with Z/E non-zero (0). u' = 1014 s-', p = 10" m-', D = lo-* m2 s-', R = 0.5 nm. (Trap depth = 1 eV.) similarly charged reactants.Fig. 3 demonstrates that the analytic estimate is generally quite good except for Z/E > 2.5 x The correction to the Z/E = 0 rate constant (fig. 4), arising from the combined effects of charge and tunnelling, is con- siderably greater than the error involved in using the analytic expression [eqn (12), (13)]. The accuracy of the computational estimates of the steady state rate constant may be assessed by comparing the value for Z/E = 0 with that from eqn (1 l), which is exact; the error is only 0.5%. 2.0 0. 0.81 I I I I I I -12 -10 -8 -6 l o g ( t / s ) FIG. 4.-Time dependence of the rate constant, k, for reaction with a scavenger of polarizability up. 0, polarization potential included; 0, polarization potential excluded. u' = 4 x 1014 s-l, /3 = 0.7 x 10" m-', D = 7.2 x nm3,12 f = 0.69.11 (Trap depth = 0.5 eV.) m2 s-', R = 0.592 nni, aI, = 6.55 xP .ROBIN BUTLER AND MICHAEL J . PILLING 43 Quite crude approximations have been made regarding the structure of the dielectric medium; no allowance has been made, for example, for the effects of di- electric saturation. The range of values of Z/E used is also quite limited. The major aim of this study was to examine the effects of charge and tunnelling on electron reactions in water at 298 K. The results show that a good estimate of the rate constant for a diffusion-controlled reaction may be obtained by combining eqn (1 1)- (14). At very short times, the reaction is dominated by static tunnelling6 and the charge effect is small. C. CHARGE-INDUCED DIPOLE INTERACTION Baird l2 recently suggested that the charge-induced dipole interaction between an electron and a neutral scavenger is important in low polarity solvents.He examined the reaction between e; and SF6 and solved the steady state equation [cf. (1) with as/& = 01 for reactant diffusion, but did not include a tunnelling term. He found that, for delocalized electrons, ie., those with a velocity correlation length, I, >0.1 nm, the rate constant is less than the diffusion-controlled value, confirming that tunnelling from distances >R is indeed unimportant. For localised electrons ( I < 1 A, e.g., in ethane) the rate constant (k) is greater than the diffusion-controlled value (kdiff) (table 2). This is reminiscent of several reactions in aqueous solution, TABLE 2.-A COMPARISON OF EXPERIMENTAL RATE CONSTANTS FOR ELECTRON SCAVENGING BY SF6 IN AJXANES WITH THOSE CALCULATED FROM EQN (13) (R = 0.592 nm) hexane13 297 2.1 x 10-7 3.3 x 10-15 273 1.0 x 10-7 1.6 x 1045 248 4.0 x 7.5 x 217 2.6 x 10-6 5.3 x 10-14 175 7.2 x 10-7 1.3 x 10-14 142 4.9 x 10-8 1.7 x 10-15 110 1.2 x 10-9 8.3 x 10-17 ethane l4 200 1.6 x 2.5 x 2.1 2.2 2.5 2.7 2.1 2.4 4.7 9.4 2.Ob 2.2b 2.5b 2.3d 2Sd 2.7d 3.P 5.0d 2.1' 2.3" 2.5" 2.3" 2.5' 2.6" 3.3" 4.2" where large rate constants have been rationalized by invoking t~nnelling.~ In particular, it should be noted that k/kdiff increases as D falls, a property associated with a long-range rea~tion.~ The diffusion equation is insoluble even in the steady state if the tunnelling term is included, and numerical techniques must be resorted to.The interaction potential is now: where up is the polarizability (assumed isotropic) of the scavenger andfis the screening function12 given by f = ( E + 2)/3~.44 NUMERICAL STUDIES OF ELECTRON TUNNELLING I N LIQUIDS Thus is defined as before and Ri = a,e2f/(8m0kT). The interaction potential is of such short range (RT - 0.7 nm) that, in the light of the results discussed in section C, the effect of the polarization potential on the barrier permeability was neglected. Because of the strong dependence of u on Y, the barrier is quite strongly perturbed in the immediate vicinity of the scavenger, but our lack of a detailed knowledge of the electron or scavenger trap limits the utility of incor- porating such an effect.Fig. 4 shows a plot of the rate constant against time both with and without the polarization term. The rate constants are designed to simulate roughly Bakale et aZ’s14 data for e- + SF6 in ethane at 175 K. Tunnelling has a marked effect, increasing the rate constant by 90% (5.36 x m3 s-l to 1.02 x m3 s-l). The polarization effect is, however, quite small, enhancing k by only a further 10%. This arises because the effective encounter distance, Reff, is greater than RT, the distance at which the polarization energy is comparable with thermal energies. Thus, for localized electrons in alkanes, the charge-induced dipole interaction may be neglected for rapid reactions and the analytic eqn (1 l), for a diffusion-controlled reaction with a tunnelling contribution, used to estimate the rate constant.Table 2 contains estimates of the rate constants on this basis. The calculated rate constants reproduce the experimental variation reasonably well, with realistic tunnelling para- meters, given the probable errors associated with the experimental data and the simplifying assumptions of constant a’ and p. p = 7 and 5 nm-l correspond to trap depths of 0.5 and 0.25 eV respectively. The mechanism of diffusion of electrons in alkanes is thought to involve thermal promotion to the continuum followed by retrapping.13 For liquids with Z < 1, this mechanism closely approaches the Brownian motion assumed in Fick’s second law. The treatment outlined in this section assumes that reaction takes place from the electron trap; it presumes that there is a low probability of reaction from the con- tinuum.An alternative mechanism may be proposed in which the electron reacts during the time that it is in the continuum. The alternatives could only be distinguished if the trapping frequency were known. D. THE DEPENDENCE OF THE ENSEMBLE-AVERAGED RATE CONSTANT ON a’ Using Fermi’s golden rule, the distance dependent scavenging probability may be expressed in the form2’l6 where W(r) is the electronic matrix element for electron transfer at a distance Y, p is the vibrational overlap integral for the scavenger ion and the scavenger vibrational levels, Ef and El are the total h a 1 and initial energies of the reacting system and q R is the vibronic partition function of the reactants. l h e electronic matrix element contains both the electronic wavefunctions for the scavenger and scavenger anion and the vibronic wavefunctions for the occupied (initial) and unoccupied (final andP .ROBIN BUTLER AND MICHAEL J . PILLING 45 unrelaxed) electron trap. The assumed exponential distance dependence of Zs(r) is contained in I W(r)I2. The vibrational wavefunctions involved in the overall Franck- Condon factor refer to both high frequency molecular modes and low frequency medium modes? Some attempts have been made to calculate the ensemble averaged rate constant, k, on a somewhat similar An important preliminary step, however, is an assessment of the sensitivity of k to changes in a'. Dainton et al.4 examined the dependence of the electron decay kinetics on a' for scavenging in rigid media.They showed that the decay is very insensitive to a' for t > s, and is determined primarily by p. This conclusion presents difficulties in accommodating the variation in scavenger efficiency found under these conditions. Miller ascribed the variation entirely to the Franck-Condon factors, but very large changes in these were required to explain the experimental results. A further un- resolved problem arises since the decay curve is expected to deviate2p3 from the linear form proposed by Dainton et aL4 if a' is small. There are, in consequence, difficulties which remain in a pure tunnelling description of electron scavenging in glasses. It has been suggested, although not proved, that these problems can be resolved if trap to trap tunnelling is incorporated.19 This uncertainty in the overall mechanism is absent from descriptions of liquid phase reactions, since diffusion, by whatever mechanism, is catered for.A reaction in solution which takes place by a contact interaction, and which does not involve a long range mechanism, may be described by the scheme: kd kr A + B P- {AB} + products k-d where kd is the rate constant for formation of the encounter pair, (AB} and k, and k-d are the first order rate constants for reaction of (AB) and for diffusive separation respectively. Since the reaction does not involve tunnelling, p = co and k, = Z,(R) = a', [provided the description contained in eqn (15) remains valid]. The dependence of the overall rate constant, k, on a' is shown in fig. 5. For a' < k-d, k is linear in a' (k = a'kd/k -d) and provided kd/k -d, the equilibrium constant for formation of the encounter pair, can be estimated,20 a' can be deduced.When a' 9 k - d , k = kd and log I o('/s- ' 1 FIG. 5.-The dependence of the rate constant, k, on u'. - -, /? = 00, t = co; -.- , B = m , D = 1 0 - 8 mz s-1, t = 10-1' s; 0, /j = loLo m-l, t = co; 0, B = 1O1O m-l, t = lo-" s. R = 0.5 nm.46 NUMERICAL STUDIES OF ELECTRON TUNNELLING I N LIQUIDS the reaction is diffusion-controlled with a rate constant independent of a’. At short times, k is larger in the diffusion-controlled region, because reaction takes place from an unrelaxed reactant distribution. s for a reaction involving tunnelling (j3 = lolo m-l). The steady-state rate constant con- tinues to increase even when the reaction is diffusion-controlled, because the effective encounter distance [eqn (12)] increases with a’.k remains, however, comparatively insensitive to variations in a’, a five-decade increase in a’ leading to only a four-fold increase in k. The sensitivity is increased somewhat at shorter times; at s the same increase leads to a ten-fold change in a’. The determination of the rate constant for an electron reaction can lead to accurate information on a’, the frequency term in the distance dependent scavenging probability, only provided a’ < loll s-’, in water at 298 K, i.e., provided the reaction rate is limited by the rate of reaction in the encounter pair. This could arise if the Franck- Condon factor were small, or, alternatively, if the reaction were endoergic.For larger values of a’, the rate constant is most sensitive to variations in a’ at short times. k still contains no information on a’. Fig. 5 also shows plots of k against a’ in the steady-state and for t = We thank Dr. Stephen A. Rice for helpful discussion and the S.R.C. for a student- ship to P. R. B. J. R. Miller, J. Phys. Chem., 1975, 79, 1070. M. J. Pilling and S. A. Rice, Chem. Reu., to be published. M. Tachiya and A. Mozumder, Chem. Phys. Letters, 1974, 28, 87. F. S. Dainton, M. J. Pilling and S. A. Rice, J.C.S. Faraday 11, 1975, 71, 1333. M. J. Pilling and S. A. Rice, J.C.S. Faraday 11, 1975, 71, 1563. P. R. Butler, M. J. Pilling, S. A. Rice and T. J. Stone, Cunad. J. Chem., to be published. C. D. Jonah, J. R. Miller, E. J. Hart and M. S. Matheson, J. Phys. Chem., 1975,79,2705. G. V. Buxton, F. C. R. Cattell and F. S. Dainton, J.C.S. Faruday I, 1975, 71, 115. M. J. Pilling and S . A. Rice, J . Phys. Chem., 1975, 79, 3035. lo J. Crank and P. Nicolson, Proc. Canzb. Phil. Soc., 1947, 43, 50. l1 P. Debye, Trans. Electrochem. Soc., 1942, 8, 265. J. K. Baird, Canad. J. Chem., to be published. l3 A. 0. Allen, T. E. Gangwer and R. A. Holroyd, J. Phys. Chem., 1975,79,25. l4 G. Bakale, U. Sowada and W. F. Schmidt, J. Phys. Chem., 1975,79,3041. l5 B. Brocklehurst, Chem. Phys., 1972, 2, 6. l6 J. Ulstrup and J. Jortner, J. Chem. Phys., 1975, 63, 4358. A. Henglein, Ber. Bunsenges. phys. Chem., 1974, 78, 1078. A. Henglein, Ber. Bunsenges. phys. Chem., 1975, 79, 129. l9 G. V. Buxton and K. Kemsley, J.C.S. Faraday I, 1975, 71, 115. 2o A. M. North, The Collision Theory of Chemical Reactions in Liquids (Methuen, London, 1964), p. 37.

ISSN:0301-7249    

DOI:10.1039/DC9776300038

出版商:RSC    

年代:1977

数据来源: RSC

摘要:

Yield and Decay of the Hydrated Electron in Proton Tracks A Pulse Radiolysis Study BY W. G. BURNS AND R. MAY Radiation and Surface Chemistry Group, Chemistry Division, A.E.R.E. Marwell, Oxfordshire, OX1 1 ORA AND G. V. BUXTON AND G. S. TOUGH University of Leeds, Cookridge Radiation Research Centre, Cookridge Hospital, Leeds LS 16 6QB Receitled 20th December, 1976 Direct observation of the time dependence of the yield of e, produced in water by nanosecond pulses of 3 MeV protons is reported for the first time. From the effect of added hydroxide ion, G(eJ is estimated to be at least 3.6 at lo-'' s, falling to 2.3 at lO-'s. The experimental measure- ments do not agree with predictions of the present diffusion model; modificatioiis to improve the model are discussed. Reaction (1) describes the radiolysis of water at about lo-' s after the passage of the ionising particle through the medium H20 --v-+ e,, H, OH, H02, M2, H202, H+.(1) Most of the G-values" of these species have been measuredl in steady state radiolysis experiments for a wide range of radiation LET (linear energy transfer). It is generally believed that the primary chemical species, e,, H, OH and H+, are generated inhomo- geneously in local volumes called spurs, blobs and tracks and that the products measured at s are those which exist when the primary species have either reacted together or diffused to a point where the probability of their reaction with a partner generated in the same local volume is negligible. On this basis the diffusion model has been developed with the aim of predicting both (a) the effects of scavengers2 and (b) the eEect of radiation type3 and LET on the steady state G-values of reaction (1).This is achieved by the inclusion of known or estimated values for some parameters (rate and diffusion constants) and selecting others (track or spur radii, initial G-values and energy per spur) to give agreement with experimental data. In the course of proceeding from the initial to the final state, the spatial distribution of each species modelled, and therefore the quantity of each present, is necessarily calculated from zero time onwards as a function of time. This predictive aspect of the model may now be tested since recent developments in pulse radiolysis methods * The C-value is the number of a species created or destroyed per 100 eV of absorbed energy.Steady state yields which apply to reaction (1) at low scavenger concentrations are written as G,; under other conditions yields are written as G(X).48 YIELD A N D DECAY OF THE HYDRATED ELECTRON I N PROTON TRACKS have made possible direct observation of e; from 10-I' s onwards for low LET radiation, typically G0.23 eV nm-l. Such experiment^^-^ clearly show that the early decay of e; is about an order of magnitude slower than that predicted by the diffusion model. No pulse radiolysis data have been reported for high LET radiation, so that the time profile of G(e;) is not known under such conditions. We are investigating, therefore, the pulse radiolysis of water and aqueous solutions at high LET by using 1 ns pulses of 3 MeV protons, which have a mean LET of 29 el? nm-l, to obtain information on the decay of e& and to test the diffusion model under these conditions.In this paper we report our preliminary findings which show features similar to those of the low LET data.4-7 EXPERIMENTAL Full details of the techniques and equipment used in this work will be described elsewhere,8 and only a brief description of the novel features is given here. 3 MeV protons were produced in nominally 1 ns pulses by the Harwell Van de Graaff IBIS facility9 at a repetition rate of 0.01, 0.1 or 1 MHz and a peak current of about 5 mA. The path length of 3 MeV protons in water is only 0.15 mm, so it was advantageous to restrict the sample to this thickness. This was achieved by irradiating the sample in the form of a streamlined jet flowing at about 8 m s-l across the proton beam as in fig. 1.The proton beam was focused at the jet into a 1 mm diameter spot, but slight instability in the beam transport system caused the position of this spot to wander within an area about 2 mm in diameter. Analysing light from a 2 mW He/Ne CW laser was focused with a 20 cm focal length lens so that it passed coaxially through the jet acting as a light pipe. The transmitted light intensity was measured with a photomultiplier and oscilloscope combination which had an overall risetime of about 5 ns. All solutions were prepared from distilled water and were irradiated under de-aerated conditions at ambient temperature. All solutes were AnalaR grade and were used as supplied.E from laser U lmrn I I FIG. 1.-Detail of proton pulse radiolysis system. A: Stainless Steel Jet, B: Spectrosil window, 1 mm thick, C: 633 nm light from He/Ne Laser, D: Water Jet, E: Proton Beam. RESULTS AND DISCUSSION Fig. 2 shows the formation and decay of the e& absorption signal produced in de-aerated water by a 1 ns pulse of 3 MeV protons. We attribute most of the time taken for the signal to reach its maximum value to the risetime of the detection system since the pulse width was 1 ns (fwhm), e i is expected to be formed in <4 ps,l0 and the major effect of the detection system risetime is to delay the signal by that amount, although some distortion of the early decay of the signal occurs. We have not deconvoluted the signal from risetime effects and, for the present, we shall assume thatW.G . BURNS, R. M A Y , G . V. BUXTON AND G . S . TOUGH 49 245 rnV 235 U 20 ns FIG. 2.-Oscilloscope trace from proton pulse irradiated deaerated water; ;1 = 633 nm; pulse width 1 ns. the signal maximum in fig. 2 occurs at 1 ns in order to place the ensuing decay on an absolute timescale. Evidently the decay of e; is very rapid for about 5 ns and then much slower for the next 100 ns or so. At this time under low LET conditions, the residual e& signal is generally identified4-7 with G,, in reaction (1). It seems reasonable, therefore, to adopt a similar procedure here for high LET radiation. No value of Ge, for 3 MeV protons appears to have been published, but a value of 1.07 can be interpolated from data on the dependence of Ge, on LET determined by Appleby and Schwarz" who used 7 x mol dm-3 nitrous oxide to scavenge e;.Equating this value with our measured absorption at 140 ns, we estimate that G(eJ = 2.35 6 0.20 at the signal maximum in fig. 2. Table 1 shows the effect of added solutes, which scavenge Hf and OH, on the yields of e i at the end of the pulse and at 70 ns after the pulse. 1 mol dm-3 OH- increased G(e4) significantly at both times, but addition of 1 mol dm-3 MeOH had TABLE ELECTRON YIELDS IN WATER AND AOUEOUS SOLUTIONS PULSE IRRADIATED WITH solution H2O 1 rnol dm-3 MeOH 1 rnol dm-3 OH- 3 MeV PROTONS G(e,)"lb AG(t)' AG(s)* end of pulse (1 ns) 2.35 & 0.20 1.18 k 0.19 1.17 f 0.28 - 2.67 f 0.17 1.20 k 0.25 1.47 f 0.30 0.32 f 0.26 3.58 f 0.31 1.86 i 0.27 1.72 f 0.41 1.23 & 0.37 after 70 ns 1 rnol dm-3 MeOH + 1 rnol dm-3 OH- 3.66 f 0.43 a Assuming G(e,) in H20 at 140 ns = 1.07 (see text).2.19 f 0.29 1.47 f 0.52 1.31 f 0.47 Quoted uncertainties are 1 standard deviation. AG(s) is the change in G(e&) at end of pulse produced by the scavengers. little effect on the yields in water or in 1 mol dm-3 OH- solution. At these solute concentrations the lifetime of H+ is s and that of OH is s. It is not surprising, therefore, that 1 mol dm-3 MeOH has no appreciable effect on G(ez) at the end of the pulse, but the large increase produced by 1 niol dm-3 OH- shows that reaction (2) is important between AG(t) is the change in G(eJ between end of pulse and 70 ns. s and low9 s. ea; + H+ -j H. (2) Table 1 also shows that the decrease in G(e;) after the pulse is rather similar for all the solutions, which suggests that reactions (2) and (3) are not very important in the decay of e; after s.Presumably the decay which we observed at these times is due largely to other reactions such as (4) and (5): ea; + OH -+ OH- (3)50 YIELD AND DECAY OF THE HYDRATED ELECTRON I N PROTON TRACKS Although we have only been able to measure G(ei) at a few nanoseconds we can get a good approximation of the yields at shorter times from the observed change in G(e,) and the lifetime of the scavenged reaction partner of the electron following the procedure used for low LET r a d i a t i ~ n . ~ Thus we equate the observed end of pulse G(ei) in 1 mol dm-3 OH- solution with G(eJ at lo-" s since this is approximately the lifetime of H+, We justify the use of this procedure since in the low LET case it provided good agreement with values subsequently measured by direct method^.^*^*'^ Fig.3 compares the time dependence of G(e,",) for water obtained directly for fast electrons'' and obtained as described above for 3 MeV protons. Two features are noteworthy. First, the initial values of G(ei) for the two qualities of radiation differ by about one unit, possibly because reaction (3) occurs before s; experiments to test this point are planned. Other possibilities are (a) that e; decays earlier than s in the higher LET case, (b) that reactions involving unsolvated (dry) electrons13 may reduce the initial G(e&) and (c) that the observed difference is due to the relatively slow response of the detection system.Deconvolution procedures are being in- vestigated. The second feature is that e; decays earlier and to a greater extent at a higher LET, in qualitative accord with diffusion model predictions. We now discuss 5 the implications of the results in fig. 3 on the diffusion model. 1 I I I 1 I 0 Cr 10 4 I I I I 1 10" lox) ld9 1 2 Ki7 106 t / s FIG. 3.-Decay of e, in proton and electron pulse irradiated water; 0-3 MeV proton pulses; this work, 0-fast electron pulses: Jonah et al., ref. (12). The model as developed by Schwarz2 predicts rather exactly the effects of scavenger concentration on low LET G-values. It is not satisfactory at higher LET, introduced into the model as the energy averaged LET, despite the attempt to reduce the dis- crepancies by including the influence of the impact parameter on spur separati0n.l' The Kupperman method l4 incorporates the distribution in the magnitude of energy loss events, which depends on the radiation quality, and if the modelling concept is valid this method should be more capable of simulating LET effects.The Schwarz method includes a similar consideration at low LET but does not take account of changes in energy degradation spectrum with radiation type. Parameters for both models, as used in our calculations, are given in table 2. Both methods fail to agree with experimental observation on the early decay of e; for low LET radiation, predicting too early a decay which can be corrected3 byW . G . BURNS, R. M A Y , G . V . BUXTON AND G .S . TOUGH 51 TABLE 2.-PARAMETERS OF DIFFUSION MODELS Schwarz model Kupperman model initial species G-value e , 4.78 H 0.62 H2 0.15 OH 5.70 H202 0 H + 4.78 energy per spur spur enlarge- ment factor diffusion constant/ cm2 s-l 4.5 x 10-5 7 x 10-5 5 x 10-5 2.8 x 10-5 2.2 x 10-5 9 x 10-5 62.5 eV 2.5 spur or track radius/nm 2.46 1.15 1.15 1.15 1.15 1.15 initial G-value 4.33 0.38 0 4.7 1 0 4.33 diffusion spur or constant/ track cm2 s-l radius/nm 4.5 x 10-5 1.875 8 x 0.625 2 x 10-5 0.625 10 x 10-5 0.625 - - - 1.4 x 10-5 48 eV 3.6 multiplying both the energy per spur and the spur radius by a spur enlargement factor (see table 2). The factor is smaller for the Schwarz model (2.5) than for the Kupperman model (3.6) because the spur size and energy are larger in the former case.Fig. 4 shows the decay predicted by the unmodified Schwarz model for water alone, and by the corrected model for water alone and water containing scavengers for OH and H+ 5 4 3 h v 1 0 e J Y 2 1 0 10" 10" lo" lo' t / s FIG. 4.-Comparison of observed and calculated decay of e, at low LET. (1) Unmodified Schwarz model, water alone; (2) expanded spur Schwarz model, water alone; (3) expanded spur Schwarz model, water + 1 mol dm-3 MeOH; (4) expanded spur Schwarz model, water + 1 mol dm-3 NaOH; ( 5 ) expanded spur Schwarz model, water + 1 mol dm-3 MeOH + 1 mol dmP3 NaOH; 0 Experi- mental data from ref. (12). separately and together. Agreement with experiment12 is good for water alone (see fig. 4) and is also reasonable for the case with alkali present.12 The predicted effect with OH scavenger present is discussed later.A suitable e i yield against time curve for 3 MeV protons could not be calculated by inserting the mean LET (29 eV nm-l) into the Schwarz model since, with the scavenger concentrations used by Appleby and Schwarz,ll a G,, of 0.55 was obtained compared with the interpolated experimental value of 1.07. The Kupperman model52 YIELD AND DECAY OF THE HYDRATED ELECTRON I N PROTON TRACKS was, therefore, applied using the energy degradation spectrum obtained by the BurchIS method, the Kupperman rate constant^'^ and the parameters in table 2. The Ganguly- Magee-Schwarz equations were used, thereby avoiding a possibly arbitrary join between spherical and cylindrical geometry, but incurring the approximation of modified prescribed diffusion. The results (see table 3) were 0.64 for the normal spur, and 1.11 for the expanded spur, in better agreement with experiment.However, to avoid excessive computation in a preliminary comparison with experiment, the method used was to select a mean LET for the Schwarz model (12.3 eV nrn-l) which gives the experimental value of G e ~ = 1.07 when the model contains the scavenger concen- trations used by Appleby and Schwarz." With these parameters the model gives predictions within 12% of values of the other measured yields (those of H, H2 and H2Q2) interpolated for 3 MeV protons. Fig. 5 shows the calculated e i decay so obtained for the conditions indicated, together with the experimental points from fig. 3. The plotted points agree better with the unmodified Schwarz model, curve 1, than with the expanded spur curve 2, although the points for times below 5 ns may have to be revised upwards, as discussed earlier, so that they approach closer to curve 2.5 1 I I I 4 0 1 I I I I 1 16'' l P lo9 Xie 1 6 ~ li? t / s FIG. 5.-Comparison of observed and calculated decay of e, at high LET. (1) Unmodified Schwarz model, water alone; (2) expanded spur Schwarz model, water alone; (3) expanded spur Schwarz model, water + 1 mol dm-3 MeOH; (4) expanded spur Schwarz model, water + 1 rnol dm-3 NaOH; ( 5 ) expanded spur Schwarz model, water + 1 mol dm-3 MeOH + 1 mol dm-3 NaOH; 0, experimental data-this work. There are a number of reasons, however, why the enlarged spur is not a satisfactory solution to the problem of these discrepancies.When applied to the effects of scavengers on low LET steady state yields, it causes changes in G-values of l0-15%, vitiating the original very good agreement of the Schwarz model.2 In addition it causes the changes resulting from increased LET to occur more slowly (table 3). This latter effect may be advantageous, especially for the Schwarz model. However, the large spur increases an already significant error in unacceptably overpredicting the effect of OH scavenging. In the low l2 and high LET cases the large effect predicted fur 1 rnol d ~ n - ~ methanol (Comparable with that for 1 mol dm-3 hydroxide) is not observed experimentally. This discrepancy is due to the predominance assigned byW. G. BURNS, R . MAY, G . V . BUXTON AND G .S . TOUGH 53 the geometry of the model to reaction (3) compared with (2). This predominance, which is marginally less in the Kupperman model, and which our results suggest should be inverted, arises because of the higher rate constant of reaction (2) and because the higher diffusion constant of H+ compared with OH quickly reduces the concentration of H+. Another modification which should now be considered, in view of this discrepancy, is to change the shape of the initial electron distribution so as to give initially a low concentration at the spur or track centre and higher concentration zones displaced from the centre. In the early stages of track expansion OH + e; will then be sup- pressed relatively more than e; + H+ since the faster diffusing H+ will have a greater probability of occupying the zones of higher concentrations of e;.Preliminary calculations with a two species model suggest that such trends can be observed, in addition to a delay in the decay of the species with low initial concentration at the spur or track centre, which is also a desirable feature of any improved model. Further work on modelling will be reported elsewhere. TABLE 3.-DEGRADATION SPECTRUM FOR 3 MeV PROTONS AND G(e&) CALCULATION FOR SCAVENGER CONCENTRATIONS OF APPLEBY AND SCHWARZ." (fis the fraction of energy deposited by ionizing particles in the energy range given) PROTONS mean total local f G(e,) f x G(eJ G(e,) f x G(e&) energy energy/ LET/eV LET/eV normal spur expanded spur range/MeV MeV nm-' nm-l 2.6-3.0 2.2-2.6 1.8-2.2 1.4-1.8 1 .o-1.4 0.6-1 .O 0.2-0.6 0 .2 2.8 12.5 2.4 14.1 2.0 16.1 1.6 19.05 1.2 23.5 0.8 31.5 0.4 49.6 0.1 93.2 6.90 7.79 8.94 10.65 13.22 18.0 29.13 72.70 0.0736 0.0737 0.0740 0.0745 0.0750 0.0762 0.0789 0.0520 I= 1.25 0.0920 1.17 0.0869 1.10 0.0814 1.00 0.0745 0.85 0.0638 0.70 0.0533 0.47 0.0371 0.23 0.0119 f x G(eJ 0.501 1.90 1.80 1.72 1.60 1.44 1.20 0.92 0.54 0.1398 0.1327 0.1273 0.1 192 0.1080 0.09 14 0.0726 0.0281 0.8191 ELECTRONS (produced by protons) range/keV energy/keV LET/eV normal spur expanded spur energy mean local .f G(eJ f x G(eG) G(eJ f x G(eJ nm-I - 4.8-6.4 3.2-4.8 1.6-2.4 2.4-3.2 1.2-1.6 0.8-1.2 0.6-0.8 0.4-0.6 0.3-0.4 0.2-0.3 0.15-0.2 0.1-0.15 0-0.1 5.6 4.0 2.8 2.0 1.4 1 .o 0.7 0.5 0.35 0.25 0.175 0.125 0.05 2.46 3.30 4.34 7.00 11.00 16.6 25.3 36.6 50.4 66.6 81.0 90.2 93.0 0.0005 0.0036 0.005 1 0.01 23 0.01 18 0.0223 0.0192 0.0337 0.0279 0.0468 0.0357 0.0577 0.1459 1.88 0.0009 2.38 1.75 0.0063 2.27 1.55 0.0079 2.15 1.25 0.01 54 1.90 0.98 0.01 16 1.57 0.73 0.01 63 1.36 0.53 0.0104 1 .00 0.40 0.0135 0.80 0.32 0.0089 0.67 0.24 0.01 12 0.56 0.20 0.007 1 0.49 0.18 0.0104 0.45 0.17 0.0248 0.43 Z f x G(e,) 0.1445 electrons Zf x G(eJ 0.6455 electrons and protons 0.0012 0.0082 0.0109 0.0234 0.01 85 0.0303 0.0192 0.0269 0.01 87 0.0262 0.01 75 0.0259 0.0627 0.2896 1.108754 YIELD AND DECAY OF THE HYDRATED ELECTRON I N PROTON TRACKS CONCLUSIONS The decay of the hydrated electron formed by 3 MeV protons in water alone and containing scavengers for OH and H+ has been observed on the nanosecond time- scale, and some implications for the subnanosecond timescale have been derived.The decay is observed to be delayed compared with predictions of the current diffusion model, but the modification of the model (use of enlarged spur) which is successful in bringing the observed and calculated decays together for low LET appears not to be so successful for 3 MeV protons. We thank the S.R.C. for the award of a studentship to G. S. T., and Dr. F. Wilkin- son for helpful discussions on the signal detection equipment. J. W. T. Spinks and R. J, Woods, An Introduction to Radiation Chemistry (Wiley-Interscience, London, 2nd edn, 1976), p. 258. H. A. Schwarz, J. Phys. Chem., 1969,73, 1928, and references therein. A. Kupperman, in Physical Mechanism in Radiation Biology, ed. R. D. Cooper and R. W. Wood, (Technical Information Centre, Office of Information Services, USAEC, 1974). J. K. Thomas and R. V. Bensasson, J. Chem. Phys., 1967,46,4147. G. V. Buxton, Proc. Roy. SOC. A, 1972, 328, 9. R. K. Wolff, M. J. Bronskill, J. E. Aldrich and J. W. Hunt, J. Phys. Chenz., 1973,77, 1350. C. D. Jonah, E. J. Hart and M. S. Matheson, J. Phys. Chem., 1973, 77, 1838. A. T. G. Ferguson, Contemporary Phys., 1964, 5, 269. * W. G. Burns, R. May, G. V. Buxton and G. S. Tough, to be published. lo P. M. Rentzepis, R. P. Jones and J. Jortner, J. Chem. Phys., 1973, 59, 766. l1 A. Appleby and H. A. Schwarz, J. Phys. Chem., 1969,73, 1937. l2 C. D. Jonah, M. S. Matheson, J. R. Miller and E. J. Hart, J. Phys. Chem., 1976,80, 1267. l3 W. H. Hamill, J. Phys. Chem., 1969, 73, 1341. l4 A. Kupperman, in Radiation Research, ed. G. Silini (North-Holland, Amsterdam, 1967), p. 212. P. R. J. Burch, Rcdiation Res., 1957, 6, 289; Brit. J. Radiol., 1957, 30, 524.

ISSN:0301-7249    

DOI:10.1039/DC9776300047

出版商:RSC    

年代:1977

数据来源: RSC

摘要:

Primary Trapping and Solvated Electron Yields Part 1. Recombination Kinetics BY F. KIEFFER, J. K L E I N , ~ C. LAPERSONNE-MEYER AND M. MAGAT Part 2. Correlation between G-Value and Neutralization Efficiency BY J. BELLONI, F. BILLIAU, P. CORDIER, J. DELAIRE, M. 0. DELCOURT AND M. MAGAT Laboratoire de Physico-Chimie des Rayonnements, associk au CNRS 9 1405, Orsay, France Received 29th November, 1976 The isothermal decay of luminescence emitted in frozen hydrocarbons as a result of the recombina- tion of trapped electrons with aromatic solute cations has been studied in relation to theoretical models for the primary processes of radiolysis. The kinetics were determined on a time scale extending from -20 to 800 ns, with radiations of different LET. In polar liquids the influence of physical as well as chemical properties of the solvent on the yield G(e,,,) at a given time ( t = 3 ns) has been investigated: in particular, evidence was found of the existence of a correlation between the survival probability of eGlv and the efficiency factor of the eS;,,-cation recombination, which had so far been assumed to be limited only by diffusion.It is generally agreed that the initial distribution of events in condensed matter submitted to ionizing radiation is in no way homogeneous. On the one hand the primary species are initially distributed in spurs and blobs, related of course to the LET, and on the other hand, even in the limiting case of an isolated pair, the spatial distribution is not uniform since the secondary ion and radical, resulting from an ion-molecule reaction, are closely spaced.We were especially interested in the following two problems : a) for how long a time this inhomogeneous spatial distribution reflects itself in the b) in what way this non-uniform ion radical distribution may influence the G(e&) kinetics of consecutive processes such as ion neutralisation, value, measured at a given time. PART 1 : RECOMBINATION KINETICS: LUMINESCENCE AT 77 K O N THE NANOSECOND SCALE The kinetics of neutralization can be conveniently investigated by studying the decay of deferred luminescence at constant temperature (ITL), arising from the recombination of trapped or pretrapped electrons with positive ions of added aromatic compounds, formed by charge transfer from the primary aliphatic ions of the matrix.It was shown earlier' that often the decay kinetics of ITL can be described by a hyperbolic law (1.1) A I(t) = - t Centre de Recherche Nucleaire, Strasbourg, Cronenbourg, France.56 PRIMARY TRAPPING A N D SOLVATED ELECTRON YIELDS where I(?) is the intensity of luminescence at time t and A a constant. In a " hard " glass such as methylcyclohexane (q = 10'' P at 77 K) this law was found to apply over a time range of 9 orders of magnitude from 2 p s to 1 h; in a " softer " glass, 3-methylpentane (q = 10" P at 77 K), this law is valid only for a shorter period, 2 pus to a number of minutes, increasing with solute concentration,' whereas at longer times the decay law becomes3 B ? '' I(?) = - Whenevei. eqn (1.1) applies at 77 K, other experimental results, such as identical decay kinetics at 4 K, or the effect of cooling from 77 to 4 K, are consistent with a tunnelling mechanism for electron-cation re~ombination.'*~ Tunnelling probability depends exponentially on the electron-cation distance. A spatial distribution of electrons and cations, therefore, implies a distribution of tunnelling probabilities.Tachiya and Mozumder5 have shown that this can account for the form of (1.1). On the other hand eqn (1.2) seems to correspond to a thermally activated process. The validity of the same decay law from 2 ps to minutes or hours means that all information about the primary ion-electron distribution in a track is already lost 2 p s after an ionizing pulse lasting 3 ns. In order to obtain such information it was necessary to extend experiments to the nanosecond range, the lower time limit being only determined by the lifetimes of excited states formed directly.Such an extension proved possible with a technique based on coincidence counting of photons, and we thank Prof. Voltz for kindly letting us do this work in his laboratory. EXPERIMENTAL The glassy matrix was methylcyclohexane doped with one of two solutes, bPBD and PTP,* having a short fluorescence lifetime (z 1 ns) and high fluorescence yields. They were used in concentrations 10-5-10-3 mol dm-3. Sources of B or a particles (90Sr90Y and 210Po respectively) of about 1 pCi were either sealed inside the sample or placed in a thin- walled finger protruding into the silica tube containing the sample. Degassing was done by the usual freeze-pump-thaw procedure.The details of the technique have been described previously by Paligoric and Klein6 and Heisel, Fuchs and V01tz.~ The equipment was adapted to the work at liquid nitrogen temperature mainly by the adjunction of a Styrofoam container fitted with two silica light guides between which the sample tube was held in position. Coincidences between two photomultipliers, one of which operated in single photo-electron counting, were analysed by a time-to-amplitude converter and the results were stored in a multichannel analyser. The resolution time was about 5 x s. RESULTS The data, corrected for background noise and the instrument function, show a decay consisting of two components : a) a " rapid " one which is exponential and has the lifetime of solute fluorescence; it i s attributed to excitation transfer from solvent to solute molecules.6 b) a " slow " one, attributed to the electron-cation recombination.Initial results obtained with bPBD seemed to indicate a significant difference between samples irradiated with a- and P-particles respectively.8 However, further experiments have shown that the differences we observed were due in part to poor reproducibility. A study of fluorescence spectra and fluorescence lifetimes of bPBD in methylcyclo- * bPBD = 2-(4-butylphenyl) 5-(4-biphenylyl)l,3,4-oxadiazole. PTP = p-terphenyl.F. KIEFFER, J . KLEIN, C. LAPPERSONNE-MEYER AND M. MAGAT 57 hexane showed a variation with concentration from which we concluded that aggre- gates can be formed in methylcyclohexane glass at concentrations as low as mol dm-3.For this reason we replaced bPBD by PTP, which we found to form aggregates only at concentrations close to Fig. 1 and 2 show the mol dm-3. 100 1000 - I I I 1 I i 1 1 1 I I l l I 1 1 1 , - - - - c - L - - - 10 c - - A - - - + I I I l l I I 1 1 1 I 1 1 1 1 I I I 1 20 60100200 6OOlooO t / n s FIG. l.-ITL decay of p-terphenyl in methylcyclohexane after ionization with a-particles at 77 K. p-Terphenyl concentrations (mol dm-3): A-10-5, B-3 x lo-’, C-10-4, D-3 x For clarity, curves C and D are shifted to the right by one and two decades respectively. slow component of the decay kinetics with 01- and /3-particles respectively. For clarity some of the curves have been displaced by one or two decades. In fact, they all cover the time span 20-800 ns.These curves can be described by the Debye- Edwards equation I(t) cc t - m . (1 -3) For m = 1 this equation is of course identical with (1.1) and is hence valid for t > 2 ,us. TABLE VA VALUES OF rn FOR M and p IRRADIATIONS AS A FUNCTION OF PTP CONCENTRATION /mol dm-3 U P 1.3 (t = 20-55 ns) { l.Os (t > 55 ns) 10-5 1.40 3 x 10-5 1.43 1.28 (t = 20-60 n ~ ) 1.2,, (t > 60 ns) 10-4 1.31 1.3 3 x 10-4 1.34 1.358 PRIMARY TRAPPING AND SOLVATED ELECTRON YIELDS t / n s FIG. 2.-ITL decay of p-terphenyl in methylcyclohexane after ionisation with P-particles at 77 K. p-Terphenyl concentrations (mol dm-3): A-10-5, B-3 x lo-’, C-10P4, D-3 x loP4. For clarity, curves C and D are shifted to the right by one and two decades respectively.Table 1 shows the values of m for the two cases (a and p irradiations) as a function of PTP concentration in the time range from 20 to 800 ns. DISCUSSION In all cases, at the shortest times, the m-values lie between 1.3 and 1.4, these “ high ” values of rn being valid in the case of a particles up to 800 ns (our upper time limit). On the other hand, in the case of j? irradiation at low PTP concentrations, the m-value decreases markedly after some 50-60 ns, while it remains high at higher concentrations. Comparing these results with our previous results at t > 2 ,us, we can say that there exists a significant track effect, the primary charge distribution influencing the decay kinetics up to at least 800 ns in the case of a particles and up to times between 50 and over 800 ns for p particles.However, the effect is just beyond the limits of experi- mental error and does not justify complicated theoretical calculations. It can be stated, however, that it appears in the expected sense: in the case of a j?-particle track, single pair recombinations become predominant well before 800 ns, whereas in the case of a particle tracks multiple recombination possibilities seem to exist for all solute cations. PART 2: CORRELATION BETWEEN G-VALUE AND NEUTRALIZATION EFFICIENCY Since the discovery of the solvated electron in irradiated water, it has appeared possible to describe the primary effects of radiation on condensed matter through theBELLONI, BILLIAU, CORDIER, DELAIRE, DELCOURT AND MAGAT 59 fate of e;,,. Accordingly, the proposed diffusion models calculate a survival prob- ability of eGlV, as a result of its diffusion in the Coulomb field of the cation and its recombination with this counter-ion, in an isolated pair or in multipair spurs. The survival probability depends on the initial spatial distribution and on the physical properties of the liquid, such as viscosity, dielectric properties, etc.However, the models only consider the disappearance of the electron by reaction with the cation, and, moreover, they imply that the neutralization occurs at each encounter. Experimental results concerning the radiolysis of liquid ammonia9 have led us to consider in more detail the reaction efficiency of electron-cation collisions and it appeared possible to explain, at least qualitatively, the rather high yield of eLm by the sluggishness of the neutralization reaction eTm + NH:.The efficiency of neutraliza- tion at each encounter could be accounted for, using a diffusion model, like the one considered by Schwarz:lO the recombination rate constant includes an efficiency factor and an average value of the diffusion velocity in a Coulomb field. However, as in previous radical diffusion models, the displacement of charged species experiences no direction effect. We have now extended our investigations to some other liquids in order to determine the influence of the neutralization probability (reaction efficiency) at each encounter on the G(eilV) value at a given time, and in cases where this efficiency was low, to decide what was the main process of electron disappearance.In order to approach the ideal situation as assumed in models, we have chosen, on the one hand, solvents in which reactions such as eGlv + e;,, and e,,, + solvent do not occur. On the other hand, in order to avoid secondary chemical reactions, specific for each liquid, we have examined the correlation between recombination efficiency and G(eGl,) values observed a few nanoseconds after irradiation. EXPERIMENTAL We used as radiation source a 706 Febetron, delivering 3 ns pulses of 600 keV electrons.11a The standard dose per pulse was 5 x lozo eV dm-3 as determined from the initial absorption of the hydrated electron, taking G(eJ = 3.3 and E(e,g) = 1.3 x lo4 dm3 mol-' cm-' at 600 nm.12 The reaction cells were entirely made of silica with an electron entrance window of 200 xm thickness, withstanding internal pressures up to 10 bar.11b The recombination efficiency f was determined by comparing the experimental and the calculated (diffusion-controlled) rate constants of the reaction between e,,, and the main cation: f = kexp/kdiff.Solutions of the cations were prepared either by addition of a salt (N2H4, 2HC1; n-propylamine, HCl) or by acidification with HCl (case of 1,2-dimethoxy- ethane). Solvated electron yields have been determined indirectly using biphenyl (Ph,) as a scavenger and assuming ~ ~ ~ , , ( P h l ) = 1.21 x lo4 dm3 mol-' cm-'.13 The oscillograms of fig. 3 show the decay of solvated electrons measured at 900 nm in pure solvents : 1,2-dimethoxyethane, n-propylamine and hydrazine.RESULTS (a) 1,2-DIMETHOXYETHANE (DME) Due to the proton affinity of this kind of ether, and the high rate constant of the on-molecule reaction l4 DME+ % DMEH+, (2.1) it can be safely assumed that the initial DME+ cation is very rapidIy replaced by the stable cation DMEH+. These DMEH+ cations can also be produced by addition of HCl to DME, If one now assumes that in the concentration range of 6 x to60 PRIMARY TRAPPING AND SOLVATED ELECTRON YIELDS icl (6) n-propylamine (50 ns div-l); (c) hydrazine (1 ,us div-l). FIG. 3.-Solvated electron decay at A = 900 nm in (a) 1,ZDimethoxyethane (50 ns div-l); 4 x mol dm-3 all these cations are present as isolated ions, one can deduce from the experimental rate of disappearance of solvated electrons a rate constant :15 ke,lv+DMEH+ = 2 x 10" dm3 mol-' s-l.Since however, it is more likely that only a fraction of DMEH+ cations are isolated, this figure for ke.lv+DMEH+ is to be considered as a lower limit of the rate constant at infinite dilution (see discussion), The rate constant for electron scavenging by added biphenyl molecules was found to be kesolv = 1.1 x 10l1 dm3 mol-' s-l. The dependence of G(PhF) on scavenger concentration permits an estimate of the electron yield at a mean reaction half time. We have determined15 a value of G(eGlv) = 0.8 5 0.1 at 3 ns ([Ph,] = 8 x mol dm-3) and deduced the extinction coefficient: E(eElv) = 6000 dm3 mol-1 cm-l at 900 nm. (b) n-PROPYLAMINE In irradiated n-propylamine (PrnNH2) the main cation is PrnNH3+; the recom- bination rate between this cation and the solvated electron was measured.The solvated electron decay rate does not depend on concentration of cations below lo-, mol dm-3. In the range 10-2-10'1 mol dm-3, the experimental rate constant is kPrnNH3+ +esolv = 7 x lo8 dm3 mol-I s-l. The static dielectric constant of Pr"NH, is close to that of 1,ZDME (see table 2). Therefore, assuming that ion pairing and ionic strength effects are the same in the two liquids, the recombination rate constant in PrnNH, is 30 times lower than in 1,2-DME. (1.3 &- 0.1) x lo1' dm3 mol-1 s-l. We also deduce, as above: G(esYlV) = 1.40 & 0.15 at 3 ns and e(eLlV) = 13 000 dm3 mo1-l cm-' at 900 nm. The rate constant for the biphenyl-electron reaction in PrnNH, is: kesolv+Phz - - (C) HYDRAZINE The experimental rate constant kNzH+ +e;lv has been determined earlier16 at infinite dilution as koexp = (5 & 3) x lo7 dm3 mol-l s-l. It is quite low, leading, as inBELLONI, BILLIAU, CORDIER, DELAIRE, DELCOURT AND MAGAT 61 the case of ammonia, to a low value o f f , since kdiff - loll dm3 mol-' s-l.The solvated electron yield at 3 ns is correspondingly rather high: GCe;,,) = 3.4 and has been interpreted as resulting from the slowness of the recombination reaction. As in the case of ammonia again, the main process determining the observed disappearance of eZlv [see fig. 3(c)] has as yet not been established. No pertinent data are available concerning the main radical N2H3' produced at an early stage and its possible reaction with the solvated electron according to N2H4 -+ N2H4+ + e' (2.2) (2.3) (2.4) N2H4+ + N2H4 ----+ N2H5+ + NzH3* N2H3' -k eZlv -+ N2H3.However, the reaction between e & , and N2H5 + would ultimately lead, as in water,17 to a production of M,. We have measured the yields of stable products resulting from the irradiation of pure hydrazine: G(H,) = 1.90; G(N,) = 2.60; G(NH3) = 4.1. These figures are in good agreement with the results of Prosch18 who has also found that G(H2) is but slightly influenced by the presence of CC14, used as electron scavenger. Therefore, inolecular hydrogen does not originate from the N2H5 + neutralization reaction and the disappearance of e,,, must occur through reaction with the radical. (d) AMMONIA For ammonia both the solvated electron yield and the neutralization rate constant are knownlg (table 2).It was also suggested that eLm, poorly reactive towards NH4+, was disappearing by a second order reaction with the main oxidizing radical NH2*. The expected product, amide ion NH,, however, has never been observed at low temperature (-50°C) and is less abundant at room t e m p e r a t ~ r e ~ ~ than expected from the stoichiometry. For this reason, the rate constant of the eZlv + NH2' reaction at -50°C was determined15 from the decay of elm at 650 nm and the correlated increase of NH, at 330 nm (LmaxNH, at -50°C) in metal ammonia solutions, containing stable solvated electrons in excess [4.3 x loe4 mol dm-3 (Na+, e,) compared with 7.5 x lod5 mol dm-3 elm produced by radiolysis at the end of the pulse]. The decay at 650 nm, faster than in pure ammonia, and the increase at 330 nm are correlated pseudo first order processes (fig.4). Due to the respective values of G(NH2) and G(NH), the observed decay depends mainly on the reaction rate of eFm + NHo2. At -5O"C, it was found that ke,-,+NH2 = 3.5 x 1O1O dm3 mo1-I s-l. At the same temperature, the rate-constant of the diffusion controlled reaction would be: kdlff - 4.35 x 10" dm3 rno1-I s-'. The efficiency factor of the reactionf = kexp/kdiff is very close to 1. This result confirms that, in pure ammonia, the decay of e, is also due to reaction with the radical at practically every encounter. Fig. 5 presents the absorption variation at il = 800 and 330 nm for temperatures -50, -20 and 0°C. While at -50°C the decays at both wavelengths are similar, at -20 or 0°C the decay of e&, is faster than at -50°C and at 330 nm one observes the slowing down of the decay, indicating a slight increase super-imposed on the decay and attributed to NH2-.The differences between metal-ammonia solutions and pure solvent in the experi- mental evidence of the NH; formation can only be understood as a result of the different distribution of reacting species. (i) In metal ammonia solutions, solvated electrons are homogeneously distributed and in large excess relatively to NH2 radicals or NHZ ions. Both of these species62 PRIMARY TRAPPING AND SOLVATED ELECTRON YIELDS 0 .I 0.01 0.001 0 0.1 0.2 0.3 t l p s FIG. 4.-Decay of elm absorption at 650 nm in sodium-ammonia solutions: 0-3.2 x mol dm-j x - 4 . 3 x mol dm-3. Initial radiolytic concentration of e&: 0.75 x mol dm-3.Inserts: absorption changes in 3.2 x mol dm-3 Na solution (a) at A = 650 nni; (b) at A = 330 nm. 01 I FIG. 5.-Decay t / p s of absorption at (a) 800 and (6) 330 nm in pure ammonia. -- --- -50°C. ooc, . . . .. I 2O0C,BELLONI, BlLLIAU, CORDIER, DELAIRE, DELCOURT AND MAGAT 63 interact with added e, to give NH, or a loose pair NHZ . . . erm, but the anion NH; disappears after diffusion towards the neutral pair (NHZ . . . e,) with weakened Coulomb attraction. (ii) Conversely, in the pure solvent, where the charges are only those produced by radiation, the solvated electron is strongly attracted by the parent cation (kdiff is high) and, since they are poorly reactive, they diffuse as a pair without disappearing.After encounter of this pair with NH;, the reaction occurs and the NH; produced is then right in the Coulomb field of the very close NHZ. Consequently, the rate deter- mining step is the formation of NH, and its concentration remains low, although at higher temperature it becomes just detectable. This picture is valid as well for non- homogeneous early reactions as for the later homogeneous process NH; + (NH;. . . In fact, we conclude that the same situation is to be expected whenever the recom- bination rate constant is much smaller than the diffusion controlled rate constant while the formation of a geminate pair, e z l v . . . cation, is certainly fast. ea-m). DISCUSSION Table 2 contains data for the solvents presented above as well as for water and alcohols which are well known as regards their respective ezl,-cation and eLl,-radical reactions.Diffusion-controlled rate constants were calculated from Smoluchowski-Debye relations, using for diffusion coefficients D data from the literature and estimates obtained by comparison with similar solvents (see table 2). Experimental rate constants ke;lv+c+ in DME and PrnNH, have been corrected for ion pairing and ionic strength effects to obtain the constant at infinite dilution koexp. Considering that in these solvents the low static dielectric constant favours the forma- tion of triple ions,2o these coexist with the neutral ion pairs at the concentrations used. The conductance of such solutions is usually not lower than one tenth of that at infinite dilution: assuming that the monovalent triple ions have the same reactivity as isolated ions, koexp is -10 times higher than the actual experimental values.Other values are taken from original papers. Qualitative comparisons between solvents of table 2 show that G(e;,,) is higher in Pr"NH, than in 1, 2-DME although dielectric constants and kdiff values are nearly equal. Ammonia may be compared with alcohols, although diffusion coefficients and kdiff values are higher in ammonia; the electron yield is still higher than in alcohols, due to the low efficiency factor f = kexp/kdiff. Finally G(e&) is practically the same in hydrazine as in water, although &N2H4 < cHZ0. This again is probably due to a low value offin hydrazine. In order to improve present theoretical models3' and to explain our experimental data, we tried to take into consideration the fact that every collision does not effectively give rise to neutralization, as was shown by values of theffactor different from unity.We, therefore, solved the Smoluchowski equation using a boundary condition similar to that formulated by some a u t h o r ~ ~ l g ~ ~ in the theoretical treatment of reaction rates in solution. According to these authors the boundary condition expresses the assumption that there exists, near the cation or near the radical associated with the ion, a partially absorbing (or reflecting) boundary on which reaction proceeds by pseudo first order kinetics. In a first approximation, this model supposes that the radical and the cation are located inside a common barrier defined by one reaction radius. In the case of an isolated pair and using an approximation method known asch P TABLE 2-RATE AND DIELECTRIC CONSTANTS AND G(eGI,) VALUES FOR VARIOUS SOLVENTS 1,2-DME n-propylamine n-pro pano ethanol methanol ammonia 25°C h ydrazine water - 50°C 5.5 1 x 10-521 20.1 1.0 x 10-5 33.6 5.3 x 10-5 17 5.5 x 10-4 25 1.7 x 10-4 78 1.4 x 10-4 5 25.1 2.3 x 52 ~~~ 2 x loll l5 2 x 10l2 0.8 f 0.1 * l5 1.40 f 0.15* 1.27.1.77. 2.07. 3.4 & 0.2" l6 2 x 10'2$ 2.2 x 1Olo 22 4.1 x 1O1O 22 6.9 x 1O1O 22 5 x 10" $ 1 x loll (7 1) x 109 (2.5 f 0.2) x 10'' 22 (4.5 f 0.2) x 1O1O 22 (6.8 & 0.6) x 1O1O 22 3.2 x 10724 1.4 x l0l2 2.9 x 1011 3.2 f 0.3 25 1.2 x lo6 l9 (5 5 3) x 107 16 2.2 x 1O'O 27 1.03 x 10" l2 3 x 1O1O 27 2.3 x 1O1O 3.3 l1 3.5 x lolo l5 4.35 X lolo 3.0* 9; 3.2 2 5 ; 3.3 26 ~~~~ * These values were determined relatively to the same G3 ns (e,) value in water and so are self-consistent.f Values of free-ion yield as discussed in the review ref. (29). $ These values were estimated by comparison between solvents of close dielectric constant.BELLONI, B I L L I A U , CORDIER, DELAIRE, DELCOURT A N D MAGAT 65 prescribed diff~sion,~~ our preliminary calculations show that the G-value of the solvated electron at a given time is strongly dependent on a parameter y characterizing the boundary. Fig. 6 shows the typical case of liquid ammonia at -50°C: reaction radius = 5.8 A, mean thermalization lengths = 30 and 45A, diffusion coefficient = 1.5 x lo4 cm2 s-’. Abscissae have been expressed in terms of y, and for convenience they have been converted into second order rate constant k’ according to ref.(32). l.C 2 0 . 5 7 I I I I u 0 12 8 4 0 FIG. 6.4-value (normalised to initial ionization yield Go) versus the parameter y and reactivity k’ at 3 ns and infinite time, for two thermalization lengths (- 45 A, ---- 30 A). G-value at infinite time is calculated with (lower curves) and without (upper curves) electric field. log k‘l dm3mo~-1 It can be seen from fig. 6 that, at a given time, the G-value starts from a total escape when there is no reaction with either the cation or the radical (k’ = 0), that is when there is a total reflectivity of the boundary ( y --+ co) and then decreases with the reactivity k’. It should be noted that the limiting value of G for longest time and for lowest y agrees exactly with that calculated from the Onsager escape pr~bability.~~ This value corresponds to the ultimate probability for the solvated electron to escape radical capture or ion neutralization.Moreover in the presence of a Coulomb field, this limiting G-value is reached for a value of y higher than in the absence of the field. The authors thank Dr. P. Penel for his help and advice concerning the mathematical This work was made possible by grants from the Centre National de la treatment. Recherche Scientifique. l P. Cordier, F. Kieffer, C. Lapersonne-Meyer and J. Rigaut, Radiation Research (Proc. 5th Int. Congr. Radiation Res., Seattle, 1974, Academic Press, 1975), p. 426. C. Lapersonne-Meyer and J. MCnigaux-Rigaut, unpublished results.66 PRIMARY TRAPPING AND SOLVATED ELECTRON YIELDS F.Kieffer, C . Lapersonne-Meyer and J. Rigaut, Int. J. Radiation Phys. Chem., 1974, 6, 79. B. G. Ershov and F. Kieffer, Nature, 1974, 252, 118. M. Tachiya and A. Mozumder, Chem. Phys. Letters, 1975, 34, 77. D. Paligoric and J. Klein, Int. J . Radiation Phys. Cheni., 1972, 4, 359; 1975, 7, 731. F. Heisel, C. Fuchs and R. Voltz, J. Physique, 1973, 34, 203. F. Kieffer, C. Lapersonne-Meyer and J. Klein, Chem. Phys. Letters, 1976, 40, 492. (a) J. Belloni and E. Saito, Coll. Weyl. III, 1972, in Electrons in Fluids (Springer Verlag, 1973), p. 461 ; (b) J. Belloni, P. Cordier and J. Delaire, Chem. Phys. Letters, 1974, 27, 241. lo H. A. Schwarz, J. Phys. Chem., 1969,73, 1928. l1 (a) J. Delaire, ThBse 32 cycle (Orsay, 1973); (b) E. Saito and J. Belloni, Rev. Sci. Instr., 1976, l2 J. K. Thomas and R. V. Bensasson, J. Chem. Phys., 1967,46,4147. l3 K. H. Buschow, J. Dieleman and G. I. Hoijtinck, Mol. Phys., 1963, 7, 1 ; D. Gill, J. Jagur- Grodzinski and M. Szwarc, Trans. Faraday SOC., 1964,60, 1424. l4 S. K. Gupta, E. C. Jones, A. G. Harrison and J. J. Mymer, Canad. J. Chem., 1975, 45, 3107. l5 F. Billiau, ThBse de 36 cycle (Orsay, 1976). l6 J. Delaire, P. Cordier, J. Belloni, F. Billiau and M. 0. Delcourt, J. Phys. Chem., 1976,80, 1687. l7 J. Belloni and M. Haissinsky, Znt. J. Radiation Phys. Chem., 1969, 1, 519. I s U. Prosch, 2. Chem., 1964,4, 395. l9 J. M. Brookes and R. R. Dewald, J. Phys. Chem., 1971, 75,986. 2o R. A. Robinson and R. H. Stokes, Electrolyte Solutions (Butterworth, London, 1959). 21 J. P. Dodelet and G. R. Freeman, Canad. J . Chem., 1975,53,1263. 22 P. Fowles, J.C.S. Faraday I., 1971, 67,428. 23 L. M. Perkey and Farhataziz, Znt. J. Radiation Phys. Chem., 1975, 7, 719. 24 G. I. Khaikin, V. A. Zhigunov and P. I. D o h , High Energy Chem., 1971,5, 72. 25 Farhataziz, L. M. Perkey and R. Hentz, J . Chem. Phys., 1974, 60, 717. 26 W. A. Seddon, J. W. Fletcher, J. Jevcak and F. C. Sopchyshyn, Canad. J. Chenz., 1973,51,3653. 27 S. Gordon, E. J. Hart, M. S . Matheson, J. Rabani and J. K. Thomas, Disc. Faraday SOC., 28 M. Matheson, Ado. Chern. Ser., 1965, 50,45. 29 G. R. Freeman, Act. Chim. Biol. Radiations, 1970, 14, 73. 30 A. Hummel, in Advances in Radiation Chemistry (Wiley, N.Y., 1974), 4, 1 . 31 F. C. Collins and G. E. Kimball, J. Colloid Sci., 1949, 4, 425; T. R. Waite, Phys. Rec., 1957, 32 M. J. Pilling in Lasers in Physical Chemistry and Biophysics (Elsevier, Amsterdam, 1975), p. 485. 33 A. Mozumder, J . Chem. Phys., 1968,48, 1659. 34 L. Onsager, Phys. Rev., 1938,54, 554. 47, 629. 1963,36, 193. 107,463,471.

ISSN:0301-7249    

DOI:10.1039/DC9776300055

出版商:RSC    

年代:1977

数据来源: RSC

摘要:

GENERAL DISC USSIO N Prof. M. C. R. Symons (Leicester) said: In a qualitative sense one can understand that C-H protons can, by rotational or librational movement, give rise to a modula- tion of the dipolar coupling, whilst strongly hydrogen bonded 0-H groups may not be able to do this so efficiently. However, it is far more difficult to obtain a physical model for significant movement of Na+ ions and still more so for phosphorus atoms in phosphate ions. Have you any feel for the nature of the tunnelling movements envisaged ? Prof. L. Kevan (Detroit) said: In general we postulate the importance of tunnelling modes in electron spin-lattice relaxation in glassy matrices because of the linear de- pendence on temperature of the spin-lattice relaxation rate over a fairly wide tempera- ture range.This postulate not only accounts for the dominant low temperature dependence of electron spin-lattice relaxation but also for nuclear spin-lattice relaxa- tion in glassy matrices such as B203 and for the specific heat in metal oxide glasses and organic polymers. Thus there is evidence in a variety of disordered systems that argues for the importance of such tunnelling modes even involving heavy atoms. I would point out, however, that our D2 coefficients which are a measure of the efficiency of spin-lattice relaxation by tunnelling modes are much larger when proton tunnelling is indicated compared with heavier atom tunnelling. In ethanol and MTHF matrices where deuteration supports proton tunnelling, D2 is 20-30 s-l K-I while in aqueous glasses, where we indirectly implicate Na+ tunnelling, D2 is only 2 s-l K-l.This contrasts with the spin-lattice relaxation of silver atoms in aqueous Michalik and L. Kevan, unpublished results). Also, for trapped H atoms in acid glasses where we suggest P atom tunnelling since there is no deuteration effect on T,-l we find D2 is only 0.2 s-l K-l. matrices in which proton tunnelling is implicated and D2 is 100-200 s-I K-l (J. Prof. J. Kroh, Dr. A. Pfonka and Dr. K. Wyszywacz (Ebdi, Poland) said : In connec- tion with the suggested correlation between electron spin-lattice relaxation times and hydrogen atom decay kinetics we would like to add some new data on the reactions of trapped hydrogen atoms with alcohols. Following the kinetics of trapped hydrogen atom decay at 63-90 K in y-irradiated 6 mol dm-3 sulphuric acid glasses it seemed to us2 that trapped hydrogen atoms are released from the traps as a result of the relaxation processes proceeding in the frozen matrices.Here we would like to present, in a very preliminary form, some more data supporting our suggestion. The probability for trap destruction arising from molecular rearrangement should be lower not only in the more rigid matrix but also should be lower in a more relaxed matrix containing fewer frozen configurations which are unstable at the given tem- perature. Both these phenomena are seen in fig. 1. The matrix irradiated at a tem- perature higher than that of observation is more relaxed and the matrix irradiated at a temperature lower than that of observations is less relaxed than that irradiated L.Kevan and A. Plonka, J. Phys. Chem., 1977,81,963. J. Kroh and A. Plonka, J. Phys. Chem., 1975,79,2600.68 GENERAL DISCUSSION FIG. 1.-Decay of trapped hydrogen atoms at 77 K in 6 mol dm-3 (0, A, m) and 13 mol dm-3 (0, A) sulphuric acid glasses y-irradiated (1.3 Mrad h-I, 1 h) at 77 (0, O), 63 (A, A) and 90 (w) K. at the temperature of observation. Furthermore, the matrix of 13 mol dm-3 sulphuric acid glass is more rigid at 77 K than that of 6 mol dme3 sulphuric acid. If everything depends on matrix relaxation phenomena then the nature of the hydro- gen acceptor is of secondary importance. This suggestion is fully supported by the data presented in fig. 2. 100 uo-o---o--o-o 50. @-fBn-o- 13 rnol dm-3 6 mol dm-3 2 0 50 100 150 200 tirnelmin FIG.2.-Decay of trapped hydrogen atoms at 77 K in y-irradiated (1.3 Mrad h-I, 1 h) at 77 K; 6 and 13 mol dm-3 sulphuric acid glasses containing 1 rnol dm-3 of methanol (A), ethanol (A), pro- panol (a), isopropanol (@), butanol (O), isobutanol (.) and t-butanol ((3).GENERAL DISCUSS ION 69 50 (D 1 c 0 r; 10. - - 91 : - 5. t z I 4- U V lo Finally, if the relaxation is promoted there should be greater differences among acceptors than when the relaxation is hindered. This phenomenon is seen in fig. 3. :: 6 50 100 150 200 Prof. L. Kevan (Detroit) said: I do think that spin lattice relaxation measurements offer an additional parameter for trying to understand radical detrapping or decay in glassy matrices. Ti is a measure of the strength of the coupling between the trapped radical and the dynamic motions of the matrix and/or of the radical.Thus, if H atoms generated at different irradiation temperatures decay with different rates at the same temperature I would expect the Ti’s of these H atoms generated at different temperature to differ. In this sense Ti can probe the degree of structural relaxation of the matrix molecules around a trapped radical. Dr. G. A. Salmon (Leeds) said: The comparison of initial solvated electron yields with yields escaping spur processes which is shown in table 1 of Fletcher’s paper has a number of interesting features. In particular, whereas for ammonia and methylamine G(e,’) = G(e;)esc, for ethylamine, isopropylamine and hydrazine G(e;) > G(e;)esc. Is there any explanation for these differences between apparently similar solvents ? Dr.J. W. Fletcher (Chalk River) said: The differences in escape probability should be related to the relative rates of diffusion from the spur as compared with the rates of recombination within the spur. InSNH3 the high mobility of e; combined with its slow rate of reaction with the positive ion leads to a high escape probability. Methylamine appears to be similar. Our results indicate that for the higher amines the rate of reaction of e; with the corresponding positive ions is faster. Dr. J. Belloni (Orsay) said: Fletcher explained the relatively high values obtained for G(e;) in the amines by a probable slow rate of the electron reaction with the positive70 GENERAL DISCUSS I ON ion. Was this recombination rate constant of e; measured in the presence of added alkyl ammonium cations? Dr.J. W. Fletcher (Chalk River) said: No. This needs to be done. However we also need reliable dissociation constants for the appropriate bases before these can be measured. Dr. J. Belloni (Orsay) said: In our paper (p. 56) dealing precisely with the cor- relation existing between the electron yield and the rate constant of the electron-cation recombination, we present the example of n-propylamine for which a high yield is associated with a slow neutralization, as already shown for other amino compounds : ammonia and hydrazine [ref. (9) and (16) p. 661. Since this correlation seems now well established, I suggest that one ought systematically to determine the experimental cation-electron rate constant, which could be far from diffusion controlled, thus having necessarily an influence on the electron escape.Prof. A. K. Pikaev (Moscow) said: What is the reason for the low initial yield of e, for CH30H in comparison with initial yields of this species for H20, N2H4 and NH3 (see table l)? Dr. J. W. Fletcher (Chalk Riuer) said : The data are from Salmon’s work, perhaps he would like to comment. Dr. G. A. Salmon (Lee&) said: The yields of e, in liquid methanol mentioned by Fletcher and Seddon were obtained by my co-worker, Dr. D. W. Johnson and were reported at the Banff Conference on Electrons in Fluids1 The yield of escaped solvated electrons*, G(e;)eso that is the yield of e; escaping both geminate-ion re- combination processes and radical-radical spur processes, was determined by measur- ing the yield of nitrobenzene radical-anion formed on scavenging these electrons with small concentrations of nitrobenzene.The extinction coefficient of the radical-anion of nitrobenzene was determined in alkaline solution where its yield can be equated with the total yield of reducing radicals. Since the product G(e;)esc EL? for the solvated electron can readily be measured by the technique of pulse radiolysis we were, therefore, able to determine the extinction coefficient of e;. The total yield of sol- vated electrons, G(e;), was determined by applying our measured value of EL to the data of Lam and Hunt who reported yields of solvated electrons at short times before the occurrence of spur processes.Since the yield of scavengeable electrons is known to be at least 4.6, one must infer that in pure methanol at least 50% of the electrons recombine without being solvated. Dr. A. Singh (Pinawa, Manitoba) said: What is the role of excited states in Fletcher’s system? Is there any contribution to ionization from excited states? Dr. J. W. Fletcher (Chalk Riuer) said: This certainly cannot be ruled out. In the presence of solvent base an increase in yield is observed which in total corresponds to D. W. Johnson and G. A. Salmon, Canad. J. Chem., 1977,55, 2030. K. Y . Lam and J. W. Hunt, J. Phys. Chem., 1974,78,2414. * We prefer this terminology to the more generally used “ free ion yield ”, G(e;)f.i., since in polar media the yield of electrons escaping spur processes is not governed solely by Coulombic interactions.t G(e;)escEL is the product of G(e;)esc, the radiation chemical yield of slowly decaying solvated electrons in units of molecules (100 eV)-’, and EL, the molar decadic extinction coefficient of e;l at wavelength 2.GENERAL DISCUSSION 71 that expected for the ionization yield. Part of this yield could be due to long lived excited states. Work along this line is continuing. Prof. M. C. R. Symons (Leicester) said : With reference to fig. 1 of Fletcher's paper, it may be significant that, in some unpublished work in my laboratory by Mrs V. Thompson, using lH n.m.r. and infrared spectroscopy, it has been shown that systems comprising methanol and inert solvents retain the properties of the bulk alcohol until very low mole-fractions of alcohol are reached.This can only mean that the alcohol is present as clusters, and provides a ready explanation of curve A. Clustering is still probably significant for weakly basic media, but certainly not for strongly basic solvents such as HMPA. My question is, has Fletcher any explana- tion for the apparently strong preference of the electron for ammonia molecules in the 0-50% range? For normal anions, water is a better solvent than ammonia, and it seems possible that the explanation of this unusual trend could be a key to under- standing the high stability of electrons in ammonia. I do not think that the formation of NH4+OH-, which is negligible in water + ammonia systems, can make any major contribution to the spectra. Dr. J.W. Fletcher (Chalk River) said: We feel it is due to the formation of un-ion- ized NH40H. Prof. M. C. R. Symons (Leicester) said: There is no evidence that undissociated NH40H exists. Prof. U. Schindewolf (Karlsruhe) said: The ionization constant is too small for ions mol dmW3) to cause the effect. Dr. A. Singh (Pinawa, Manitoba) said: Is ammonia uniformly distributed or does it aggregate ? Prof. U. Schindewolf (Karlsruhe) said : It is uniformly distributed. Dr. J. W. Fletcher (Chalk River) said: Although there is no evidence for compound formation, NH3 is considered to be hydrated. The phase diagram indicates the formation of the two hydrates NH3 * H20 and 2NH3 H20 (also possibly NH2 2H20). An estimate of hydrate formation has been made.l NH3 + H20 = H3N . . . H-OH K % 0.21 If hydrate formation effectively prevents electron solvation by H20 then this could account for the negative deviation.Prof. L. Kevan (Detroit) said: Singh suggests that vibrationally excited water may give OH- and in a second step that electronically excited H20 may transfer energy to OH- to ionize it. Has he tried to estimate what lifetime of the electronically excited H20 is required to make this a significant process and whether such a lifetime is reasonable in the liquid phase? Dr. A. Singh (Pinawa, Manitoba) said: The lifetime of H20*V has been estimatedS9t s. These H20*V and the resulting OH& should be spread over most to be 1 x W. L. Jolly, The Inorganic Chemistry ofNitrogen (W. A. Benjamin, New York, 1964). t References as in-Singh's paper.72 GENERAL DISCUSSION of the spur.The lifetime of electronically excited water, H20* in the liquid phase has been mentioned17Jf to be < s. On the basis of y-radiolysis of aqueous solu- tions of Fe2+, Matthews20 has estimated the lifetime of H20* to be >lo-' s. No direct lifetime measurements of H20* (triplet or singlet) by emission studies seem to have been made. Most likely, the singlet excited state lifetime is < s as men- tioned by Sworskyl' and the longer lived excited water (>lom7 s) postulated by Matthews 2o is 3HzO*. Therefore, electronically excited states of water should be available in spurs for reaction with OHcq produced from H20*V [reaction (6) in the paper], in close proximity to each other. Incidentally, some OH; would also result in the spurs by reaction of electrons to hydroxyl radicals.Precise evaluation of the contribution of reaction (7) requires diffusion kinetic calculations which have not been done. Dr. G. V. Buxton (Leeds) said: What number and spatial distributions of H20*V and H20* are required in the spur if reactions (5)-(7) are to account for G(eG) = 0.3 in H20 and 1.2 in D20 ? Would not the reaction of H& with OH; compete effectively with reaction (7), particularly as H& is produced in reactions (3) and (4) as well as (6)? Dr. A. Singh (Pinawa, Manitoba) said: I expect both H,O* and H20*V to be more homogeneously distributed in spurs than either H& [from the primary ionization event, reaction (3)] or e;. Accepting the known quantum yields of eiq formation61*62Jf from OH;, the minimum yields of H20* and H20*V will have to be G = 2, each so that the yield of e z is G = 0.3 from reaction (7).For precise evaluation, of course, proper diffusion kinetic calculations should be done taking into account all the competing reactions. Further, it should be noted that the efficiency of reaction (7) may increase dramatically with the energy of H20*. The quantum yield for eLq formation from OD; is not known, so the required yields of D20* and D20*' cannot be evaluated. Dr. G. V. Buxton (Leeds) said: What is the effect of added sodium hydroxide? Dr. A. Singh (Pinawa, Manitoba) said: Addition of 1.5 mol dm-3 NaOH to H20 and NaOD to D20 increased the observed e; yield by -10%. This is consistent with the role of HzO* in eTQ formation in spurs, that we have postulated.Dr. G. V. Buxton (Leeds) said: Would you not expect a bigger increase in the yield of e& on the addition of these alkalis? Dr. A. Singh (Pinawa, Manitoba) said: Yes; since G(H20*) is expected to be -4, one would expect a larger increase. I think that the following three reasons limit the size of the increase. (i) The quantum yield of e z formation from OH; [reaction (12)] is low [ 4 . 1 , ref. (61), (62) and J. Barrett, M. F. Fox and A. L. Mansell, J. Chem. SOC. A, 1966, 4891. (ii) In the solutions containing the alkalis, the structure of water around the sodium and hydroxide ions will not be the same as in pure water. The energy re- quirement of ionic dissociation of these water molecules (water of hydration) t References as in Singh's paper.GENERAL DISCUSSION 73 should be higher than that in pure water.As a result, in these solutions, the extent of ionic dissociation of water would be smaller. The increased e i yield that we see then, is the net effect of increased yield due to added OH& and decreased yield due to lower ionic dissociation of water. (iii) There is a possibility that e- may be captured by the sodium ions followed by regeneration of Na + [reactions (1 3), (1 4)] Na& + e- + Na + nHzO (13) Na -% Na& + e;. This could lead to a decreased yield of e; immediately after the pulse if reaction (14) is slow (i.e,, requires > lo-” s). Though complex formation between alkali cations and solvated electrons in polar solvents has been suggested (C. Gopinathan, E.J. Hart and K. H. Schmidt, J. Phys. Chem., 1970,74,4169; A. K. Pikaev, T. P. Zhest- kova and G. K. Sibirskaya, J. Phys. Chem., 1972, 76, 3765; B. Rockrath and L. M. Dorfman, COO-1763-44), specific information on reactions (13) and (14) is not available. Dr D. Meyerstein (Israel) said: I wonder how the observation of J. R. McNesby et al. (J. Chem. Phys., 1963, 36, 605) that at 123.6 nm the process H20-%H2 + 0 (1 5 ) is of major importance affects the results in table 2. Furthermore, how do these observations affect the conclusion that the geminate recombination eG + OH, is low in photolysis ? Dr. A. Singh (Pinawa, Manitoba) said: Photolysis of water vapour has been re- viewed by Dixon,l6? which includes McNesby et aZ.’s work. The following points emerge from this review16 about photolysis of water vapour in the 1240 A band: (i) The probability of the reaction (15) is about one third of that of reaction (16) H ~ O -L H + OH.(16) (ii) As the vapour pressure of H,O is increased in the range of 5-15 Torr (0.67- 2 kPa), the relative importance of the reaction (15) decreases in comparison to the reaction (1 6). It can thus be concluded that collisional deactivation affects reaction (15) much more than it does reaction (16). Therefore, reaction (15), should play a very minor role in liquid water, if any, and should have little bearing on the results in table 2. Whether geminate recombination of e; + OH takes place in photolysis, or not, should depend on the irradiation intensity (dose rate). In photolysis, energy is not deposited in spurs.If a molecule of water absorbs energy, it will either produce OH or ee;, assuming that these are the only two choices available. H20 -% H + OH (17) Therefore, combination of e& and OH would only occur, if the two H20 molecules undergoing reactions (17) and (18) are quite close to each other. This, apparently, is not the case in the work of Getoff and S~henck.~’ -f References as in Singh’s paper.74 GENERAL DISCUSSION Dr. D. Meyerstein (Israel) said: I do not think that a solution of 2 mol dm-3 sodium salicylate can be considered as a dilute aqueous solution. In such a solution about 25% of the energy is directly absorbed by the solute and not by the solvent. This is even more so if the hydration sphere of the solute is taken into account as solute.Thus the observed increase in yield might be due to the nature of the solute and not to primary processes in the radiolysis of water. Dr. A. Singh (Pinawa, Manitoba) said: We have applied corrections for the changes in electron densities of the solutions and the yields given in table 1 are based on the total energy absorbed by the solutions. When processes in spurs are being investigated experimentally by scavengers, they must be used in high concentrations, since the concentration of transients that can react with each other in the spurs is quite high. For example, using the parameters of the average spur according to Schwar~,~f. the initial concentration of many of the transients (H&, e;, OH, etc.) should be 4 . 1 mol dm-3. Use of a concentrated solution poses some complexities, as shown in the following approximate data for NaOH solutions [NaOH]" [H201 of [HZ01 % energy absorbed by solvation " free " NaOH (NaOH),,, H20 '' free " 1 8 47 4 20 80 5 20 15 20 80 20 a All concentrations are in mol drnp3.Thus the primary acts of energy deposition will be different in the NaOH solutions from those in pure water. However, the situation is less complex than it appears to be. Let us consider a simple general case of a concentrated solution in which no chemi- cal interaction between the transients of one component with the other takes place (A = solvent, D = solute). It is also assumed that the deposition of energy in the two components is according to their electron fraction. Ionization and excitation of the two will proceed as follows: A---+A++e-+A* (1 9) D--+D++e-+D*.(20) Assume that I.P. (A) > I.P. (D) and that E(D*) > E(A*). The following reactions will take place: A+ + D - t A + D+ (21) D* + A + D +A*. (22) Thus, in terms of ionization, the solution will behave as if it were almost all D and, in terms of excited state formation, it will behave as if it were all A; that is, the lowest ionization and excitation potentials in the solution would be emphasized. Therefore, results from concentrated aqueous solutions in water can be extra- polated to the case of pure water, if the various physical and chemical reactions between the transients of the two components have been taken into account. In the case of Nasal solutions, we have done that. The reactions: and H20* + Na& -+ H20 + Na;: i- e: H20+ + Na+ -+ H20 + Naf,' 7 References as in Singh's paper.GENERAL DISCUSSION 75 cannot take place since the ionization potential of Na+ is quite high [ -47 eV; L.G. Christophorou, Atomic and Molecular Radiation Physics (Wiley-Interscience, New York, 1971, p. 614)]. The more important reactions involving Sal,, have already been taken into account in discussing the mechanisms of e z formation, in the paper. Prof. K.-D. Asmus (Berlin) said: It has been noted that G(es) in D20 is higher than in H20. The proposed mechanism [reactions (5)-(7)] involving the formation of OH; through excited water molecules could also help to explain this difference. In addition to reactions (5)-(7), neutralization H+ + OH,-% H20 will occur. the neutralization reaction in D20, i.e., This process competes with reaction (7) for OH;.Now, it is known that DZq + OD&--% D20 is slower by almost a factor of two than the corresponding process in H20. (G. Ertl and H. Gerischer, 2. Elektrochem., 1962, 66, 560.) If the rate constant for is not decreased by the same factor the difference in k, and kD should lead to a higher yield of solvated electrons in D20. Dr. A. Singh (Pinawa, Manitoba) said: I thank Asmus for pointing out this addi- tional consistency between our results and the postulated mechanisms. Eventually, of course, detailed diffusion kinetic calculations are needed to evaluate the effect of the various competing reactions that can take place in spurs, on the yields and decay of e; at early times (51 ns).Prof. A. Henglein (Berlin) said: Concerning the energetics of the reaction H20 + OH- -+ H20 + OH + e;, I would like to mention that thermodynamically a free energy of E"(0H) - E"(eG) = 4.9 eV is required (E": standard redox potentials). In order to drive the reaction as a very fast process, the excess energy of the water mole- cule must be significantly larger than 4.9 eV, since the strong solvent reorganization energy in the formation of OH- must also be considered (OH- will be formed with a highly unrelaxed hydration shell). The triplet state of water could not drive the reaction. Dr. A. Singh (Pinawa, Manitoba) said: A previous estimate of the energy required to produce e; from OH,, is 23.3 eV by Merkel and Hamill.60T Photoionization of OH,, has been observed61y62 on irradiation at 6.7 eV.The energies of the various excited states of water are all 26.6 eV, except for the lowest triplet and possibly a very weak singlet, which are both at -4.5 eV.63-65 Thus the reaction H20* + OH,,---+ H20 + OH + eFQ (25) is energetically favoured, except perhaps from the energy states at -4.5 eV. How- ever, its efficiency is low (9 -0.1) for an excitation energy of -6.7 eV, as has been mentioned earlier. This should be applicable to reactions in spurs. The quantum f References as in Singh's paper.76 GENERAL DISCUSSION yields for the other relevant excitation energies are not known; these could well be higher for the excited states of water. The higher excited states of water should be able to react with OH- or OH; in spurs to produce e i .Dr. M. A. J. Rodgers (Texas) said: The question as to whether excited water mole- cules play any significant role in the radiolysis of aqueous solutions has been a con- troversial one for several years and new data, while welcome, must be carefully scrutinized for alternative explanations before categorically assigning it as evidence for the existence of H20*. In this context, how firm is it that the whole of the optical absorption at 460 nm is in fact due to SaIG? Have you considered the possibility that a second species perhaps formed as a result of *OH radical attack on a salicylate ion is contributing optical density at the monitoring wavelength ? Since the argument for H20* rests solely upon the measurement and assignment of the 460 nm absorption I would recommend a serious experimental attempt to dis- count all the alternative interpretations before invoking excited water molecules as chemically significant intermediates.Dr. A. Singh (Pinawa, Manitoba) said: The absorption spectrum of S a l ~ at 460 nm contains small contributions from the transients formed by the attack of H* and *OH on Sal,,. This can be e ~ t i r n a t e d ~ ~ * ~ ~ J f to be <2% at 30 ps. We have not applied this small correction to the data given in table 1. Some of the possible mechan- isms that have not been discussed in the paper due to space limitations, are represented by the following reactions. We have considered other mechanisms, as well. H+OH;--+H,O+e~ (26) (27) (28) H20* + H20* .+ eFq + H30,+, + OH H20* + H20*’ -+ e& + H30& + OH.The rate of reaction (26) is quite low1’ (k -2 x lo-’ mol-l dm3 s-l). So, the yield of e;, derived from thermal H atoms by this reaction would be small and spread out over the ns - ,us region. However, initially, hot H atoms may be formed in some of the reactions in spurs, and their rate to produce e; by reaction (26) may be higher. Energy pooling reactions (27) and (28) are possible but remain to be investigated in the case of H,O. A version of reaction (4) has been proposed by Peers and Cottin (J. Chim. phys., 1966, 63, 1346), as follows: H20* + H20 -+ H30+ + OH + e- [E(21) > 11.5 eV] which might also occur in spurs. Evidence for the participation of electronically excited states of water, in radiolysis, has been reported.10~1”18~20 As pointed out in the discussion of fig.5, there is continued formation of S a l i in the time period 30 to 350 ps after the electron pulse. This is reasonably explained only by reactions [(8)-(1 l)] given in the paper, involving energy transfer from H20*. It could be argued that Sal; is formed directly during the pulse but it undergoes reaction (1 1) comparatively slowly (over several hundred ps), followed by reaction (2). However, the experiments of Rentzepis, et al. (P. M. Rentzepis, R. P. Jones and J. Jortner, J. Chem. Phys., 1973, 59, 766) strongly suggest that the time required to produce e& from energetically allowed excited states is only a few ps. I am not aware of any other evidence suggesting a much longer time for e,; formation from excited states.7 References as in Singh’s paper.GENERAL DISCUSSION 77 The results on emission from solutions of Nasal in water, obtained recently at Pinawa are mentioned in the paper and are consistent with the energy transfer mechanism. Recently, G(Sa1;) -6 has been r n e a ~ u r e d ~ ~ t in a 2 mol dm-3 solution of Nasal in HzO. This is consistent with the growth of Sal; seen in ps pulse radioly- sis work (fig. 5). In my opinion, there can be no question of whether H20* is formed on radiolysis, or not. The question is : what reactions do electronically excited water molecules undergo? The reactions relevant to our system are given in the paper, and earlier in the discussion. Dr. M. A. J. Rodgers (Texas) said: Is it not possible that the emission of Nasal seen by you is due to direct solute excitation and the Cerenkov radiation? Dr.A. Singh (Pinawa, Manitoba) said: Yes, both these sources will contribute to shows that the energy transfer from H20* is emission. However, our quite important. Dr. M. A. J. Rodgers (Texas) said: There is a possibility that some of the G(Sa1;) For ex- may come from attack of another reducing species other than eG or H20*. ample, what about the scheme H* + Sal + SalH* + Sal- + H& where SalH* is an H* adduct to the aromatic nucleus? Dr. A. Singh (Pinawa, Manitoba) said: We have not considered this reaction seriously. However, it seems to me that H atom addition should be to the aromatic ring giving a cyclohexadienyl radical3 from which loss of a proton to give Sal; will be difficult. I might point out that the free radical formed on H atom addition is k n ~ w n ; ~ ~ .~ ’ it decays much more slowly than SalFq. Dr. H. A. Scbwarz (Brookhauen) said : What is the quantum yield of ionic dissocia- tion of vibrationally excited water? Dr. A. Singh (Pinawa, Manitoba) said: The measured quantum yield has been reported59 to be 4 x 10- 7. However, this value is measured on a ,us timescale follow- ing a nanosecond pulse from a neodymium laser. I have extrapolated the quantum yield to zero time (po - 1) assuming a combination of 1st and 2nd order recombination of Hf and OH;. It would of course be better to have a direct measurement on a sub- microsecond timescale. Perhaps this will emerge in the future from ns conductivity work or by an extension of the approach used by Pikaev et al., this Discussion, to measure yields of HA.Dr. H. A. Schwarz (Brookhaven) said: Even if reaction (7) proceeds, it will be followed by ea, + KO,+, + H20 + H (30) so that there will be no net effect of reaction (7) on e z formation. Dr. A. Siugh (Pinawa, Manitoba) said: You are quite right. However, the net effect of these two reactions is to spread out the time of formation of e; over a longer period than the ps electron pulse from the accelerator. Both the smaller than expected t References as in Singh’s paper.78 GENERAL DISCUSSION decay of e z over the first 1 ns region1yl27 and the lower G(exc + ion) in liquid H20 as compared to H20 vapour suggest that simultaneous formation and decay of e; probably take place during this period; reactions (7), (30) and (26), most likely con- tribute to it.Dr. H. A. Schwarz (Brookhaven) said: What is the efficiency of H atom formation in water? I think Platzman pointed out that at energies greater than the ionization potential of water, H atom formation is not important. Dr. A. Singh (Pinawa, Manitoba) said: The ionization efficiency of H20, for reac- tion (26) H,O + hv -+ H20 + e- (31) has been measured (P. H. Metzger and G. R. Cook, J. Clzem. Phys., 1964, 41, 642). Though the ionization potential of H20 35 is 12.62 eV, at 12.8 eV and 20 eV excitation energies, the efficiency of reaction (31) is only 30% and 70%, respectively. So, H atom formation from superexcited states of water should be important. It is relevant to mention that, according to the optical spectrum of water,1° the energy distribution of the primary events will be as follows: < 12.6 eV, (-2573, < 15 eV (-50%), > 15 eV (-50%).Dr. M. G. Simic (Natick, Mass.) said: At what pH did you do the experiments? Dr. A. Singh (Pinawa, Manitoba) said: At natural pH, which is 7-7.2 in the case of solutions containing 1-2 mol dmW3 Dr. M. G. Simic (Natick, Mass.) said: If you did the experiments at about neutral pH, you probably have to consider the protonation of the electron adduct to salicylate. The starting solute carries unit negative charge, hence Sal- + e- -+ *Sa12- -Sa12- + H+ s *HSal- whatever e- is. The protonation should be fast and take place at the carboxy group as in the case of benzoate (M. G. Simic and M. Z . Hoffman, J. Phys. Chem., 1972,76,1398).The pK, = 12 for benzoate and it should be very similar for salicylate. Since the pro- tonation results in a spectral shift of about 10 nm, some of the absorbance as time changes could be due to protonation. For an experiment like yours, it would have been more convenient to have an electron acceptor whose electron adduct has pK, < 4. Dr. A. Singh (Pinawa, Manitoba) said: Hunt and c o ~ o r k e r s ~ ~ f have done extensive work on Sal; from which it has been concluded that protonation of Sal; is not important, at the pH of the solutions we used (natural pH), within the 30 ps resolution time of the pulse radiolysis system. Some protonation of SalG during the subsequent period (30-350 ps) would occur. The extinction coefficient of protonated SalG is smaller4' than that of Sal;, at 458 nm.So, if some protonation of Salz has already occurred in our system which re- quires correction, then the corrected values of G(Sa1;) will be even higher than those reported in the paper. t References as in Singh's paper.GENERAL DISCUSSION 79 Prof. R. H. Schuler (Notre-Dame) said : I simply want to point out that the old ps data from Argonne, that you just referred to, did not show any decay of eG for 1 ns after the pulse. However, further very careful work there has shown that there is -10% decay of e; over 1 ns after the pulse. Dr. A. Singh (Pinawa, Manitoba) said: You are quite right. The point I wish to emphasize is that for the reasons already mentioned it is quite possible that there is simultaneous formation and decay of elq in the post-pulse period.Dr. B. C. Webster (Glasgow) said: In consideration of the lifetimes mentioned for the excited state of water, s for a singlet state, it might be commented that if the coupling between the electronic excitation and the intermolecular vibrations in the condensed phase is undisturbed, then the residence time for the excitation on a particu- lar molecule is This implies that, during the lifetime of the excited state, the excitation could traverse an order of 10 000 molecules, and so be removed far from the spur. The probability of the excitation being preserved for this period would appear rather low, in view of excitation loss events that could occur. Accordingly to postulate that the excited state of water is involved in this way in the formation of e,; seems untenable.A lucid description of the possible role of excited states in the formation mechan- isms of excess electrons in condensed polar media has been given recently by H. B. S teen s. Dr. A. Singh (Pinawa, Manitoba) said : Migration of the excitation energy of H20* to areas outside the spur would occur efficiently only if there were no scavengers for H20* in the spur. I expect the concentration of OH; in spurs to be -0.1 mol dm-3. So, there will be competition between energy migration, and reactions of H,O* in spurs by the various reactions given in the paper and earlier discussion. It may be mentioned that singlet excited states can transfer energy to acceptor molecules at considerable distances (up to 50 A), without direct collisions.This mechanism makes singlet energy transfer from donors to acceptors an efficient process. I am happy to note that Steenl has discussed possible role of excited states in e,, formation and I look forward to reading his paper when it becomes available to me. Prof. M. C. R. Symons (Leicester) (communicated) : In the light of the discussion on Singh's paper, it may be worth noting that, in general terms, there are two distinct types of electronic excited states for water molecules, both of which will be greatly perturbed by the hydrogen-bonded network of the liquid. One has the excited electron essentially in the 8 orbitals which will naturally lead to 0-H bond fission. The other will be a Rydberg type orbital centred on the H20+ ion, and will naturally lead to electron loss.This latter state could, during its life-time (singlet or triplet) act as a powerful electron-donor, and could fulfil the role Singh's results seem to require. Prof. J. Kroh, Dr. W. Bartczak and Dr. Cz. Stradowski (I%&, Poland) said: It seems that time dependence of trap depth due to matrix relaxation should be included in the model presented by Butler and Pilling. Our calculations suggest that two relaxation mechanisms must be considered. The first one is simply the rotation of the nearest-neighbour molecular dipoles. The second mechanism consists of two steps. H. B. Steen, in Electron-Solvent and Anion-Solvent Interactions, ed. L. Kevan and B. C . Webster (Elsevier, Amsterdam, 1976), p. 175-21 1.80 GENERAL DISCUSSION The primary one is the electron tunnel transfer from an occupied, unrelaxed trap to a slightly deeper empty pre-existing trap.This is followed by local heating of the matrix allowing the molecular dipole rotation. The local “ heating ” is due to the energy ex- cess, possessed by the electron on arrival into deeper trap. It appears that the calcu- lated relaxation time is of the order of 10-12-10-11 s in liquids at 300 K, being com- parable with the time of tunnel transfer. Hence the parameters a and b should be time- dependent; this leads to the change in the shape of the kinetic curves. The calculations of Butler and Pilling are mainly referred to hydrocarbons. Ac- cording to our evidence for glassy matrices the tunnelling to neutral scavenger mole- cules in such systems is rather doubtful.The equations describing electron tunnelling from the trap to a scavenger ion were checked experimentally in aqueous glasses, mainly alkaline ice. There is no positive evidence of direct applicability of the same model to other systems. The study of the decay of electrons trapped in hydrocarbon glasses containing biphenyl shows that the increase of concentration of biphenyl anion during the decay of trapped electron is probably due to photobleaching. Also, the lack of the growth of benzyl radical during the decay of electrons trapped in alcohol glasses (containing benzyl chloride) indicates that the role of the scavenger in electron decay may be complex. One must admit, however, that the situation in liquids may be very different from that in low temperature glasses.Dr. M. J. Pilling (Oxford) said: The decay of electrons in alkane glasses by random scavenging is not so closely symptomatic of tunnelling as it is in more polar glasses (although this qualification does not apply to, say, recombination luminescence of solutes in alkane glasses.) This arises because of complexities in the overall transport process. In liquid alkanes, where diffusive transport can be described in terms of known diffusion coefficients, it is feasible to invoke and model tunnelling reactions provided the electron transfer takes place from the trap and not from the conduction band. It is true that trap relaxation should be taken into account in a full description of the scavenging process at short times. There are significant problems involved, however, partly because the trap relaxation process has been little characterised, either experimentally or theoretically, and partly because the overall reaction energetics change as relaxation proceeds, so that different acceptor levels are sampled as the trap deepens; because of this, a’ changes as well as b, but in an unknown way.In liquids, trap relaxation occurs at short times, where the reaction is most sensitive to changes in a’. Hunt and co-workers have suggested that short time (< 30 ps) scavenging in liquids involves dry e1ectrons.l We have shown that for the more efficient scavengers, e.g., Cr042- in water, tunnelling, even from fully relaxed traps, can account for a signifi- cant fraction (-50%) of the scavenging.2 Tunnelling from partly relaxed traps pre- sents an intermediate process, whose importance depends on the rate of trap relaxa- tion.Prof. A. Henglein (Berlin) said: I have two questions for Pilling and one comment: Question 1 : What potential barrier do you assume in your calculation of tunnelling Question 2: Must the long-distance reaction be explained by tunnelling or could it reactions of hydrated electrons ? K. Y . Lam and J. W. Hunt, Int. J. Radiation Phys. Chem., 1975,7, 317. P. R. Butler, M. J. Pilling, S. A. Rice and T. J. Stone, Cunad. J. Chenz., to be published.GENERAL DISCUSSION 81 be an electron jump on a single potential surface as in ordinary electron-transfer reactions ? Comment: Regardless of the question of tunnelling or electron transfer, the accep- tor has to reach a molecular configuration suitable for the acceptance of the electron.In the case of very weak interaction, one may describe this effect approximately by regarding the distributions of unoccupied redox levels in the acceptor-system and of the occupied ones in the donor system,I as it has been done in electrochemistry.2 The problem of the electronic redox levels for excess electrons in water thus arises.l I would like to draw attention to the fact that the views of radiation chemists and of electrochemists are different. In electrochemistry, electron photoemission experi- ments have led to certain ideas about the energies at which water is able to accept electron^.^^^ The following figure describes the two views. radiation chemist's view electrochemist's view t -' 2 i2 -2 ,/ distribution of occupied levels of e i q -3 1 I I [ A I / / , ,'unocc.' '\ s 1 \ \ 1 / / occ. 0 / ,/ levels I I I ibl 4 I I I \ \ \ \ I \ '\ /' \ 1 / / / occ. ; levels A ext I Eint 1 FIG. 1 .-Three diagrams of electronic levels of excess electrons in water. The electronic energy E (left) is set at zero for an electron in the gas phase. A free energy AGs of hydration of - 1.63 eV is accepted in all models.s The vertical external ionization energy IE,,, of eG (energy to remove e- from e& into the gas phase in a A. Henglein, Ber. Bunsenges. phys. Chem., 1974, 78, 1078. H. Gerischer, 2. phys. Chem. N.F., 1960,26,223. G. C. Barker, Ber. Bunsenges. phys. Chem., 1971, 75, 728. A. M. Brodsky and Y . Y . Pleskov, Progr. Surface Sci., 1972, 2, 1 .First calculated by J. N. Baxendale, Radiation Res. Suppl., 1964, 4, 139; more refined calcula- tions by A. J. Swallow, Radiation Chemistry (Longman, London, 1973)) p. 150.82 GENERAL DISCUSSION fast transition) is larger by the reorganization energy RE of the water trap left behind. The energy RE is not accurately known. Taking a value of 0.3 eV,l the most probable external ionization energy of e& would be 2.0 eV. The most probable state of erq in water would thus be characterized by E = -2.0 eV on the scale e (most probable occupied electronic state). A distribution of occupied levels shown by the dashed line must exist since various transition energies in the vertical ionization are possible. The internal ionization energy IEint of e& [energy to move e- from its trap into the level Vo (energy of the delocalized electron in water)] is also not known.It is often assumed in radiation chemistry that this internal ionization energy is equal to or a little larger than 1.73 eV, i.e., the quantum energy in the absorption maximum of e;. The following relationship exists between these various energies : Vo - AH, = IEint - RE (AHs: energy of hydration). Approximating AHs by AGs and using IEint = 1.73 eV and RE = 0.3 eV, Yo = -0.2 eV is obtained. This situation is depicted on the left side of the figure. The second dashed line indicates a possible distribution of un- occupied levels for excess electrons in water (characterized by the energy E that is liberated), if an electron from the gas phase is suddenly localized at a configuration of water molecules that has a positive electron affinity.(Such configurations are formed and disappear during the thermal fluctuations in water.) These levels will generally lie above E = - 1.63 eV (an electron in such a shallow trap would rapidly relax). Threshold determinations in the electron photoemission experiments led to a much lower Yo value of -1.2 eV. In these measurements, it was assumed that electrons can only be emitted into the Vo level or higher levels of the continuum. (The possi- nuclear configuration L nuclear configuration reaction. (6) Exoergic reaction. FIG. 1 .-Potential energy profile for electron transfer from reactants (R) to products (P). (a) Isoergic K. Fueki, D.-F. Feng, L. Kevan and R. E. Christoffersen, J.Phys. Chem., 1971,75, 2297; a similar value was used in calculations of B. C. Webster (personal communication).GENERAL DISCUSSION 83 bility of emission into shallow traps was not considered.) The right hand side of the figure shows two extreme cases of electronic levels in water that can be conceived for Vo = 1.2 eV. (a) The external ionization energy is 2.0 eV as in the model of the left side. Light absorption at a quantum energy of 1.73 eV would produce a state eG* far above the Vo level. (b) The internal ionization energy of e i is equal to about 1.73 eV as in the model of the left side. The external ionization energy of e z would then be much larger, i.e., 3.0 eV. No unambiguous experimental evidence has yet been given for either of the con- ceivable electronic level diagrams in water shown in fig.1. Dr. M. J. Pilling (Oxford) said: (i) I would suggest that a value of 2 eV for the trap depth is an overestimate, even if one takes Henglein's value for the photoconductivity threshold of 1.73 eV. Photoionization produces an electron with kinetic energy at least equal to the zero point energy, To. This corresponds to energy in excess of the top of the potential barrier; estimates of its magnitude vary, but values as high as 1 eV have been suggested. In addition, the trap is left in a non-equilibrium configura- tion and this increases further the excess energy required to produce a mobile electron. Finally, evidence from studies of some aqueous glasses (e.g., 10 mol dm-3 NaOH) show that the photoconductivity and optical absorption spectra coincide,l showing that, in these glasses, the photoionization threshold is 51.2 eV.Even if we have grossly underestimated the potential well depth, the description we have used remains valid. A value for V, - E of 2 eV gives p = 14 nm-l. An Reff value of 1.2 nm for reaction between an electron and an uncharged scavenger (R = 0.5 nm) requires a' = 1.1 x 1W6 s-', compared with the value of 3.3 x 1014 s-l for p = 10 nm-' [corresponding to (Vo - E ) = 1 eV]. This is further illustrated by the data shown in table 2 of our paper, which reproduce the experimental data in alkanes for assumed trap depths of 0.25 and 0.5 eV. Our ignorance of the form of the potential in the disordered materials we study presents a major problem in our attempts to describe electron tunnelling.The poten- tial is difficult to derive and even to visualise in a 3-dimensional system and it is likely that regions of high potential exist, in excess of the total energy of a mobile electron. This will be especially true for glasses formed from aqueous salts. In a liquid, fluctuations in the potential will certainly arise; it seems unlikely, however, that these fluctuations will be comparable with the depth of the electron trap. Provided this stipulation holds good, the exponential distance dependence of the tunnelling proba- bility will remain and will be little changed from that derived assuming a constant potential (/3 will be altered somewhat). This is illustrated by the insensitivity of the barrier penetration to the introduction of a coulomb term.(ii) Electron transfer reactions are generally discussed using the potential energy surface shown in fig. l(a) (of this Comment), which describes the potential energy of the reactants and products as a function of a generalised nuclear configuration in- corporating both medium and solute coordinates.2 Electron transfer occurs primarily at the intersection of the two curves; the activation energy arises because of the rapid- ity of the electron transfer and of the necessity of the nuclei to move some way towards the equilibrium configuration of the products. The probability of electron transfer at the intersection and the size of the splitting between the upper and lower curves S . A. Rice and L. Kevan, J. Phys. Chem., 1977, 81, 847. R.A. Marcus, Ann. Rev. Phys. Chem., 1964,15, 155.84 GENERAL DISCUSSION depend on the electronic matrix element, which, since it depends on the overlap of electronic wave functions, falls off roughly exponentially with the reactant separation. In other words, a full description of the reaction in our terms would require the con- struction of a family of curves such as that shown in fig. l(a), for different reactant separations, r. As r decreases, the transfer probability increases, as does the splitting between the adiabatic potential curves. Our model simply assumes that the transfer probability depends exponentially on r. I still maintain, however, that it is meaningful to employ the term “ electron tunnel- ling ’’ in a description of the process, provided the trap walls remain intact during transfer.Under these circumstances the electron passes from its trap to the well provided by the screened Coulomb potential of the scavenger anion. In so doing it tunnels through a classically forbidden region. The term becomes inappropriate only if the fluctuations in the potential provide a barrierless route for the electron from reactants to products; the magnitudes of the fluctuations required suggest that such a mechanism is unlikely. (iii) The model we have presented has been aimed primarily at describing the dist- ance dependence of electron transfer for the more efficient electron scavengers. As was pointed out in the paper, the parameter a’ contains, inter alia, Franck-Condon factors for the transfer process. The faster electron transfer reactions are highly exoergic and occur with a small or zero activation energy; fig.l(b) is more applicable in this situation and is accommodated in our description by a large value for a‘. As fig. 4 of our paper shows, the overall rate is dominated, under these circumstances, by the diffusive approach of the reactants and is relatively insensitive to changes in a’. If one moves to a less favourable situation, when d is small, then the rate con- stant varies linearly with a’ and, conversely, provides information on a’. Prof. M. C. R. Symons (Leicester) said : I wonder if it is significant that some elec- tron acceptors in solvents such as water may also present a broad band of energies into which e- can be captured, rather than the narrow bands expected for species in inert media or the gas-phase? For example, CrOd2’ ions have very broad absorption spectra in aqueous solution, with no resolved vibrational structure.Dr. M. J. Pilling (Oxford) said: As the electron trap relaxes, the energies of the resonant acceptor quantum states change. For a scavenger with a low electron affinity, it is possible that the product is formed in an activationless step, with a large Franck-Condon factor. As the trap relaxes the scavenging process may become activated because of the decrease in the reactant energy. Henglein has discussed the importance of the overall energetics for the reaction between e& and M2,T.l Prof. L. Kevan (Detroit) said: I might point out that in our early experiments on direct observation of electron spurs from detecting nonhomogeneous trapped electron concentrations in aqueous glasses by e.s.r.saturation methods, we concluded that an upper Eimit to the spur radius is 4.1 nm. (J. Zimbrick and L. Kevan, J. Chem. Phys., 1967, 47, 2364 and revised in L. Kevan, Adu. Radiation Chem., 1974, 4, 181 - Table XIV). This radius, of course, reflects the situation after the electrons have been trap- ped and should be larger than the initial electron spur radius needed for the liquid phase diffusion model. However, the enlarged spur radius needed in the diffusion model to fit the short time decay of e; for low LET radiation is 6.1 nm, which exceeds our experimental upper limit. Consequently, I am interested in Burns’ suggestion A. Henglein, Ber. Bunsenges. phys.Chem., 1974,78,1078.GENERAL DISCUSSION 85 this this enlarged spur is not now compatible with his new LET data and that he suggests a different initial distribution for e;. In the suggested e; distribution with a hole in the middle, will the initial spur radius be increased much from the 2.5 nm value that Schwarz used in order to fit all the data? Dr. A. Singh (Pinawa, Manitoba) said: I am glad to see that Burns et al. are not satisfied with the results obtained when they increase the size of the spur, in their diffusion model calculations. Kuppermann has done diffusion kinetic calculations for water and he finds that the calculated elq decay curve fits the observed ps pulse radio- lysis data better, when the spur radius is increased from 18.75 to 67.5 and the energy per spur is increased from 48 to 175 eV.I However, there is a great deal of evidence which suggests that the spur size and the spur energy are smaller than those required for Kuppermann’s expanded spur, as follows : (i) Rauth and Simpson found that the first collision energy losses for 20 keV electrons in Formvar were as follows: most probable energy loss, 22 eV; average energy loss, 60 eV.2 (ii) Several theoretical calculations result in the spur energy being < 100 eV.3 (iii) As Kevan has just mentioned, the spur radii deduced by his group, by e.s.r.measurements are < 36 A, < 36 A and < 37 A for alkaline ice, methyletetra- hydrofuran and triethylamine, respectively, at 4.2 If anything, the thermalization distance for electrons in liquids at room temperature should be smaller.So, even though in some cases an expanded spur model might give a better fit to the observed data, its acceptance will depend on what fresh evidence becomes available to support its energy and size parameters. Dr. W . G. Burns (Harwell) said: In agreement with Singh’s comment and that of Kevan, our paper indicates a number of reasons why the enlarged spur is difficult to accept, in line with Kevan’s e.s.r. results and those of other^.^ In our preliminary calculations using two species, one species having a minimum centre concentration, the trends referred to were obtained using approximately normal spur sizes, see table 2 of the paper, and not the expanded spur. Dr. A. Singh (Pinawa, Manitoba) said: Burns has mentioned the frequently quoted result of Rentzepis, et al.[ref. (10) of Burns’ paper] of solvation of e; in <4 ps. The energetics of most photoionization events (at excitation energies below the gas phase ionisation potentials in solution require the availability of the energy of A. Kuppermann, Physical Mechanisms in Radiation Biology, ed. R. D. Cooper and R. W. Wood (USAEC, Division of Biomedical and Environmental Research, Washington, 1974; Conf. 721 OOl), p. 155. A. M. Rauth and J. A. Simpson, Radiation Res., 1964,22, 643. A. Mozumder, Adv. Radiation Chem., ed. M. Burton and J. L. Magee (Wiley, New York, L. Kevan, in Actions Chimiques et Biologiques des Radiations, ed. M. Haissinsky (Masson, Paris, (e.g., B. G. Ershov and G. P. Chernova, 2nd L. H. Gray Conference Proceedings, Institute of W.G. Burns, M. J. Hopper and C. R. V. Reed, Trans. Faraday SOC., 1970,66,2182. 1969), vol. 1 , chap. 1 , p. 1 ; I. Santar and J. Bednhf’, Ifit. J. Radiation Phys. Chem., 1969, 1, 133. 1969), vol. 13, p. 57; D. P. Lin, P. Hamlet and L. Kevan, J. Phys. Chem., 1972,76,1226. Physics and the Physical Society, ed. G. E. Adams, D. K. Bewley and J. W. Boag, p. 93.)86 GENERAL DISCUSSION solvation of ions, for the ionization event1 The energy required for the ejection of electrons from aqueous solutions of ferrocyanide anion into the gas phase is 5.7 0.5 eV.2 The photon energy used by Rentzepis, et al. [ref. (10) of Burns' paper] was -4.7 eV suggesting that ionisation of the ferrocyanide ion was assisted by the solva- tion energy of the product ions.-+ (Fe(CN);-),, + ez. In radiolysis, thermalization time of electrons is likely to be - 10-l' s, as shown by the following consideration. It is generally agreed that most of the electrons get thermalized before hydration, though Mozumder has suggested that a significant portion may become hydrated prior to thermali~ation.~ So, the time required for thermalization would seem to be the minimum time required for hydration. The time for thermalization is not precisely known, though most suggested values are in the range of 10-12-10-11 s.3-6 Let us consider thermalization of a subexcitation electron (-6 eV). Initially, it will lose energy in units of -1 eV to the vibrational modes of ~ a t e r ~ ? ~ -5 steps e'* (-6 eV) + nH20 ____+ H20*" + e-* (<l eV); H,O*" represents vibrationally excited water molecules.The time required for each event of vibrational excitation should be s since the vibration times in H20 are small (-1 X s). The rest of the energy of the electron will be lost to the rotational modes3p7 of H20 in units of -0.01 eV -100 steps e-( -1 eV) f- nH20 H,O*I + e- ; H20*' represents rotationally excited water molecules. A typical rotation time for H20 is -1 x 10-l' s; so the time required for the thermalizing electrons to lose the last -1 eV of energy should be -10 x s. It leads to the conclusion that hydra- tion of thermalized electrons will require --10-lo s, which is much longer than the figures reported by Rentzepis, et al., [Burns, ref. (lo)]. I might also mention that on the basis of more recent work, Christophorou has concluded that the average loss to rotational modes of water is -0.005 eV, which will increase the thermalization time by a factor of ~ 2 .~ It may be argued that if the thermalization time for electrons is -1O-lO s, a growth of e; formation should be seen in ps pulse radiolysis work. I have stressed during the discussion of my paper that in the post-ps-pulse period, there should be simultaneous M. Kasha, Comparative E'ects of Radiation, ed. M. Burton, J. S. Kirby-Smith and J. L. Magee (John Wiley, New York, 1960), p. 62; L. I. Grossweiner and H.-I. Joschek, Soluated Electron (Ad- vances in Chemistry, Ser. 50, American Chemical Society, Washington, 1965), p. 279, P. Delahay, P. Chartier and L. Nemec, J. Chem. Phys., 1970,53,3126. A. Mozumder, in Adu.Radiation Chem., ed. M. Burton and J. L. Magee (Wiley-Interscience, G. R. Freeman, Quaderni Ric. Sci., 1972, 2, 55. J. K. Thomas, in Adu. Radiution Chem., ed. M. Burton and J. Magee (Wiley-Interscience, N.Y. I. Dvornik and D. Razem, in International Discussion on Progress and Problems in Contemporary R. P. Blaunstein and L. G. Christophorou, Radiation Res. Rev., 1971,3, 69. L. G. Christophorou, personal communication based on the results described in: L. G . Christo- New York, 1969), vol. 1 , chap. 1 , p. 1, and references therein. 1969), vol. 1, p. 103. Radiation Chemistry, ed. J. Teply (Prague, 1971), p. 255. phorou, K. S. Grant and J. K. Baird, Chem. Phys. Letters, 1975,30, 104.GENERAL DISCUSSION 87 formation and decay of eLq; one of the mechanisms of formation of eG in this period would be the solvation of e‘ as thermalization occurs during 10-l’ - s.Dr. W. G. Burns (Harwell) and Dr. 6. V. Buxton (Leeds) (communicated) : Although Singh’s suggestion of radiolytic solvation times extending to 0.1 ns might help to explain the constancy of G(e&) up to such a time, the lack of decay continues until -1.0 ns; [J. W. Hunt, Advances in Radiation Chemistry, ed. M. Burton and J. L. Magee (Wiley, New York, 1976), vol. 5, p. 1851 his proposition is, therefore, useful only for a small fraction of the relevant time. The simplest interpretation of the evidence is of solvation within 3 ps of the radiolytic event, (W. J. Chase and J. W. Hunt, J. Phys. Chem., 1975,79,2835), similar to Rentzepis’ time for photoionization, and implying direct solvation of subvibrational electrons.Dr. G. V. Buxton (Leeds) said: On behalf of my co-authors I wish to state that we have recently made some preliminary measurements of eFq yields in proton tracks in D,O. In contrast to the data for HzO, G(e;) in D20 is comparable with G(e,-,> in 1 mol dm-3 OH- in H20, and is unaffected by addition of 1 mol dm-3 OH- to D,O. These observations indicate that e; + Da+s is not an important track reaction in D20. This is consistent with the spatial distribution of e; being broader in D20 as suggested by others.1.2 Dr. P. Cordier (Orsay) (communicated): I would like to make a comment on the relationship existing between the efficiency factory and the parameter y characterizing the partially absorbing boundary.I have recently found that f and y are correlated as follows: yf= R(l -f), where R is the reaction radius. Such an equation stems from the boundary condition by introducing the experimental rate constant as calcu- lated by Collins and Kimball.31T Unfortunately, this equation takes no account of the Coulomb field and we are carrying out a radiation boundary treatment in the presence of a Coulomb field. As suggested by Pilling (personal communication), the only possibility at present of improving the above equation is arbitrarily to introduce the Debye rate constant expression. As concerns the value of R, it should be worthwhile to note that the reaction radius plays an important role in the mathematical treatment and in the related numerical results. However its role diminishes as y tends to infinity, i.e., when the reactivity of reacting species is low.Dr. M. J. Pilling (Oxford) (communicated): Reactions with rate constants less than the diffusion-controlled limit may be treated using the radiation boundary condition. This gives an overall rate constant of the form: 4nRD 4nRD -‘ k e x p = ( 1 + 4nRD/k’) (I +&(’ ‘7) } where R is the encounter distance and D the mutual diffusion coefficient. Belloni et al., via the equation The equation may be related to the radiation boundary coefficient, y , used by y = D/k E. M. Fielden and E. J. Hart, Radiation Res., 1968, 33, 426. Cz. Stradowski and W. N. Hamill, J. Phys. Chem., 1976,80,1431. t Reference as in our paper.88 GENERAL DISCUSSION where k has units m s-l and corresponds to a velocity across the boundary of the encounter sphere, radius R.Thus k’ = 4nR2k. This development assumes no interactions, such as Coulomb forces, between the reactants. There is no treatment of the radiation boundary condition for charged species, but for very slow reactions, such as those considered in fig. 6 of Belloni’s paper, the interactions may be included simply by incorporating the equilibrium constant for formation of the encounter pair, which is -e-U(R)’kT where U is the Coulomb inter- acti0n.l The rate constant for transport over the encounter radius is unaffected, but the concentration of encounter pairs is increased in the presence of a favourable Coulomb field. Thus, For (NH,‘ + e-), e-U(R)/kT - 10+3.5 at room temperature and the experimental rate constant corresponding to a given value of y is increased by several orders of magnitude.These considerations affect substantially the form of fig. 6 of the paper by Belloni et al. Dr. G. A. Salmon (Leeds) (partly communicated) : In table 2 of Belloni’s paper you compare the electron yield at 3 ns with the dielectric constant of the medium, thus apparently suggesting that the yield of electrons is determined by Coulombic inter- actions, i.e., the Onsager escape probability is relevant. The work of Johnson and myself on methanol2 suggests that a significant amount of spur decay occurs for electrons which escape the Coulombic field of the parent ion; that is, some of the electrons which escape the Coulombic field of the parent ion are still in a non-homo- geneous distribution relative to the counter cation and other radicals such as CH30* and that spur reactions such as CH,OH,+ + e; -+ CH30H + H and CH30* + e; -+ CH,O- are important.A further problem is that in methanol it seems that about 50% of the electrons do not become fully solvated and that rapid ion-recombination processes, probably involving ed,, or ezmp, compete with the solvation process. It seems likely that the high frequency dielectric constant is one of the relevant factors in this early recombination process. Thus the yield of e; at 3 ns is determined mainly by this early process, but also in part by spur processes involving Coulombically escaped species. It is probable that similar considerations may be important in some of the other media you have studied. Dr.J. Belloni (Orsay) said: To study the influence on the electron yield of the efficiency of the e--cation neutralization at each encounter, we compared the yields measured at a same given time, t = 3 ns. For simplicity, the only physical property mentioned in table 2 is the static dielectric constant, which gives a rough estimate of the diffusion controlled e; -cation reaction rate. Actually, other peculiar physical parameters of the liquid must also be accounted for as proposed in theoretical models. Since, however, they are unable to explain the high G(e;) values found in NH3, N,H4 and amines and, moreover, imply that the neutralization is diffusion controlled, which A. M. North, The Collision Theory of Chemical Reactions in Liquids (Methuen, London, 1964), p.38. * D. W. Johnson and G . A. Salmon, Canad. J. Chem., 1977, in press.GENERAL DISCUSSION 89 conflicts with experimental evidence at least in these solvents, we propose to include the parameter of the neutralization efficiency in the calculation of escape probabilities. When this efficiency is low, the role of the reaction of e; with the geminate radical has obviously more importance and the escape probability now results from the relative diffusion of the species e,, cations and radicals. In such a situation, due to the individual mobility of the cation being higher than es- in water and alcohols, contrary to pNH4+ < pe,, in ammonia, the separation of radical and cation by diffusion is much slower in ammonia and the ion encounter occurs inside the spur; then the probability of e; -+ NH2 radical reaction after attraction by NH; is higher. In that, I agree with Burns et al., who admit that the diffusion of H30+, faster than that of OH, causes an accelerated reduction of the cation concentration in the spur.Dr. E. Collinson (Leeds) said: Belloni et al. assume that any reaction between N2H$ and e,,, in hydrazine would ultimately lead to production of H2, and that, because the electron scavenger CCIQ causes only a small change in G(H2), the dis- appearance of ezIv in irradiated hydrazine must occur by the reaction e; + N2H3(l) rather than by e, + N2HS+ (2). Mr. J. Dodsworth and Mr. D. Perfect, in collabora- tion with Dr. D. Smithies and myself, investigated the radiation chemistry of liquid hydrazine and obtained results which support a somewhat different conclusion.We agree that electron scavengers have no significant effect on the yield of H2, indeed one of the most striking features of the radiation chemistry of hydrazine was our finding that G(H2) remained close to 2 whatever was added to the liquid, electron scavenger or not. We also agree that reaction (2) is relatively slow; we found the rate constant to be 6.7 x lo7 dm3 mol-1 s-l. But when the concentration of N2HS+ in hydrazine was increased by adding hydrazine hydrochloride, the yields obtained changed in the following way G(HJ G(N2) G(NH3) pure N2H4 2.05 2.44 5.6 N2H4 + 0.1 mol dm-3 N2H4 2HC1 2.22 3.70 11.5 Therefore we suggest that reaction (2) occurs predominantly as follows es- + N2H$ .+ NH, + NH2.The part played by reaction (2) in the radiation chemistry of pure hydrazine is less certain, but the results obtained from solutions of N20, CH3SH, and C,H,CI in hy- drazine lead us to believe that reactions (1) and (2) proceed at approximately equal rates. Dr. J. Belloni (Orsay) said: As to the products of the reaction e; + N2H$ in hydrazine, it is well known that in water the corresponding reaction leads to molecular hydrogen. This is not the case in N2H4 solvent and I would like to point out similar behaviour in liquid ammonia, where the addition of an ammonium salt and of a solute such as ethanol or hydrazine, absolutely fails to give a supplementary H2 yield corresponding to e; scavenging. Although the NHZ + e; reaction is slow, it occurs at high ammonium concentration, and the lack of supplementary H2 formation in the presence of ethanol again raises the question on the nature of the product of the recombination NH: + e:.Prof. A. Charlesby (Shrivenham) said : Many organic materials when allowed to warm up after irradiation emit light at characteristic temperatures. This thermo-90 GENERAL DISCUSSION luminescence has been studied in detail for a number of polymers, where large scale molecular motion is not possible at the temperatures involved. The temperature peaks of light emission are characteristic of the polymer structure and morphology, but the emission spectrum corresponds to the fluorescence and phosphorescence of an additive or impurity, usually an aromatic present in very small concentration.My Question I concerns the mechanism by which this additive (termed lumines- cence centre, L.C.) becomes excited on warming. (a) Both electron and hole are trapped in the polymer matrix following irradiation. On warming one is released, and after charge recombination, the energy re- leased is then transferred to the luminescence centre, which is thereby excited. (b) Following irradiation the electron is trapped in the polymer matrix, while the hole migrates to the L.C. (+LC+). On warming, the electron is released and travels to LC+ to give L.C*. Alternatively it is the hole which is trapped while the electron migrates to L.C. (c) The L.C is itself ionised (directly or indirectly), and the electron (or hole) becomes trapped in the polymer matrix, to be released later on warming.I would be glad to hear of any evidence supporting any of these, or other mechan- Several alternatives are possible. isms. Question 2. A possible method of deducing trap depth involves the imposition of an electric field during isothermal luminescence decay. This produces a sudden increase in light emission which rapidly decays to its initial value. A second or third imposition produces only a slight enhancement. If, however, the field is reversed a strong enhancement is again observed. This points to a strong directional effect, readily ascribed to geminate recombination via a tunnel effect. The theoretical relationship between trap depth, immediate enhancement and subsequent decay has been evaluated quantitatively. Dr. F. Kieffer (Orsay) said: So far as your first question is concerned, I agree that, quite generally, thermoluminescence peaks are characteristic of the matrix studied (not only in polymers), whilst the emission spectrum corresponds to the fluorescence and phosphorescence of the aromatic solute or impurity, although we recently ob- served a less simple pattern for the emission spectrum in the case of scintillators in MCH glass.As regards the mechanism by which the additive becomes excited, I would per- sonally favour your alternative (b), part 1, for hydrocarbon matrices, although neither (a) nor (c) have ever been really disproved and (c) seems to be valid in some polar glasses. There is quite conclusive evidence for the presence, in an irradiated hydro- carbon matrix containing an aromatic solute, of the following species : “ physically ” trapped electrons with characteristic optical and e.s.r.spectra; “ chemically ” trapped electrons, i.e., radical-anions formed by attachment of an electron to a suitable solute molecule ; radical-cations of aromatic solutes having a lower ionization potential than the matrix molecules. A considerable literature has been devoted to these trapped species in the nineteen sixties (cf. Hamill; Albrecht etc.). The presence of anions and cations resulting from the capture of an electron or a positive charge by radicals formed during radiolysis is probable, but less firmly established. It can probably be neglected in low dose experiments. The presence of trapped holes or matrix cations and anions does not seem to have been demonstrated.In “pure” hydrocarbon matrices trapped electrons are present after irradiation, but nothing seems to be known about the nature of the positive species which necessarily coexist with them. TheirGENERAL DISCUSSION 91 deferred luminescence is very weak and its origin is a matter of conjecture: trace impurities or radiolysis products could be responsible. Your question is a fundamental one and it would certainly be well worth while to devise the means for replying to it properly. Prof. L. Kevan (Detroit) (communicated) : The new time dependence reported for recombination fluorescence kinetics for high LET radiation is very interesting. Perhaps it is worthwhile to compare it with the theoretical model of Tachiya and Mozumder,l (Chem. Phys.Letters, 1975, 34, 77). They obtain the expression where a - 1 A in alkanes and refers to the inverse square root of the electron trap depth and b is the distance parameter characterizing an exponential electron-cation distance distribution. For low LET radiation, like prays, b is estimated as 50 A. This predicts a time dependence for the luminescence intensity of t - l V o 2 which agrees with that found experimentally. With high LET radiation in the same matrix one expects b to decrease and a to remain unchanged. Thus a time dependence o f t - l m 3 as reported in this paper implies b - 3 A. This seems rather short but the theoretical model and experimental results are certainly qualitatively consistent. It also appears that one would not expect to find a larger effect on the luminescence time dependence than reported in this paper.I ( t ) t-(l+alb) Dr. F. Kieffer (Orsay) said: This is a very interesting comment. Unfortunately we have no data on the decay kinetics of ITL at times longer than xl ,us after a-particle irradiation. Hence we do not know whether the time dependence will remain as t-1.3 or will tend towards t as it does with p particles at times at the utmost of the order of a microsecond. Whereas Tachiya and Mozumder's model concerns single pair recombinations, we think that multiple recombination possibilities exist in the initial track, especially with high LET radiation, and this could explain a faster initial luminescence decay. This is in fact observed and does seem to go on for longer times with a-particles than with p particles. Dr.J. Belloni (Orsay) said: In response to a remark by Schindewolf, the value of the reaction radius is not too different, I believe, from values given by other authors for the same erm + NHZ reaction. At 20 "C the encounter radius results from ream = 3.5 A. A more consistent value at -50 "C should be near 3.0 A. However, I must say that the experimental rate constant is so far from the diffusion controlled limit calculated from this radius, that our conclusions do not depend on the accuracy of this parameter. On the contrary in 1,2-DME where k,, +.c+ is high, the result of the comparison with the calculated rate constant depends mainly on the value chosen for this radius. Prof. J. Kroh (Addi, Poland) said: A few years ago2 in our laboratory the influence of LET on intensity ( I ) of isothermal luminescence (ITL) of acetone was investigated. It was found that (IfTL)/(IfTL) = 2.5.This result was ascribed to the difference in track structure of T P-particles and Compton electrons produced by 6oCo prays. Prof. J. Kroh, Dr. J. Mayer and Miss M. Szadkowska (Lo'di, Poland) said: It has M. Tachiya and A. Mozumder, Chem Pliys. Letters, 1975, 34, 77. J. Kroh and M. Wypych, Itit. J. Radiation Phys. Chem., 1974, 6, 67.92 GENERAL DISCUSSION been commonly accepted that isothermal luminescence, ITL of organic solids contain- ing aromatic additives is caused by the recombination of trapped electrons with positive ions of added aromatic compounds, formed by charge transfer from a matrix cat ion.It was shown earlier2 that in the 3-methylpentane (3 MP) + biphenyl (Bph) system at 77 K the ITL intensity (measured in the phosphorescence band) increases with Bph concentration, in the range 10-5-2.5 x mol dm-3, then reaches a maximum and at higher concentration (up to 7.5 x loh3 mol dmd3) decreases. It was suggested by us that such dependence results from the competition between the scavenging and trapping of dry electrons. In order to test the general validity of the above mechanism the 3MP + naphthal- ene system has been investigated. Naphthalene (Nph) is a particularly adequate scavenger because of its ability to form excimers; additionally the solubility of Nph in 3MP at low temperature is much better than that of Bph. t FIG. 1.-The dependence of ITL intensity as a function of [Nph], without and in the presence of 5 x mol dm-3 CC4.P, (filled symbols) phosphorescence band; F, (open symbols) fluores- cence band. The dose is 9.0 x 10l6 eV 8-l. mol dm-3 CC4. 0 0 , 3 MP + Nph; A A, 3MP + Nph + 5 x In the presence of Nph the ITL of 3 MP glass at 77 K consists of fluorescence (300-360 nm) and phosphorescence (445-600 nm) of the solute. The influence of [Nph], in the range 5 x - 2.5 x mol dm-3 on the intensities of both emis- sion bands, measured 4 min after the end of irradiation, is shown in fig. 1. The fluorescence and phosphorescence intensities at first increase with [Nph] and then for P. Cordier, F. Kieffer, C. Lapersonne-Meyer and J. Rigaut, Proceedings of the Fifth International Congress of Radiation Research, ed.0. F. Nygaard, H. I. Adler, W. K. Sinclair (Academic Press, N.Y., 1975), p. 426. J. Kroh, J. Mayer and M. Szadkowska, Proceedings of the IVSymposium on Radiation Chemistry (Keszthely, Hungary, 1976).GENERAL DISCUSS I ON 93 [Nph] w 2.5 x mol dm-3 start to decrease passing a minimum at ~ 7 . 5 x mol dm-3. For higher [Nph] the intensities of both bands sharply increase. The traces of excimer emission (365-430 nm) can be observed only for [Nph] 2 mol dm-3. In order to explain the very complex shape of the curve shown in fig. 1 the follow- ing experiments were carried out. At first the ITL of the y-irradiated 3MP + Nph system was investigated in the presence of Ccl4(5 x mol dm-3). According to our current results the electron scavenging efficiency of CC14 is higher by one order of magnitude than that of Nph.The intensity of ITL for 3MP + Nph system containing CC14 increases steadily with In the next series of experiments the samples after irradiation were kept for 2.5 “Phl, (fig* 1). ‘ 104 102J 10-2 lo-’ > log “Phl FIG. 2.-The influence of sample treatment on the dependence of ITL intensity on mph]; P, phos- phorescence band; F, fluorescence band. The dose is 9.0 X loi6 eV g-’. OP, OF, no treatment; AP, AF, kept at 85 K ; 0 excimers, kept at 85 K; OP AF, photobleaching with light at 1300 nm. min at about 85 K and then the ITL was examined at 77 K. The results of these measurements are shown in fig. 2 and again one obtains a curve without a maximum. It is interesting to note that in the [Nph] range 7.5 x 10-3-2.5 x lou2 mol dm-3 a considerable amount of Nph excimers is produced in irradiated samples kept at 85 K.The formation of dimer cations in partly warmed hydrocarbon + Nph systems after irradiation was proved using the spectroscopic meth0d.l Finally the irradiated samples were subjected to bleaching with infrared light (1300 & 40 nm, bleaching time -2.5 min) and, as before, no maximum on the ITL intensity-[Nph] dependence is observed (fig. 2). The infrared light as well as the storage of the samples at 85 K (the temperature A. Kira, M. Imamura and T. Shida, J. Phys. Chem., 1976,80,1445.94 GENERAL DISCUSSION of the first ITL peak of 3MP) cause the disappearance of trapped electrons in 3MP glass. The above results strongly suggest that there are two different reaction paths responsible for ITL in 3MP glass.According to the first one, which explains the maximum, the luminescence is caused by trapped electron-cation recombination whereas the residual emission is due to charge transfer between Nph- and Nph+ most probably by tunnelling. Such an explanation seems to be reasonable because the highest intensity of ITL is observed at 2.5 x mol dm-3 Nph in 3MP. No electrons are found in such a system on spectrophotonietric examination. The decay kinetics of ITL measured in both emission bands can be described by Bagdasar’yan’s equation at least in the time range up to -30 min after the end of irradiation, which is consistent with tunnelling, although some contribution of I 4 8 12 16 20 24 28 32 > ti min FIG. 3.-The influence of [Nph] on the decay kinetics of ITL for the 3MP + Nph system. P, (filled symbols) phosphorescence band ; F, (open symbols) fluorescence band. Numbers indicate concen- trations of Nph. thermally induced process, i.e., diffusion cannot be excluded in soft glasses such as 3MP (fig. 3). The decay is faster for higher [Nph] as was found before in the case of biphenyl; it seems reasonable in view of the proposed mechanism. Dr. F. Kieffer (Orsay) said: The hump observed in your curves representing ITL intensity at 77 K as a function of naphthalene concentration is surely a peculiarity of 3-MP, resulting from its glass transition temperature being precisely 77 K. Some years ago we observedl in methylcyclohexane (MCH) glass (Tg N” 85 K) a linear increase of integrated ITL intensity with biphenyl concentration, while the integrated intensity of the first thermoluminescence peak (90 K) presented a maximum at mol dm-3. Both ITL and the first thermoluminescence peak are due to electron- F. Kieffer, C. Meyer and J. Rigaut, Chern. Phys. Letters, 1971, 11, 359.GENERAL DISCUSSION 95 cation recombination : electrons involved in ITL are trapped sufficientIy close to cations to allow recombination by tunnelling, whereas those involved in the first glow peak are trapped at greater distances and recombine only when the trapping structures are disrupted at Tg (thermally activated process). As is well known, the total number of trapped electrons, observed spectrophotometrically, decreases with increasing scavenger concentration,' and this can account qualitatively for the decrease of the first glow peak with concentration at values In 3-MP you have undoubtedly a superposition of the two processes which, in MCH, can be observed separately as ITL and first glow peak. The fact that ITL intensity goes on increasing at concentrations at which excimer formation becomes important does seem to suggest electron tunnelling from naphtha- lene anions to naphthalene cations. This could occur in aggregates, which are certainly formed at these concentrations and are responsible for the formation of dimer cations and the appearance of excimer fl~orescence.~ Tunnelling from anion to cation is also suggested by the fact that, when excimer fluorescence is observed in MCH3, it occurs not only in the second glow peak (anion- cation neutralization), but also at temperatures (77 and 90 K) where electrons are the only mobile species. Hence excimer emission requires the presence of dimer cations. It does not seem to result from the reaction of monomer cations with anions and, therefore, it can be assumed that the electron tunnels from the anion to the cation when thermal diffusion brings both ions to a close enough distance. What puzzles me is the linear relationship which you observe in 3-MP for &/I against time at a naphthalene concentration of only 5 x mol dm-3. With biphenyl in 3-Mi? we found this relationship to apply only at concentrations higher than about 3 x mol dm-3, on the same time scale. At lower concentrations a linear relationship was observed for against time. Your observation would seem to suggest that aggregate formation already occurs at this low naphthalene con- centration, with possible tunnelling from anion to cation, but then one should also expect excimer emission, which you did not observe in this case. mol dm-3. J. B. Gallivan and W. N. Hamill, J. Chern. Phys., 1966, 44, 1279. B. Brocklehurst and R. D. Russell, Trans. Faraday SOC., 1969,65, 2159. C. Deniau, A. Deroulide, F. Kieffer and J. Rigaut, J. Luminescence, 1971, 3, 325.

ISSN:0301-7249    

DOI:10.1039/DC9776300067

出版商:RSC    

年代:1977

数据来源: RSC