摘要:

6ENERAL DISCUSSIONS OF THE FARADAY SOCIETY Date 1907 1907 1910 191 1 1912 191 3 1913 1913 1914 1914 1915 1916 1916 1917 1917 1917 1918 1918 1918 1915 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 Subject Volume Osmotic Pressure Trans. 3 The Constitution of Water 6 Hydrates in Solution 3 High Temperature Work 7 Magnetic Properties of Alloys 8 Colloids and their Viscosity 9 The Corrosion of Iron and Steel 9 The Passivity of Metals 9 Optical Rotary Power 10 The Hardening of Metals 10 The Transformation of Pure Iron Methods and Appliances for the Attainment of High Temperatures in a Laboratory 12 Refractory Materials 12 Training and Work of the Chemical Engineer 13 Osmotic Pressure 13 Pyrometers and Pyrometry 13 The Setting of Cements and Plasters 14 EIectrical Furnaces 14 Co-ordination of Scientific Publication 14 The Occlusion of Gases by Metals 14 The Present Position of the Theory of Ionization 15 The Examination of Materials by X-Rays 15 The Microscope Its Design Construction and Applications 16 Basic Slags Their Production and Utilization in Agriculture 16 Physics and Chemistry of Colloids 16 Electrodeposition and Electroplating 16 Capillarity 17 The Failure of Metals under Internal and Prolonged Stress 17 Physico-Chemical Probleins Relating to the Soil 17 Catalysis with special reference to Newer Theories of Chemical Actioil 17 Some Properties of Powders with special reference to Grading by Elutriatioo 18 The Generation and Utilization of Cold 18 19 19 11 Alloys Resistant to Corrosion 19 The Physical Chemistry of the Photographic Process The Electronic Theory of Valency Electrode Reactions and Equilibria 19 Atmospheric Corrosion.First Report 19 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 20 The Physical Chemistry of Steel-Making Processes Photochemical Reactions in Liquids and Gases Explosive Reactions in Gaseous Media 20 20 20 20 21 21 22 Physical Phenomena at Interfaces with special reference to Molecular Orientation 22 Atmospheric Corrosion.Second Report 23 The Theory of Strong Electrolytes 23 Cohmion and Related Problems 2 GENERAL DISCUSSIONS OF THE FARADAY SOCIETY Date 1928 1929 1929 1929 1930 1930 1931 1932 1932 1933 1933 1934 1934 1935 1935 1936 1936 1937 1937 1398 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 Subject Homogeneous Catalysis Crystal Structure and Chemical Constitution Atmospheric Corrosion of Metals. Third Report Molecular Spectra and Molecular Structure Optical Rotatory Power Colloid Science Applied to Biology Photochemical Processes The Adsorption of Gases by Solids The Colloid Aspects 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 (b) Solutions The Properties and Functions of Membranes Natural and Artificial Reaction Kinetics Chemical Reactions Involving Solids Luminescence Hydrocarbon Chemistry 35 The Hydrogen Bond 36 The Oil-Water Interface 37 The Mechanism and Chemical Kinetics of Organic Reactions in Liquid Systems 37 The Structure and Reactionsof Rubber 38 Modes of Drug Action 39 Molecular Weight and Molecular Weight Distribution in High Polymers.(Joint Meeting with the Plastics Group Society of Chemical Industry) 40 The Application of Infra-red Spectra to Chemical Problems 41 Oxidation 42 Dielectrics 42 A Swelling and Shrinking 42 B Electrode Processes Disc. 1 The Labile Molecule 2 Surface Chemistry. (Jointly with the SucMt6 de Cbimie Physique at Colloidal Electrolytes and Solutions The Interaction of Water andporous Materials The Electrical Double Layer (owing to the outbreak of war the meeting was abandoned but the papers were printed in the Transactions) Bordeaux.) Published by Butterworths Scientific Publications Ltd. Trans. 43 Disc. 3 4 Lipo-Proteins 6 The Physical Chemistry of Process Metallurgy Crystal Growth 5 Chromatographic Analysis 7 Heterogeneous Catalysis 8 Physico-chemjcal Properties arid Behaviour of Nuclear Acids Spectroscopy and Molecular Structure and Optical Methods of In-Trans.46 vestigating Cell Structure Disc. Hydrocarbons Disc. 10 Electrical Double Layer Trans. 47 The Size and Shape Factor in Colloidal System Radiation Chemistry 12 11 13 14 15 16 57 The Physical Chemistry of Proteins The Reactivity of Free Radicals The Equilibrium Properties of Solutions of Non-Electrolytes The Physical Chemistry of Dyeing and Tanning The Study of Fast Reactions Coagulation and FIocculation 18 Volume 24 25 25 25 26 26 27 28 29 29 30 30 31 31 32 32 33 33 34 34 35 3 GENERAL DISCUSSIONS OF THE PARADAY SOCIETY Date Subject 1955 1955 1956 1956 1957 1957 1958 1958 1959 1959 1960 1960 1961 1961 1962 1962 1963 1963 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 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 Volume 19 20 21 22 23 24 25 26 27 28 29 30 31 32 .33 34 35 36 For current availability of Discussion volumes see back cover

ISSN:0366-9033    

DOI:10.1039/DF963360X001

出版商:RSC    

年代:1963

数据来源: RSC

摘要:

GENERAL INTRODUCTION Fundamental Processes in Radiation Chemistry : Effects of the State of Aggregation BY MILTON BURTON Dept. of Chemistry and the Radiation Laboratory, University of Notre Dame, Notre Dame, Indiana Received 26th August, 1963 The variety of processes which must be discussed in a detailed way are mentioned and comments are made on various models and mechanisms. For elucidation of the ensuant discussion, certain usages are examined and defined. The papers of the Conference are briefly reviewed and the con- clusion is reached that state of aggregation is now shown to affect fundamental processes in clearly discernible fashion. 1. INTRODUCTION Radiation chemistry derives from Roentgen's discoveries of 1895 but was nameless until 1942. Since the latter date there have been a series of major con- ferences devoted to the subject, including the Faraday Society Discussion at Leeds in 1952.A second Discussion at the present time reflects both the number of major advances made in the interval since then and a better recognized need for more detailed understanding of the basic phenomena. The titles of the papers presented reveal a concern with effects of degree of aggregation on elementary processes and the attendant consequences; they mirror a demand for more explicit statement both of major problems and of major conclusions. An introductory paper at any conference should serve a rather obvious function and, perhaps, the more obvious it is in its statements, the more useful it may be. Thus, the existence of certain processes in radiation chemistry must be accepted as axiomatic.They include, for example, energy loss by the incident particle or radi- ation, energy deposition in the medium under study, energy dissipation in that medium, and energy localization as evidenced by the occurrence of certain localized endothermal physical and chemical processes. Our knowledge of such processes is both detailed and fragmentary and is confined in each case to an aspect of a process and to the results of study of a phenomenon or phenomena in media of a particular degree of aggregation-very commonly gases in the cases where detailed visual examination has been possible. Factually, mere knowledge of the existence of processes does not indicate that the processes are understood or even necessarily isolable. Thus, one function of a conference of this nature is the examination of the frequently accepted notion that the processes of energy deposition and energy localization in radiation chem- istry are identical in all cases studied.They are most probably identical under conditions of extreme material attenuation-as in the low-pressure mass spectro- meter ; other cases may require more consideration. One objective of this meeting is to obtain more detailed understanding of fundamental processes. Another is to engage in more satisfactory discussion and 78 GENERAL INTRODUCTION exchange of ideas. For that purpose some clarification of language and employment of terms with singularity of definition is desirable. It is important to recall that there are differences in meaning between the terms process, mechanism and model -and also that the term process is to be distinguished from the term elementary process.The existence of a process (e.g., energy deposition) may be known from a priori considerations. The mechanism of a process may be in many cases entirely un- known; in kinetics the most that can be said in support of a proposed mechanism is that it is not in contradiction to (i.e., it is consistent with) the totality of observa- tions. The objectives of a model are more limited ; the model is employed because it is susceptible to mathematical treatment and affords the minimum skeleton on the basis of which some facts can be interpreted or predicted, i.e., on the basis of which rigorous calculations can be made.A model is not all-inclusive; whenever it can be applied satisfactorily to a system which, in fact, differs markedly from the model system, inquiry into the actuality of employment of all the restrictions set forth in the model is required. It is important for the development of theory that the limitations of application of a model be persistently explored. A typical example is the Magee-Samuel model of diffusion-controlled free-radical reactions in liquids consisting of individually reactive molecules.1 Its successful employment in aqueous systems is certainly cause for congratulation but, much more importantly, it is also cause for further inquiry. The function of theory, of experiment, of proposed mechanism and of abused models is to elucidate processes which, in many cases, are known to exist.The preoccupation with details of model and mechanism shown by the papers in this conference is natural and desirable. The applicability of the results for description of the processes depends in some degree on recognition of the existence of certain separate processes and for such reason, if no other, it is desirable to categorize them, 2. EARLY PROCESSES* Except for momentum transfer processes (not included in this discussion), the initial process of importance in radiation chemistry is the deposition of energy by a charged particle. The distance involved in such deposition introduces no problem for highly attenuated gases. However, for compressed and condensed systems the facts are not really known; it is possible that many molecules, indeed a large volume of molecules may be involved in a single deposition process.2 Conse- quently, when, as in this Conference, Fueki and Magee 3 attempt an a priori cal- culation of the anticipated results of a radiolysis, they not only examine a relatively simple case chemically speaking (i.e., the conversion of oxygen to ozone under con- ditions where relatively little ozone is formed) but they also select a range of pressure for their calculations which appears superficially safe in the sense of these remarks, namely, the pressure range of 1-100 atm.Nevertheless, already in this pressure range, according to some current views, a number of molecules may be simul- taneously involved in the initial process of non-localized energy deposition. In their current treatment, Fueki and Magee avoid this problem by confining their considerations to cases of initial ionization and excitation of single molecules.In spite of the practical needs and the assumptions of a priori calculations, the facts of energy deposition are not really known for compressed and condensed systems. A reasonable question is whether possible non-localized energy loss (by * In this section, there will be repeated use, without further attribution, of ideas stemming from U. Fano.M. BURTON 9 the electron) is to be identified with non-localized energy gain (by the system). Other questions concern the fraction of energy loss in non-localized (as contrasted with initially localized) processes, the manner in which energy deposited in non- localized processes becomes available for chemical processes and the relative con- tribution of such processes to the totality of observable chemical changes.Ap- parently, although a majority of all collisions seems to result in localized excitation, only a minor fraction of the total energy deposition is involved. Most of the energy, in this view, goes into collective excitation. The volume of an initial non- localized energy deposition may be (100 4 3 . One conclusion from such consider- ations is that energy in addition to that deposited in an initially localized process is effective for subsequent chemical processes ; i.e., at least some energy initially deposited in non-localized processes ultimately becomes localized and useful for the initiation of chemical processes.However, it is not to be assumed that the regions of subsequent localized excitation consist of single molecules. They may consist of groups of molecules or even of excited molecules produced somewhat remotely from each other but nevertheless within the region of initial non-localized excitation. Theory appears as yet inadequate to state what actually does happen in the deposition of energy in condensed systems. Under the circumstances, we turn to experiments which appear to reveal energy transfer of some sort at an early stage and thus to indicate the source of that transfer. Typical of such studies are those in radiation protection,4-6 sensitized luminescence 7-10 and attendant quenching phenomena,ll-13 and sensitized decornposition.l4-18 All of these processes can be explained in terms of initial non-localized deposition of energy followed by rapid localization, as in protection and some instances of quenching, in an essentially inert region or molecule; as in other cases of quenching and in sensitized lumin- escence and decomposition, in a more responsive species.However, kinetic studies only limit the types of possible explanations of the phenomena observed. They do not delineate them. Thus, the phenomena of protection, sensitization and quenching are also explicable in terms of simple ionization transfer and electron transfer reactions involving adjacent molecules in liquids. Consider a radiation sensitive species C as the solvent (cyclohexane is typical), an " inert " quencher D, a scintillator molecule X, a chemically sensitive solute Y.For all of these cases it is possible to explain certain results (e.g., the extraordinarily high specific rates of " excitation transfer " and of quenching in cyclohexane solu- tions) by a single picture : initial non-localized deposition of energy in the system S generally, followed by localization preferentially in the additive-for reasons which are deter- mined by factors which are related to relative ionization potential, excitation potential, transition probability and concentration.19 Thus, we can write simply one pattern for each of the cases : D+S+D (2) Y -+S + Y* -+products, (4) S* + C+S + C?-+products, ( 5 ) s-s*, (1) S"+ { x+s +X*+kv (3) or, if no additives be present, where C? indicates that the nature of the excited species is such that decomposition (e.g., of cyclohexane) follows immediately.10 GENERAL INTRODUCTION On the other hand, the possibility of initial localized ionization of a C molecule is not excluded.For example, we consider the case where D has an electron affinity (as in 12) and the ionization potential of D is less than that of C : C-+C+ + e D+e+D- C+ + D+C+ Df (8) D-+D+---+2D (9) and protection ensues. Such a mechanism seems to account only for " initially ionized species ". However, what about the excited molecules? These can be disposed of by another semantic device : we write that the initial excitation involves an adjacent molecule (or, if pressed by considerations of concentration, even a more remote one) In this way we may avoid the inconvenience of considering the mechanism of pro- tection of an excited species whose life is suspected not to exceed 10-13 sec.Appropriate reactions of a similar sort may be written to " explain " sensitized luminescence (e.g., of p-terphenyl), or of quenching (e.g., by phenyl bromide), or of sensitized decomposition (e.g., of lead tetraphenyl). In this Conference, the actuality of processes of negative-ion formation in inter- action of high-energy radiation with liquids is shown by Hamill, Guarino, Ronayne arid Ward.20 Also, Allen and Humme121 examine pure liquid hexane, certainly not a typical negative-ion former. They conclude that the number of ions escaping initial recombination is presumably about 4 % of the number formed transiently. In our laboratory, Dillon and Rein22 have recently produced evidence of very limited electron trapping in pure 3-methylpentane glass near 77°K ; luminescence studies as a function of temperature indicate at least two trapping sites neither of which appears to be ionic.Introduction of bromobenzene, of oxygen or of cc14 (in the latter case in concentrations as low as 5 x 10-4 M) decreases the lumin- escence perhaps because of deeper trapping which may prevent the luminescence reaction ; the luminescence reaction is broadly described as radiative capture of an electron by a positive hole, 0. @ +e+M + hv. Experiments of this nature limit the diversity of mechanisms which may be properly suggested for the initial process; they do not delineate them. In particular, if (as appears in the present view of the Dillon-Rein experiments) the electron in- volved in reaction (1 1) is actually trapped, in an early stage, near the positive hole, it is difficult to understand the very high quenching efficiency of CC14 unless it in some way competes for the electron in an initial process; e.g., as if the reactions are C, D--+C++D-. (10) (1 1) S--+S* (1) (12) S* + C6H14, CC14- S + C6H& + negative ion and are followed ultimately by neutralization processes which may yield chemical products but certainly no light.The major point that can be made for the notion that non-localized deposition of excitation precedes localization at some trapping site (a single molecule, a pair of molecules or even a spur) is that this single mechanism can without strain explain so many of the phenomena of the radiation chemistry of condensed systems.OnM. BURTON 1 1 the other hand, in contradiction to that blithe and happy conclusion are two dis- turbing facts. First, except in radiation protection and particularly in such cases where high specific rates have actually been measured, only relatively small fractions of the total energy input are involved. Secondly, the mechanism is so beautifully general that it explains everything without commitment as to detail. 3. ELEMENTARY PROCESSES Recognition of the existence of possibly separate processes of energy deposition and of energy localization * is important to radiation chemistry, just as it is im- portant to recognize that excitation processes precede chemical processes and that there can be, and in many cases are, diffusion-controlled processes, forward pro- cesses, chain processes, back reactions, etc., all of which are recognizable as vague concepts and all of which require recognition and description in elaborate fashion if it is to be stated that the attendant radiation chemistry is understood.More succinctly put, a process is a vague term; it is not understood except if its elements can be described or if it can be expressed as a single elementary process the details of which can be examined in mathematically expressible language-i.e., if it can be dzscribed quantum mechanically. Thus, the term process is itself vague; it may be used to contain vast, unanalyzed (and perhaps unanalyzable) areas of ignorance. By contrast an elementary process is presumed to have characteristics sufficiently definable for precise description and for quantum-mechanical analysis. A legitimate, indeed an important, question concerning an elementary process is the possible existence of a criterion or criteria by which one recognizes that he is, in truth, considering an elementary process.One criterion which has been suc- cessfully used is that of chronology. If it can be separated in time from the attendant processes which one invokes to understand the observed effects, it is an elementary process. If two processes so invoked seem mutually involved in the same element of time there may be no purpose to their intellectual separation ; indeed, it may prove more profitable to consider both of them as part of one closely-linked process.For example, the concept of a spur is a frequently used model in the radiation chemistry of condensed systems. In the spur, ionization (or quasi-ionization) and excitation occur and result in a number of chemical processes which we customarily arrange and interpret in a time sequence. The time sequence, almost inevitably, provokes consideration of separate ion-molecule reactions, reactions of individual excited species unimolecularly or in metathetical reaction with each other or with other molecules, step-by-step free-radical reactions (with attendant distinctions be- tween hot and thermalized free radicals) and decompositions by rearrangement. Certainly each of these processes has been shown to occur in attenuated systems. Is the extrapolation to condensed systems necessary, useful or justified? This is the type of question which confronts this conference-if not overtly, at least by implication.3.1. PRIMARY PROCESSES The primary process in radiation chemistry customarily means something related to the initial chemical act-whatever that is. As an example, it is convenient to consider a well-recognized photochemical case rather than the more obscure * The time required for a presumed energy localization process is not known from simple prin- ciples. It depends on the mechanism of localization (e.g., triplet-triplet interaction, internal con- version, etc.) but it may be guessed that the time involved does not exceed -10-13 sec and is perhaps of the order of 10-15 sec.2312 GENERAL INTRODUCTION phenomena of radiation chemistry.In the mercury-photosensitized decomposition of hydrogen we deal with a reaction which may be profitably written Hg+2H HgH + H ~ s ~ i ) + & s ( H ~ H Z ) ~ In this case the existence of an intermediate species which decomposes to yield products (one of which may be unstable) is clearly recognized24-26 and step (a) may be considered as the primary process. However, when the photosensitization reaction involves an actual transfer of excitation, as in the benzene-photosensitized decomposition 27 of Pb(C6H&, C6H6(lB2~) + Pb(C6H5)4 C6H6 + Pb(C6H5); Pb(C6H5)*,4 Pb(C6H& + C6H5 step (c), which alone involves the sensitizer, is not profitably considered to be the primary chemical reaction. In the mercury-photosensitized decomposition of cyclohexane 28 we deal with a case in which little is known. We suspect that a re- action like (a) is indeed the primary chemical process but it is nut impossible that the reaction may be more akin to (c).In the chemistry of " radiosensitized " pro- cesses it is important to determine whether the sequence of processes is (a), (b) or (c), (d), and not which should be called " primary processes ". Perhaps, the prag- matic approach is best. The word '' primary '' may well be introduced (but only tentatively) only after facts are " known "-not when they yet remain to be established. In this connection some mention should be made of a related term, namely, "primary product". A reasonable practice would be to reserve the term for products of the decomposition or reaction of excited species.In the preceding illustrations, primary products would include HgH2, Pb(C&)3, and C6H5 but not Hg, H or HgH. Many other examples of possibly proper usage could be given. However, to avoid semantic difficulties it is advisable never to use the term except with mechanistic implication. For example, the fact that a product is present initially (i.e., measurably in the earliest detectable stage) does not justify the implication that it is a primary product (i.e., the product of a primary reaction). Consider the alternative reactions in liquid cyclohexane : C6Hi2----)*C6Hio+ H2 (1) Without any attempt to establish mechanism, it is nevertheless apparent that H2 produced in reaction (3) is not a primary product in the same sense as is the hydrogen of reaction (1)-although they are both initial products.3.2. SECONDARY PROCESSES A major difficulty in the definition of the primary (chemical) process as suggested is apparent in an attempt at definition of a secondary process. The temptation is to rely solely on chronology; i.e., a secondary process is one which succeeds a primary one. However, if it is claimed that the secondary process is the sole and inevitable consequence of the first under any set of conditions, there would appearM. BURTON 13 to be no pragmatic value in the definition. It seems reasonable to surmise that a process which invariably leads to a particular set of products is indeed one continuous process. From that viewpoint, only if a part of the process can be interrupted, pre- vented or redirected by a competitive process is there value in considering it as a separate process.From that same viewpoint a process is secondary in a real and experimentally useful sense if it both follows a primary process and is subject to modification (in yield per unit time) by intrusion of a competitive process. The difficulty with such legal hair-splitting is that it accepts defeat. We now know that intermediate products of a primary reaction (e.g., free radicals) can be detected in many cases by e.s.r. measurements without any effect on the observed course of reac~ion.29 In such case the existence of a secondary reaction may be made apparent and (3 good experimental, as well as speculative, basis may be laid for investigating the possibility of competing secondary reactions.Thus, it appears that techniques now in existence and others which may yet be contrived may cause us to modify our views of a primary process, to break it (mentally) into a succession of processes the separate existence of which may be establishable by experimental techniques which do not necessarily involve intro- duction of competing process. It appears that it may be easier to establish the existence of a secondary process, by destruction of the myth of a specific proposed primary process, than it is to estab- lish usefully a real primary process in any particular case. The difficulty involved is immediately evident. A completely described process has aspects of mechanism. A mechanism is never really known. It may not, at the moment, be disproved and in that sense it is a scientifically useful model.The goal of experiments is to extrapolate the model into a realm in which its predictions fail, to discover the cause of the failure, and to present an improved model. It is in such a sense and for such a reason that we are interested in the effect of en- vironment and of degree of aggregation, on the fundamental processes of radiation chemistry-whether they be primary or secondary. 4. MECHANISMS OF PROCESSES The remainder of this introductory statement is confined to a brief review of some of the mechanisms suggested for the processes outlined in the previous part. This presentation generally accepts the validity of the chronology suggested by Magee 30 regarding the various events involved.4.1. DEPOSITION OF ENERGY AS A PRIMARY PROCESS Our knowledge of the events involved in the deposition of energy derives essenti- ally from cloud-chamber and ionization-chamber studies and from " reasonable inference ". The more recent such studies of gases have contributed very precise knowledge of W, the energy required per ion-pair produced.31-33 Also, the detailed studies of mass spectrometric data have made possible accurate statements of the nature of those ions and, in some cases, even of the times of appearance relative to the times of initial excitation. On the other hand, the casual assumption that the excess energy W-I (where I is the ionization potential) is employed in molecular excitation-to states which are rarely clearly established-is neither supported nor contradicted by experimental evidence.In a connected sense, the generally ac- cepted notion that in gases the number of initially excited species is about 2 to 3 times as great as the number of ions is unsupported by adequate experiments. The presentation of Fueki and Magee has theoretical impact in regard to these matters.14 GENERAL INTRODUCTION However, it is interesting to observe to what extent experimental approaches which, in a certain degree, have aspects of “ bookkeeping ” also bear on these matters and help to clarify our views regarding these points. For example, in the presenta- tion by Beck 34 which relates to the decomposition processes in a mass spectrometer (at very low pressures), there is an indication that in propane and in butane about one neutral molecule (presumably an excited one) is decomposed per ion produced.Beck suggests that umny of these exsited molecnh clro “ supercxcitcd ” in the scmc that the term has been eixkployed by Platzinan.33 4.2. ION-MOLECULE REACTIONS Ever since the first papers on the subject by Stevenson,36 by Lampe, Field and Franklin,37 and by Hamill 38 and their colleagues, there has been a persistent effort to explain the phenomena of the radiation chemistry of gases in terms involving exclusively the behaviour of ions and the interaction of ions and excited molecules. Indeed, the literature of radiation chemistry reveals from time to time an effort to explain the phenomena of condensed systems on a similar basis-in spite of the fact that we have not yet clarified our views regarding the nature of ions in condensed systems.To a significant degree the presentation by Beck 34 in this Conference tends to stern such a trend by the mere device of pointing out the importance of purely non- ionic processes even in attenuated gases. Ausloos, Lias and Sandoval 39 disagree with Beck regarding the relative importance of neutral molecule decomposition in radiolysis of propane ; they present evidence that increased pressure affects the path of fragmentation and the nature of products presumably because of the increased probability of ion-molecule reactions. The suggestion by Clay, Johnson and Warman40 of the occurrence of ionic chain reactions in the oxidation of carbon monoxide is reminiscent of Garrison’s suggestion for the mechanism of the polymerization of acetylene.41 Statements of the nature of those presented in the previous paragraph tend to raise the question whether the phenomena of ionization and charge transfer are the only significant starting processes in radiation chemistry whether of gases or of liquids. The fact that Mellows and this author,42 as an example, showed that gamma-induced telomerization of chloroform by ethylene is free radical and non- ionic in its nature is countered by such work as that of Williams and his colleagues 43 in this Conference, who demonstrated that positive ions with acidic properties actually can participate in the radiation chemistry of liquid hydrocarbons. Of course, whenever a demonstration depends on the determination of the effect of an additive, there is always the question whether the effects are in reality artifacts which redirect the processes studied and which relate only to such redirection.This criticism bears commonly not only on attempts to establish ionic reactions but also on the earlier demonstrations of free radical chains by the introduction of free radical “scavengers”, the entire function of which was presumed to be the capture of free-radicals in unambiguous fashion. This view has been countered by demonstration that in many cases the so-called scavengers do not act in :he predicted fashion.44-46 In this Conference, the difficulties are again pointed up by Dyne, Denhartog and Smith 47 who show that in liquid hydrocarbons the mechanism of decrease of yield differs from that in gases.4.3. WATER A N D SOLVATED ELECTRONS The exciting report by Hart and Boag 489 49 of an absorption spectrum attributable to hydrated electrons has stimulated and redirected much of the recent work onM. BURTON 15 radiolysis of liquids.* In this Conference the Argonne group give detailed informa- tion on specific rates of reactions of solvated electrons 50 in water and show their contribution in ethanolic and methanolic systems 51 while Weiss and his colleagues 52 indicate some related theoretical generalizations. Hughes and Willis 53 present some additional G values in aqueous systems. Baxendale and Gilbert 54 address themselves to a fundamental problem, the primary yield of water decomposition. Specifically, they question the oft-quoted Firestone value 55 of G(-H20) = 11.7 and, on the basis of new experiments, suggest a lower value. These data, as well as their new contributions in the case of organic vapours, are certain to affect our more general views regarding radiolysis not only of vapours but of liquids.4.4. FREE RADICAL REACTIONS It is characteristic of this Conference that, unlike many of the older ones, there is but little preoccupation with free radicals as such. Free radicals are not ignored and their role is repeatedly recognized. The comments and interpretations, however, become more involved, as in the studies of behaviour in solid matrices by Magat 56 and his school and as in the examination of the role of CH2 by Hummel,57 and very much more precise, as in the e.s.r. approach of Fessenden and Schuler.29 5.EXCITATION TRANSFER In $ 2 of this presentation, attention is addressed to distinction between energy deposition and energy localization. Localization is itself a type of transfer and ultimately the mechanism or mechanisms of such localization must be examined. The present consideration involves matters susceptible perhaps to more direct theoretical and experimental approach. We consider the situation subsequent to localization of the energy in a particular molecule or group of molecules and ask how the energy is transferred so that it becomes effective for the decomposition or chemical conversion of some other molecule. An extreme position, which may be adopted for the purpose of such analysis, is that the only type of energy deposi- tion which actually occurs is confined to single molecules ; the question which follows is whether it is possible to present mechanisms for energy transfer compatible with experimental results in even the most extreme cases.An example of such a case under recent study in the Radiation Laboratory is the radiolysis of metal perphenyls in cyclohexane sohtion.58 Although the results are most conveniently explained on the basis of non-localized energy deposition followed by energy localization in the metal perphenyl molecule, the fact is that models may be contrived which involve primary deposition of energy in single cyclo- hexane molecules followed by subsequent transfer by some effective device to single metal perphenyl molecules. One suggestion is that ionization is transferred from single @&It2 ions to metal perphenyl, M& (and that the latter may be effective also in acceptance of free electrons); the suggestion is reminiscent of the first sug- gestions made to account for protection of cyclohexane by benzene.4-6 Some years ago, in order to account for variations in the behaviour of lumin- escent solutions containing benzene at very low concentrations as contrasted with high concentrations, Nosworthy, Magee and this author 59 suggested a concept of * It should be emphasized that observation of a solvated electron is important for establish- ment of mechanism of radiolysis of a liquid system.On the other hand, the character of the observation is such as to establish the existence of the solvated electron at a tine -10-6 sec after the initial act.The time at which the solvated electron appears and the degree and the nature of its participation in the early processes remain to be established.16 GENERAL INTRODUCTION domains of benzene (already shown to exist in thermodynamic and X-ray studies) in which excitation could move freely and be ultimately localized in a collision partner (either a scintillator or a quencher). Certain predicted decay-time effects were later observed by Dreeskamp and the author 60 and others,sl and later Magee made calculations on a model system (a chain of H l ions) in which an exciton state was demonstrated to move very freely23 Support for such views has been developed recently by Mullin62 who finds that results on quenching of luminescence in mixed benzene+cyclohexane solution can be explained on the basis of benzene domains which remain above a critical size at benzene concentrations down to 20 mole %.Results of Kropp and the author 1 3 , 6 3 also are satisfactorily interpreted on the basis of domain formation in benzene (as well as in cyclohexane) and Yguerabide and the author 64 have presented evidence for the existence of two-molecule domains of scintillator solute in benzene and in cyclohexane solutions. A theoretical examin- ation of diffusion-controlled processes in liquids by Yguerabide, Dillon and this author65 will make it possible to establish whether processes involving only single molecules of scintillator can at all account for the observed effects. Recently, Mullin has been studying the effect of temperature on such domains and is at- tempting the direct study of the size of excited domains as a function of temperature. An inconvenient aspect of the work reviewed is that relatively small fractions of the total energy available are indeed involved.Perhaps, recent studies of photosensit- ized decomposition of metal perphenyls in benzene (by Walmsley and Peterson),27 in which the seems clearly involved, may be free from such criticism. It should be emphasized that ad hoc invocation of the peculiar role of an (excited) triplet state does not, of itself, prove the occurrence of excitation transfer processes. In the discussion of the paper by Cundall and Griffiths66 on radiation-induced cis- trans isomerization of 2-butene, there will probably be some discussion of this point.Straightforward kinetic studies will ultimately help to resolve questions of mechanism, in the sense that they will restrict the number and type of mechanisms which can be invoked. Doubtless, also, a somewhat more precise picture of photo- sensitized reactions (as, for example, the formation of one or more chemical inter- mediates) will ultimately evolve. For this author, it is very refreshing to have at this Conference a paper by Collinson, Conlay and Dainton 67 in which they develop in a detailed way the need for energy transfer in benzene domains via exciton states, embracing entire domains, related to the state of the isolated benzene molecule. It has been suggested by Forster 63 that the times required for such exciton motion in benzene systems are inconveniently large.Two points are, however, worthy of special mention in this respect. The first is that the decay-time of an excited domain in pure benzene (e.g., in pure liquid benzene) has been shown by Dreeskamp and this author to be about 3 x 10-8 sec.69 The second is that the time of collision of a mole- cule of quencher or reactant with the domain is to be taken as something near the relaxation time, i.e., - 10-11 sec. The latter (relatively long time as compared with the vibration period) is then the time available for movement of the excitation (in a domain) to the colliding partner. Thus, the demands which kinetic requirements may make on exciton mobility appear not too severe. Contrasted with the very high mobilities calculated by Magee 23 for the system of excited HZ ions, the needs in this case appear modest indeed.An important point which emerges from this Conference is that elementary pro- cesses in the radiation chemistry of gases may be expected to differ markedly from those necessarily invoked in the radiation chemistry of liquids. In the next few years we may expect to be confronted with models for the behaviour of liquids and otherM. BURTON 17 condensed systems which are unorthodox indeed compared with our presently developed positions and also which take into account in very refined fashion the fundamental interaction of molecules (both unexcited and excited) in the liquid state. The Radiation Laboratory of the University of Notre Dame is operated under contract with the U.S.Atomic Energy Commission. 1 Samuel and Magee, J. Chem. Physics, 1953, 21, 1080. 2 Fano, Comparative Eflects of Radiation, chap. I1 (Burton, Kirby-Smith and Magee, ed.) 3 Fueki and Magee, this Discussion. 4 Manion and Burton, J. Physic. Chem., 1952, 56, 560. 5 Burton and Patrick, J. Chem. Physics, 1954, 58,421. 6 Hardwick, J. Physic. Chem., 1962, 66, 2132. 7 Burton, Berry and Lipsky, J . Chim. Physique, 1955, 52, 657. 8 Kallmann and Furst, Physic. Rev., 1950, 79, 857. 9 Kallmann and Furst, Physic. Rev., 1951, 81, 853. 10 Kallmann and Furst, Physic. Rev., 1952, 85, 816. 11 Brown, Furst and Kallmann, Luminescence of Organic and Inorganic Materials (Kallmann 12 Lipsky, Helman and Merklin, Luminescence of Organic and Inorganic Materials (Kallmann 13 Kropp and Burton, J. Chem.Physics, 1962, 37, 1752. 14 Dyne and Denhartog, Can. J. Chem., 1962, 40, 1616. 15 Oster and Kallmann, Nature, 1962, 194,1033. 16 Burton, Chang and Reddy, Radiation Res., 1958, 8, 203. 17 Krengnauz and Bagdasar’yan, Doklady Akad Nauk., U.S.S.R., 1957, 16, 817. 18 Arakawa, Walmsley, Peterson, Zeleznik, McCusker and Burton (Amer. Chem. SOC. Meeting, 19 Burton, Strahlentherapie, 1962, 51, 1. 20 Mamill, Guarino, Ronayne and Ward, this Discussion. 21 Allen and Hummel, A., this Discussion. 22 Dillon and Rein, forthcoming publication. 23 Magee, Comparative Eflects of Radiation (Burton, Kirby-Smith and Magee, ed.) (John Wdey 24 Rollefson and Burton, Photochemistry (Prentice-Hall, Inc., New York, 1946), pp. 268 ff. 25 Cario and Franck, 2. Physik, 1922, 11, 161. 26 Laidler, J.Chem. Physics, 1942, 10, 43. 27 Walmsley and Peterson, forthcoming publication. 28 Beck, Kniebs and Gunning, J. Chem. Physics, 1954, 22, 672. 29 Fessenden and Schuler, Mellon Institute, Radiation Research Laboratories, RRL-112, 1963. 30 Magee, Ann. Rev. Nucl. Sci., 1954, 3, 171. 31 Platzman, Int. J. Appl. Radiation Isotopes, 1961, 10, 16. 32 Jesse and Sadauskis, Physic. Rev., 1955, 97, 1668. 33 Weiss and Bernstein, Physic. Rev., 1955, 98, 1828. 34 Beck, this Discussion. 35 Platzman, Radiation Res., 1962, 17, 419. 36 Stevenson, J. Physic. Chem., 1957, 61, 1453. 37 Field, Franklin and Lampe, J. Amer. Chem. SOC., 1957, 79, 6127. 38 Meisels, Hamill and Williams, J. Physic. Chem., 1957, 61, 1456. 39 Ausloos, Lias and Sandoval, this Discussion. 40 Clay, Johnson and Warman, this Discussion. 41 Garrison, J. Chem. Physics, 1947, 15, 78. 42 Mellows and Burton, J. Physic. Chem., 1962, 66, 2164. 43 Busler, Martin and Williams, this Discussion. 44 Dyne and Denhartog, Can. J . Chem., 1963, 41, 1794. 45 Hamill and Nash, J. Physic. Chem., 1962, 66, 1097. 46 Chang, unpublished work in the Radiation Laboratory, results presented at various meetings. 47 Dyne, Denhartog and Smith, this Discussion. 48 Hart and Boag, J. Amer. Chem. SOC., 1962, 84,4090. (John Wiley and Sons, Inc., New York, and London, 1960). and Spruch, ed.) (John Wiley and Sons, Inc., New York, 1962), p. 100. and Spruch, ed.) (John Wiley and Sons, Inc., New York, 1962), pp. 83-99. New York, Sept., 1963). and Sons, Inc., New York and London, 1960).18 GENERAL INTRODUCTION 49 Boag and Hart, Nature, 1963,197,45. 50 Gordon, Hart, Matheson, Rabani and Thomas, this Discussion. 51 Taub, Sauer and Dorfman, this Discussion. 52 Scholes, Simic and Weiss, this Discussion. 53 Hughes and Willis, this Discussion. 54 Baxendale and Gilbert, this Discussion. 55 Firestone, J. Amer. Chem. SOC., 1957, 79, 5593. 56 Bensasson, Durup, Dworkin, Magat, Marx and Szwarc, this Discussion. 57 Hummel, this Discussion. 58 Arakawa, Walmsley, Peterson and Burton, forthcoming publication. 59 Nosworthy, Magee and Burton, J. Chem. Physics, 1961, 34, 83. 60 Burton and Dreeskamp, Disc. Faraday SOC., 1959,27,64. 61 Burton, Ghosh and Yguerabide, Radiation Res., 1960, suppl. 2,462. 62 Burton, Dillon, Mullin and Rein, J. Chem. Physics, in press. 63 Kropp and Burton, J. Chem. Physics, 1962, 37, 1742. 64 Yguerabide and Burton, J. Chem. Physics, 1962, 37, 1757. 65 Yguerabide, Dillon and Burton, J. Chem. Physics, in press. 65 Cundall and Griffiths, this Discussion. 67 Collinson, Conlay and Dainton, this Discussion. 68 Forster, Comparative Eflects of Radiation (Burton, Kirby-Smith and Magee, ed.) (John Wiley and Sons, Inc., New York and London, 1960). 69 Burton and Dreeskamp, 2. Elektrochem., 1960, 64, 165.

ISSN:0366-9033    

DOI:10.1039/DF9633600007

出版商:RSC    

年代:1963

数据来源: RSC

摘要:

Reactions in Tracks of High Energy Particles Radiolysis of Oxygen BY KENJI FUEKI AND JOHN L. MAGEE Dept. of Chemistry and the Radiation Laboratory, University of Notre Dame, Notre Dame, Indiana Received 4th June, 1963 An a priori calculation of the radiolysis of oxygen gas in the pressure range 10-3 to 100 atm has been made. In the low background region (1-100 atm) all track effects have been considered. The calculated G(03) values seem to be in reasonable agreement with experiment if only one excited oxygen molecule is initially formed per ion pair. Eff'ects of pressure, LET and dose rate have been discussed. The radiation-induced chain decomposition of 0 3 has not been considered. 1. INTRODUCTION The present study attempts a completely a priuri calculation of chemical effects in an irradiated system.Oxygen was chosen for several reasons. It is a gaseous system composed of one type of atom and there is a chemical product which can be measured, ozone. It is also of practical interest and its radiation chemistry has been investigated experimentally. Finally, enough data were available for reasonable choices of the necessary constants. Theoretical description of an irradiated system involves solution of a set of coupled inhomogeneous partial differential eqilations. Any reasonably complete set of such equations is too complicated for solution by means other than a digital computer. In the present case the Univac 1107 in the University of Notre Dame Computing Centre was used. The range of pressures * between 1 and 100 atni was chosen for study.Track effects are not expected at the lower pressures, but at the higher pressures clustering of ions and parent-ion recapture of electrons must be considered as well as in- creased bimolecular reactions of intermediates. When the calculations got under- way it was found that for all rates of irradiation such that isothermal conditions could be maintained there was no significant overlapping of tracks. This situation is usually called the " low background " condition and tracks can be treated individually. On the basis of the computer calculations a simplified scheme was found to be approximately valid. The radiolysis of 0 2 in the lower pressure range to 10-3 atm was investigated by analytical means. It was known before the calculations were started that many of the experimental results were contradictory, and it was thought that an a priuri calculation would be of value in elucidation of the mechanism.The radiation-induced decomposition of 0 3 was found to be particularly difficult to consider. It is a chain process known to be affected by many trace impurities in the system. In this paper we only con- sider the 0 2 system in which a negligible amount of O3 has been formed. * In the calculations the pressure does not occur explicitly. We have used the term " pressure " in this paper as the pressure of perfect gas with the actual density of the system. 1920 TRACK REACTIONS I N OXYGEN 2. REACTION MECHANISM, PHYSICAL AND CHEMICAL CONSTANTS (i) REACTION MECHANISM The mechanism of radiolysis of gaseous oxygen used here is given in table 1.Processes (a), (b), (c) and (d) are those directly induced by ionizing radiations or particles. Processes (e) and (f) show parent-ion recapture of electrons. Reactions (1)-(46) are secondary reactions, in which odd and even numbers represent the cor- responding forward and reverse reactions, respectively. In reactions (3 1)-(46), the TABLE REACTION MECHANISM. THE NUMBERS AND LETTERS IN PARENTHESIS GIVE THE number of 0 atoms and 0 2 molecules formed depends on the cluster size. As will be shown in the next section, the smallest cluster dominates over the pressure range in which the contribution of ions cannot be neglected. In neutralization reactions between small clustered ions and unclustered ions or electrons molecular dissociations are likely to occur. We have therefore assumed for n, i and j in the reaction scheme the following values : reaction n i i reaction n 1 j (311, (32) 1 2 1 (391, (40) 1 2 2 (331, (34) 1 3 0 (40, (42) (3% (34) 1 2 2 (43), (44) (37).(38) 1 3 1 (451, (44) 1 1 3 1 1 - - - 1 The temperature was taken to be 300°K. (ii) CLUSTER DISTRIBUTION The cluster distribution has been calculated using the Magee and Funabashi theory.1 The molecular constants required are obtained from ref. (2) and (3). The co-ordination number of the shell has been chosen to be 6. Fig. 1 shows the cluster distribution for 0$(0& (n = 0, 1, . . ., 6).4K , FUEKI AND J . L. MAGEE 21 (iii) DIFFUSION COEFFICIENTS The diffusion coefficients have been calculated from where D is the diffusion coefficient, N is the total number density of the gas, p is the reduced mass, k is the Boltzmann constant, T is the absolute temperature and Q d is the diffusion cross-section.Q d is obtained from ref. (3) and (5) for neutral -I \ / - log10 p (atm) FIG. 1 .-Cluster distribution as a function of pressure. species, given by the Langevin formula for ions 6 and is taken as 5 x 10-16 cm2 for electrons.7 The diffusion coefficient for 02 is assumed to be the same as that for 02. The diffusion coefficients are given in table 2. TABLE 2.-DIFFUSION COEFFICIENTS (Cm2 SeC-l) P (atm) D1 0 2 0 3 0 4 DS 1 3 . 0 ~ 10-1 2.1 x 10-1 1 . 7 ~ 10-1 9.0 x 10-2 7.3 x 10-2 10 3 . 0 ~ 10-2 2.1 x 10-2 1 . 7 ~ 10-2 9 . o ~ 10-3 7 . 3 ~ 10-3 100 3 . o ~ 10-3 2.1 x 10-3 1 . 7 ~ 10-3 9 .o ~ 10-4 7.3 x 10-4 P (atm) D6 Dl D8 D9 DlO 1 6.7 x 10-2 6 . 3 ~ 10-2 7.3 x 10-2 6 . 3 ~ 10-2 2 . 6 ~ 102 10 6 . 7 ~ 10-3 6-3 x 10-3 7-3 x 10-3 6.3 x 10-3 2 . 6 ~ 101 100 6 - 7 ~ 10-4 6.3 x 10-4 7.3 x 10-4 6.3 x 10-4 2-6 D1: 0, D2: oz, D3: 0 3 , 0 4 : o+, D5: o;, Dg: 0" (0&, D7: 0 f 2 ( 0 2 ) n , D8: 05, Dg : O;(O&, Dlo : e. (iv) RATE CONSTANTS REACTIONS BETWEEN NEUTRAL SPECIES.-The rate constants k19, kzl 8 and k23 9 are experimental values; kl3, kls and kl7 have been calculated by the gas kinetic theory assuming zero activation energy.1022 TRACK REACTIONS IN OXYGEN ION-MOLECULE AND CLUSTERING REACTIONS.-~ experimental value is avail- able for kll 11 ; k25, k27 and k29 have been calculated by use of the rate formula based on the ion-induced dipole interaction.TRONS.-The rate constants kl, k3, k7, kg, k31, k33, k35, k37, k39, k41, k43 and k45 have been obtained by applying the Thomson theory in the low-pressure region and the Langevin theory in the high-pressure region.12 ELECTRON ATTACHMENT.-The effective rate constant k5 has been estimated by use of the following expression based on the Bloch-Bradbury mechanism,139 14 NEUTRALIZATION REACTIONS BETWEEN POSITIVE IONS AND NEGATIVE IONS OR ELEC- k5 = aP/(l+P/P), (2.2) where P is pressure, a and p are constants.15 REVERSE REACTIONS.-AII the rate constants for reverse reactions have been cal- culated using the equilibrium constants. Since the rate constants which are not given in table 3 are extremely small, these constants may be neglected in the computation.TABLE 3.-blT CONSTANTS kll = 2 . 5 ~ 10-11 (cm3 sa-1) k12 = 1.5 x 10-37 (CIII~ s - 1 ) k13 = 2 . 2 ~ 10-10 (cm3 sec-1) kls = 6-3 x 10-32 (cm6 sec-1) k17 = 2.3 x 10-10 (cm3 sec-1) k19 = 3 . 3 ~ 10-14 (cm3 sec-1) k21 = 2.3 x 10-34 (cm6 sec-1) k23 = 2 . 7 ~ 10-33 (cm6 set-1) k25 = 7.4 x 10-10 (cm3 sec-1) k27 = 9 . 0 ~ 10-10 (cm3 sec-1) k29 = 7-4x 10-10 (cm3 sec-1) k14 = 2 . 4 ~ 10-17 (cm3 sec-1) k22 = 1.7 x 10-26 (cm3 sec-1) k26 = 6.2 x 1011 (sec-1) k28 = 2.3 X 1011 (SeC-1) k30 = 6 . 2 ~ 1011 (sec-1) (cm3 sec-1) P (atm) k7 k9 k35 =k39 k37 1 4.3x 10-6 5.1 x 10-6 3 . 8 ~ 10-6 4 . 0 ~ 10-6 100 1 . o ~ 10-7 1.1 x 10-7 9.5x 10-8 9.8x 10-8 P (atm) k4 1 k43 k4S 1 4 . 8 ~ 10-6 3 . 2 ~ 10-6 3-5x 10-6 2 . 8 ~ 10-8 100 1-1 x 10-7 8-9 x 10-8 9-1 x lo-* 2 . 8 ~ 10-6 10 1.ox 10-6 1.1 x 10-6 9 .5 ~ 10-7 9.8 x 10-7 ki = k3 = k3l = k33 10 1.1 x 10-6 8 . 9 ~ 10-7 9.1 x 10-7 2-sx 10-7 (V) INITIAL G-VALUES OF THE PRIMARY SPECIES An a priori calculation of the initial yields of ions and excited molecules was made using the assumption that the ions are formed in their ground states. AVERAGE IONIZATION ENERGY ion relative abundance in the appearance potential 16’ 17 mass spectrum of 0 2 (eV) O+ 0.83 12-2 0 3 0.17 18.9 Using these data we can estimate the average ionization energy as = 13.3 (ev).K . FUEKI A N D J . L. MAGEE 23 AVERAGE EXCITATION EmxY.-We assume that the average excitation energy Eex = 8.5 (ev). This energy corresponds to the wavelength at a maximum absorp- tion in the Schuman-Runge bands.18 The electron impact spectrum for oxygen is similar to the optical spectrum.19 average energy of sub-excitation electrons gs = 2-5 (eV) by use of the energy dis- tribution function in ref.(20). - AVERAGE ENERGY OF SUB-EXCITATION ELECTRONS.-We have estimated that the ENERGY ABSORBED IN THE EXCITATION PROCESSES PER ION PAIR : E,, = W-(E,+Es) = 16.4 (eV), where W is the total energy absorbed per ion pair, 32.2 (ev.21 NUMBER OF THE EXCITED MOLECULES FORMED PER ION PAIR : - N,, = E , J E , , ~ 2 . We assume that the probability for the dissociative processes (02-20) is four times higher than the non-dissociative processes (02*0*). Then, the number of the oxygen atoms formed per ion pair = 2 ~ 0 . 8 ~ Nex = 3.2, and the number of the excited oxygen molecules (in stable states) formed per ion pair = 0.2 x Nex = 0.4.NUMBER OF THE PRIMARY SPECIES FORMED PER 100 (eV> OF ENERGY ABSORBED : g ( 0 : ) = 0.83 x 1OO/W = 2.6, g ( 0 ' ) = 0.17 x lOO/W = 0.52, g ( 0 ) = (3-2+0.17) x lOO/W = 10.4, g ( 0 ; ) = 0.4 x lOo/W = 1.2, g ( e ) = g(Oi)+g(O+) = 3-12. INITIAL G-VALUES OF TEE PRIMARY s~~cr~s.-Taking parent-ion recapture of electrons into account, we get the initial G-values of the primary species by use of the following relations : where <Ep) is the average escape probability for sub-excitation electrons and obtained from ref. (20). The initial G-values of the primary species are shown in table 4. TABLE IN INITIAL G-VALUES OF THE PRIMARY SPECIES P (atm) G"(e) G"(0:) G"(O+) G"(O$) G"(O) 1 3.12 2.6 0.52 1 *2 10.4 10 3.00 2.5 0-50 1.2 10.6 100 1-32 1.1 0.22 1.2 13.724 TRACK REACTIONS I N OXYGEN 3.MATHEMATICAL TREATMENT OF THE REACTIONS (i) D I F F u s I o N - KINE TI c E Q u AT I o N s In the present treatment we have eleven species and the same number of diffusion- kinetic equations. However, at the early stage of the reactions, we may regard the concentration of the normal oxygen molecule as constant. Thus, the number of coupled differential equations is reduced to ten. The general form of diffusion- kinetic equations is dCi(r, t)l& = Div2Ci - kiCi -CkijCiCj +Ck,Cl + kmnCmCn. (3.1) i 1 m, n In the equations Ci is the concentration of the ith species, Di is the diffusion co- efficient, ki and kl are the rate constants for the first-order reactions and k, and kmn are the rate constants for the second-order reactions.(ii) IN IT I A L D I s TRI B u T I o N s distribution in the radial direction. The initial distributions are defined by We have used an axially homogeneous, cylindrical track model and a Gaussian Ci(r, 0) = (NPlxrg) exp (- r2/rt) for heavy species, Ce(r, 0) = (NO,/nrZ) exp (- r2/r:) for electrons. (3.2) NP is the initial number of the ith primary species formed per unit track length and given by NP = G"(i) x (dE/dx)/100, (3.3) where dE/dx is the stopping power for a-particles in units of eVcm-1, obtained from ref. (21). The effective initial radius for heavy species (atoms, molecules and ions) is given by where P is pressure in units of atm and ro is taken to be consistent with the Samuel- Magee value used in the radiolysis of water.22 This value has been used in many later calculations of aqueous systerns.23 The effective initial radius for electrons has been estimated as follows.The thermalization process of sub-excitation electrons is described by 1-0 = 10-4/P (cm), (3.4) -d&/dt = AYE, (3.51 where E is the energy of electrons, 1 is the fraction of energy lost per collision, v is the collision rate and expressed in terms of the cross-section 6, v = ON J2~lm; (3.6) m is the mass of an electron. Assuming o and A to be constant, we can integrate eqn. (3.5) and a measure of the distance of electron travel is given by Taking cr = 2 x IQ-15 cm2 and A = 3 x 10-39 24 we get re = 5 x P (cm) = Sr,. (3.8)K . FUEKI AND J . L. MAGEE 25 (iii) CELL BOUNDARY AND TRACK LIFETIME the '' cell model " : 25 We can define a cell boundary rm and a track lifetime tm based on the concept of r m = JDotm, (3.9) (3.10) where DO is a diffusion coefficient which is of the same order of magnitude as those of heavy species, p is the density of the system and I is dose rate.The track lifetime for electrons is given by fern = JDOIDetrn. (3.11) rm and tm are given in table 5. TABLE 5 . x E L L BOUNDARY AND TRACK LIFETUlE rrn (cm) dose rate (rad/sec) P (atm) 1 102 104 1Oa 1 2.9 x 10-2 9.1 x 10-3 2 . 9 ~ 10-3 9.1 x 10-4 10 1 . 6 ~ 10-2 5.1 x 10-3 1 . 6 ~ 10-3 5.1 x 10-4 100 9 1 x 10-3 2 9 x 10-3 9.1 x 10-4 2.9 x 10-4 dose rate (radlsec) P (atm) 1 102 104 106 1 8.3 x 10-3 8.3 x 10-4 8 . 3 ~ 10-5 8.4 x 10-6 10 2 . 6 ~ 10-2 2 . 6 ~ 10-3 2.6 x 10-4 2 . 6 ~ 10-5 100 8.3 x 10-2 8 . 3 ~ 10-3 8.3 x 10-4 8.3 x 10-5 The energy of a-particles is 2 MeV.DO = 10-1/P (cm2 sec-1) ; P, atm. (iv) DIMENSION LE s s D I FF u s I o N - KI NET I c E Q u A TI o N s Let D*, r* and C* be constants having the dimension of a diffusion coefficient, a length and a concentration, respectively. We introduce the dimensionless quantities D', r', Cf, t', k: and kb defined by 23 0: = Di/D*, rt = r/r*, Cf(r', t') = Ci(r*r; r*2t'/D*)/C*, t' = D*t/r*2, kf = r*2ki/D*, = r*2C*kij/o*. (3.12) In terms of these dimensionless quantities, the diffusion-kinetic equations (3.1) where v'2 is the Eaplacian operator for the dimensionless space co-ordinates. We have chosen the arbitrary constants D*, r* and C* as being D* = Dol r* = rol J2 and C* = z.N;/nr:. Eqn. (3.13) are solved numerically by standard methods of finite differences.23 I26 TRACK REACTIONS IN OXYGEN 4.RESULTS AND DISCUSSION (i) c o M P E T I T I v E R E A c T I o N s The changes in the number of the various species per unit track length with time for three different pressures are shown in fig. 2, 3 and 4. For convenience, 5 4 3 r l *.. % 2 I I 2 tog10 9- FIG. 3.-Change in the number of the various species per unit track length N ( t ) with time ; P = 10 atm. Ordinate is in arbitrary units. we define a unit of time by z = lO-lO/P (sec), where P is given in atm. The develop- ment of a track in time is similar at various pressures in this unit, which is the same order of magnitude as the molecular collision time.K. FUEKI AND J . L. MAGEE 27 REACTIONS OF THE POSITIVE IONS The most rapid reactions are the clustering reactions which are almost complete at loglo 2-0.The proportion of clustered ions depends on pressure. Clustered ions are not formed in appreciable amounts at 1 atm, but become significant at 10 atm and dominate among the positive ions at 100 atm. The number of the 0' ions decreases by ion-molecule reaction, forming 0; ions in a reaction which is complete at loglo N 2. The neutralization reactions between the positive and negative species are not so fast and most of the positive ions survive at loglo 2-2. REACTIONS OF THE NEGATIVE SPECIES The 0, ions are formed by electron attachment to 0 2 and then clustering re- actions occur. Therefore, the clustered negative ions are not formed as fast as the clustered positive ions. The number of negative ions reaches a maximum at log10 2-2. Most of the electrons disappear by that time.REACTIONS OF THE NEUTRAL SPECIES The excited molecules 0; react with 0 2 forming 8 and 0 3 in a reaction in competition with deactivation by collision. The relative importance of these reactions depends on pressure. The formation of 0 3 at the initial stage is ascribed log10 7 FIG. 4.-Change in the number of the various species per unit track length N ( t ) with time; P = 100 atm. Ordinate is in arbitrary units. only to the reaction of 0;. The reaction rate of 0 with 0 2 to form 0 3 is also dependent on pressure and this reaction does not appreciably occur at 1 and 10 atm in the time scale of fig. 2 and 3, but is well underway at the correspond- ing time at 100 atm as shown in fig.4. The number of the oxygen atoms depends upon several reactions and varies only slightly in the time scale of the figures. SPATIAL DISTRIBUTION The variation in concentration of the various species with radial distance in a track at the scale times loglo 7- 1 and 2 is shown in fig. 5-10. The distribution of the positive ions is similar to that of the neutral species, but the distribution of the negative species is much broader. This broad distribution is due to the rapid diffusion of electrons. It should be noted that at higher pressures a depression28 TRACK REACTIONS IN OXYGEN in the concentration of negative ions occurs in the central part of the track. This is interpreted in terms of the competition between the neutralization reaction and diffusion of the negative ions.(ii) G-VALUE FOR OZONE FORMATION The computer calculations gave a complete description of the fast reactions of the initially formed intermediates. After a time of about 100 scale times in FIG. 5.Variation of the concentration C(r, t ) with radial distance r ; a : 0 3 , -; 0+, -.-a P = 1 atm loglo 7-1. Ordinate is in arbitrary units. 2 n *r T 5 l r FIG. 6,Variation of the concentration C(r,t) with radial distance r ; P = 1 atm loglo 7-2. Ordinate is in arbitrary units. each case the reaction system had a simple behaviour. The most important inter- mediate remaining was the oxygen atom and it was reacting slowly with 0 2 to form 0 3 . The ultimate fate of all species seemed to be clear and so a simplified scheme for finishing the calculations without using the computer was devised.K.FUEKI AND J . L. MAGEE 29 LOW BACKGROUND REGION After the fast reactions are over, the possible reactions of the oxygen atoms are Reaction (19) is not important in low ozone concentrations (19), (21) and (23). 5 r FIG. 7.-Variation of the concentration C(r, t ) with radial distance r ; P = 10 atm loglo 7% 1. Ordinate is in arbitrary units. a: 0 3 - ; O$,--.--. (upper) ; 02(02)n---- (lower). 2 n z c I 5 --- - - __ r0 r FIG. 8.-Variation of the concentration C(r, t) with radial distance r ; a: 0 3 , - ; O,t,--.-. (upper) ; WW, -.-. (lower). P = 10 atm loglo 7632. Ordinate is in arbitrary units. and since we are interested only in the initial yield, i.e., the G-value at negligible 0 3 concentration, we consider only reactions (21) and (23), 0 + 2 0 2 - 0 3 4- 0 2 (21) 2 0 + op202.(23) Let the effective rate constants for the reactions (21) and (23) be kl and k2, respectively, and the number of the oxygen atoms and the ozone molecules per30 TRACK REACTIONS IN OXYGEN unit track length be Nl and Nz respectively. The rate equations for N I and N2 based on the " sharp boundary model " 25 are expressed by dN,/dt = - E1N1-2E,N;, V, dNJdt = EINl, where Y is the track volume per unit track length. 6 5 4 n 8 . : 3 2 I 0 r FIG. 9.Variation of the concentration C(r, t ) with radial distance r ; a : 0 3 , - ; 0$(0~)~, ---0 (upper) ; 05, -.-. (lower). P = 100 atrn loglo T M ~ . Ordinate is in arbitrary units. I ----__ --------____ TO r FIG. 10.-Variation of the concentration C(r, t ) with radial distance r ; P = 100 atm loglo TM2.Ordinate is in arbitrary units. a : 0 3 , - ; QZ ( 0 2 ) n , -.-.K. FUEKI AND J . L. MAGEE The solution for N2 in eqn. (4.1) is given by N2(t) = ni+ni/' El exp (-klt')/(l+4(t'))dt; 0 31 (4.2) #(t) = 2k,Ni[' exp (- kit')/( V, + nL)t')dt, 0 where Vo is the initial track volume, D is the diffusion coefficient for the oxygen aton and %? and %; are the initial number of the oxygen atoms and the ozone molecules, respectively. In the low background case, we may define the G-value for ozone formation by where Ea3 is the energy absorbed per unit track length in the unit of eV-1. If #(t) < 1, we get G(03)- N2(t+ co) x 100/E,,, (4-3) N2(t + 00) 21 Ni +IT;( 1 - +), t / ~ = - (2E2Ni/7tD) exp (2E1 Vo/nD)Ei( - 21t2 VJnD), (4.4) where -Ei(-2%2Vo/nD) is the exponential integral.Nr and &' are given by Ni 21 Nf(0) + Nf(0,') +Nf(O,) R-Nf(O3). Nf(i) is the number of the ith species at the end of the computer calculation. Putting numerical values in eqn. (4.3) and (4.4), we can estimate G(O3). The upper curve shown in fig. 11 from loglo P = 0 to loglo P = 2 was obtained this way. The G-values deciease through NT and n; with increasing pressure. This ten- dency is due partly to the deactivation of the excited oxygen molecules and partly to the clustering of ions. Let us consider the effects of pressure and LET on 4 in eqn. (4.2) or $ in eqn. (4.4). In general, 4 and II/ increase with increasing pressure and LET. Under our conditions ~- 10-2 at 100 atm and the reaction (23) does not effectively com- pete with the reaction (21) up to 100atm.However, it is probable that at higher pressures and in liquid oxygen the contribution of the reaction (23) becomes ap- preciable and as a result G(03) decreases. Since LET of fission fragments is about ten times higher than that in the present case, we get $ - 10-1 at 100 atm for fission fragments. Accordingly, we may expect that G(03) for fission fragments is lower than that for a-rays. Although the experimental values of G(O3) are scattered over a wide range, the most reliable values seem to be 9-10 for y-rays 26, 27 and -6 for a-rays.28 Our G-values obtained by the a priori calculation described here are too high compared to these values. The absolute G-value is related to the initial yields of the primary species.Since we have assumed that the primary ions are formed in their ground states in estimating the initial yields of the primary species, we regard the values obtained as maximum values. If electronic excitation of positive ions is taken into account, the average ionization energy is about 5 eV higher than that estimated assuming these ions are formed in their ground states.17 Therefore, two excited molecules per ion pair is certainly an over-estimate. The calculation of G(03) has also been made, assuming one excited molecule per ion pair. The results are shown in fig. 11 and the values obtained seem to be in better agreement with experiment.32 TRACK REACTIONS IN OXYGEN HIGH BACKGROUND REGION In the high background case, we may assume that all concentrations are homo- geneous in space.Then, the equations which express the reactions of the oxygen atoms and the formation of ozone are given by dN,/dt = i'G1 -E1N1-2E2NT, (4.5) dNJdt = I'G2 -I- klN1, where G1 = G"(O)+ G"(0;)+2(G0(O~)+ Go(O+)), G2 = G"(O;), and I' is the dose rate in the unit of 100 eV cm-3 sec-1. curves 1-4, two excited oxygen curves 1'-4', one excited molecules per ion pair. oxygen molecule per ion pair. and 105 rad/sec. 1, 1': dose rate between one 2, 2' : dose rate 106 radlsec. 3, 3' : dose rate 107 rad/sec. 4,4': dose rate 108 radlsec. -3 -z - I W I log10 p (atm) FIG. 1 1.-Dependence of G(O3) on pressure and dose rate. Putting dNl/dt = 0 for steady irradiation, we get G ( 0 , ) = (l/l')dN2/dt = G2 + kl( Jk: + 8EJfG1 - k1)/4721f.If and if I'G Ef/8E,G1, G(O& G1 + G2, I'g k?/Sk2G,, G ( 0 , ) - tE1JGI/2k,If + Gz- For low dose rates, G(03) is independent of pressure and dose rate. For high dose rates, G(03) increases with pressure and decreases with dose rate, since kl is pro- portional to the square of pressure and k 2 is directly proportional to pressure. TheK . FUEKI AND J . L . MAGEE 33 dependence of G(03) on pressure and dose rate has been calculated for the range lO-3<P<lO-1 atm and is shown in fig. 11. The upper curves assume 2 excited molecules of 0 2 per ion pair and the lower curves 1 excited 0 2 per ion pair. (iii) STATUS OF a p r i o r i CALCULATIONS The calculations presented here depend strongly on the yields of the primarily formed intermediates. It would seem that the mechanism we have assumed is in satisfactory agreement with experiment for initial G values of 0 3 formation if only one excited molecule of 0 2 is formed per ion pair.On the other hand, our mechan- ism does not explain the chain decomposition of 0 3 induced by radiation. It has been suggested that this decomposition results from small concentrations of im- purities usually present. It may also be that some important intermediate or re- action of the 02-03 system itself has been omitted. It is possible that ion-molecule reactions may be involved, for example, the reaction sequence, 0, +o,+o, +o, 0; + 03+ 0; + 2 0 2 , would decompose ozone in a chain mechanism. Both of these reactions are exo- thermic according to accepted experimental data. At pressures such that clustering occurs, a positive ion reaction sequence may also be possible.Further study is required to ascertain whether such reactions are sufficient to explain the ozone production in irradiated oxygen. The authors would like to acknowledge the assistance of Mr. Renolds Paberzs who wrote the programme and ran the computations described in this paper. They also appreciate co-operation of the Notre Dame Computing Centre staff. This paper is a contribution from the Radiation Laboratory of the University of Notre Dame and is operated under contract with the U.S. Atomic Energy Commission. 1 Magee and Funabashi, Radiation Res., 1959,10, 622. 2 Herzberg, Molecular Spectra and Molecular Structure. I. Spectra of Diatomic Molecules (D Van Nostrand Co., Inc., New York, 1950), p.558. 3 Hirschfelder, Curtiss and Bird, Molecular Theory of Gases and Liquids (John Wiley and Sons, Inc., New York, 1954), p. 1111. 4 Some of these clustered ions have been observed experimentally. Burch and Geballe, Physic. Rev., 1957, 106, 188. Brederlow, Ann. Physik, 1960, 5, 414. Hirschfelder and Eliasson, Ann. N. Y. Acad. Sci., 1957, 67, 451. 6 Bates, Atomic and Molecular Processes (Academic Press, New York, 1962), p. 652. 7 Bates, Atomic and Molecular Processes (Academic Press, New York, 1962), p. 363. 8 These values were taken from Brown and Wall, J. Physic. Chem., 1961, 65, 915. k21 used is of the same order of magnitude as other experimental values. Benson and Axworthy, J. Chem. Physics, 1957, 26, 1718. Elias, Ogryzlo and Schiff, Can.J. Chem., 1959, 37, 1680. Kretschmer and Peterson, J. Chem. Physics, 1960, 33, 948. k19 used is close to Phillips and Schiff’s value, J. Chem. Physics, 1962, 36, 1509, and larger than Benson and Axworthy’s value by one order of magnitude, J. Chem. Physics, 1957 26, 1718. 9 Reeves, Manneth and Harteck, J. Chem. Physics, 1960, 32, 632. k23 used is close to Morgan and Schiff’s value, J. Chem. Physics, 1963, 38, 1495. 10 see, for example, Laidler, The Chemical Kinetics of Excited States (The Clarendon Press, Oxford, 1955), p. 97. 11 Dickenson and Sayers, Proc. Physic. SOC., 1960, 76, 137. 12 Massey and Burhop, Electronic and Ionic Impact Phenomena (The Clarendon Press, Oxford, 13 Bloch and Bradbury, Physic. Rev., 1935,48, 689. 1952), p. 623. B34 TRACK REACTIONS IN OXYGEN 14 Hurst and Bortner, Physic. Reu., 1959, 114, 116. 15 We have used a = 3 . 4 ~ 108 and fi = 0-2. P is in atm. Meyerott, Landshoff and Magee, 16 Laidler and Gill, Trans. Faraday SOC., 1958,54,633. 17 Frost and McDowell, J. Amer. Chem. Soc., 1958,80,6183. 18 Ladenburg and van Voorhis, Physic. Reu., 1933, 43, 315. The observation by Watanabe et al. shows a maximum absorption at about 8-7 eV. Watanabe, Inn and Zelikoff, J. Chem. Physics, 1953, 21, 1026. Report, LMSD-48361, 1951. Phelps and Biondi, Report, AFSWC-TR-61-91, 1961. 19 Lassettre, Radiation Res., 1959, suppl. 1, 530. 20 ElKomoss and Magee, J. Chem. Physics, 1962,36,256. 21 Lind, Radiation Chemistry of Gases (Reinhold Publishing Corporation, New York, 1961) 22 Samuel and Magee, J. Chem. Physics, 1953,21, 1080. 23 Kuppermann and Belford, J. Chem. Physics, 1962,36, 1412. 24 SCX ref. (12), p. 279. Generally speaking, both CI and A are a slowly varying function of E. 25 Magee, J. Amer. Chem. SOC., 1951,73, 3270; J. chim. Physique, 1955,52, 528. 26 Kircher, McNulty, McFarling and Levy, Radiation Res., 1960, 13, 452. 27 Johnson and Warman, Session E-5-2 : Abstr. 2nd Znt. Radiation Res. Congr. (Harrogate, 28 see ref. (21), p. 282. p. 274. 1962).

ISSN:0366-9033    

DOI:10.1039/DF9633600019

出版商:RSC    

年代:1963

数据来源: RSC

摘要:

Radiolytic Exchange of Carbon44 between Carbon Monoxide and Carbon Dioxide BY D. A. DOMINEY AND T. F. PALMER (in part) Atomic Energy Research Establishment, Hamell, Didcot, Berkshire Received 4th June, 1963 The exchange of carbon-14 between labelled CO and CO;! of normal isotopic composition has been studied by using the Harwell nuclear reactor BEPO and a source of y-radiation. The measured G value for reactor radiation is 3.0 (f0-3) and for y-radiation 4.2 (f0.5). The rate of formation of 14CO2 is increased by the presence of oxygen and decreased by the presence of nitrogen dioxide. Analysis of these results shows that the mechanism of the reaction cannot be explained in terms of reactions of oxygen atoms. A mechanism is suggested for oxidation of CO to C02 by an active oxygen species which is produced as a result of a reaction between oxygen and C o t .The same process can be used to explain the stability of C02 under ionizing radiation. The exchange of carbon isotopes between carbon monoxide and carbon dioxide for the temperature range 700-900°C has been shownl-3 to be predominantly heterogeneous and to have an activation energy 2 of about 77 kcallmole. The rate of the reaction is negligible below 100°C but Stranks 4 has shown that the exchange occurs rapidly when a mixture of 14CO with COz of normal isotopic composition is irradiated with a-particles. The work reported here is concerned with a study of this exchange reaction induced by reactor radiation and by y radiation. The work was undertaken in an attempt to elucidate the mechanism of carbon dioxide radiolysis.EXPERIMENTAL MATERIALS The purification of carbon dioxide has been described elsewhere.5 Carbon-14 labelled carbon monoxide was supplied by the Radiochemical Centre, Amersham and was diluted with inactive carbon monoxide prepared by the reaction between degassed formic acid and concentrated sulphuric acid followed by distillation at liquid- nitrogen temperature. This gas was shown by gas chromatography to contain no detectable amount of oxygen (< 50 p.p.m.). Nitrogen dioxide was prepared by thermal decomposition of lead nitrate and purified by fractional distillation. Spec. pure oxygen (British Oxygen Co.) was used without further purification. GAS HANDLING The silica bulbs in which gases were irradiated had diaphragm-type break-seals at one end and several cm of capillary tubing at the other.The bulbs were heated in air to red heat and evacuated for several hours before being filled with gas. The volume of the bulbs was 13-15 cm3. The gases to be irradiated were allowed to mix for at least 16 h. Analysis of the mixture by gas chromatography showed that the concentration of oxygen was <50 p.p.m. The mixture was transferred to pairs of silica bulbs the pressure of gas being measured with a mercury manometer. A trap at - 78°C was interposed between the manometer and the bulb in an attempt to exclude mercury vapour from the bulb. In the later stages of the work the gases were handled in the complete absence of mercury, pressure being measured with a glass spiral gauge used as a null instrument with an external mercury manometer.The method of 3536 handling mixtures containing NO2 has been described previously.5 The bulbs were immersed in liquid nitrogen and sealed off as rapidly as possible. Several analyses of unirradiated samples showed that this technique was adequate to prevent formation of W02 during the sealing of the sample. The use of a Tesla spark coil in the vicinity of the samples was avoided since this caused exchange to 0 ~ c ~ r . 4 EXCHANGE BETWEEN CARBON MONOXIDE AND CARBON DIOXIDE ANALYSIS After irradiation the total amount of gas in each tube was determined and the total activity measured as described below. This was always equal to the total activity of the gas before irradiation within experiniental error.The CO2 was frozen and the amount of non-condensable gas measured. The composition of this gas was determined by gas chromatography using hydrogen as the carrier gas (20 cm3/min), a 5 ft column of Linde 5A molecular sieve at 110°C and thermistors as detectors. The C02 was purified by fractional distillation. The carbon-14 content of this CO2 was determined in one of two ways. Initially, the CO2 was absorbed in 1N sodium hydroxide solution and barium carbonate was precipitated by the addition of barium chloride solution. The barium carbonate was counted with a thin end-window G.-M. tube. In the later stages of the work gas scintillation counting was used.68 7 A Panax liquid scintillation counter was adapted for this work. IRRADIATIONS AND DOSIMETRY Samples were irradiated in the spent fuel element irradiation pond (T.I.G.pond) at Harwell or in the BEPO nuclear reactor. Irradiations in the T.I.G. pond were carried out at a dose rate of - 1 Mrad/h (9.4 x 1017 eV g-1 min-1 in C02) measured with a magnesium-walled, argon-filled ionization chamber calibrated against the Friclce dosimeter.8 The doses in CO2 were calculated from this measurement by correction for the difference in electron density between CO2 and 0.8 N sulphuric acid, it being assumed that the boundary correction is small because of the similarity in electron density (electrons per g) of Si02 and C02. The irradiations in BEPO were carried out in the pneumatic tube devices in experimental holes E3/2 and E313. Samples could be loaded and unloaded with the reactor operating.The temperature at which the irradiations were carried out varied from 60 to 90°C over the period of the work. Thermal neutron doses were monitored by measuring the activity developed in a cobalt wire (0.01 in. diam.) attached to the samples. The wires were counted against a standard with an end-window G.-M. tube or with a scintillation counter. The thermal neutron fluxes were calculated for a mean neutron velocity of 2200 msec-1 assuming a cross section of 33.2 barns for the reaction 59Co(n, y)6OCo. This cross-section includes a self-shielding correction which has been experimentally determined 9 for the wire used. The results of the work of Anderson and Waite,lo showed that there was a direct relationship between the thermal neutron dose rate, as monitored by gold foils, and the rate of energy absorption in graphite measured calorimetrically for a particular vertical experimental hole in BEPO.However, Linacre and his co-workers 9 have since shown that the same relation- ship between thermal neutron flux and energy absorption rate in graphite did not apply to the horizontal experimental hole E3/3 at the time of the work reported in this paper. This is probably the result of changes in the fuel loading pattern of the reactor over a long period and to the fact that the pattern of fuel elements around the irradiation positions is not the same in vertical and horizontal experimental holes. This finding emphasized the necessity for direct measurement of the energy absorption rate during the course of an experiment.Con- sequently, during the experiments carried out in E3/3 in the later stages of this work, a calori- meter measuring the rate of energy absorption in a graphite sample was placed alongside the irradiation position. At the position of irradiation the total dose in graphite consists of an 85 % y scattering component and a 15 % fast neutron scattering component.10 The y dose in CO2 is equal to the y dose in carbon, for the y spectrum present assuming the Bragg-Gray principle 11 applies to these bulbs. The fast neutron dose in CO2 is equal to 0.8 of the fast neutron dose in carbon, treating the GO2 as a free gas, i.e., ignoring the effects of recoil particles close to the silica wall. The total dose in C02 is therefore 0.97 of the total dose in graphite. During this period the ratio between the measured thermal neutron dose rate andD.A. DOMINEY AND T. F. PALMER 37 the rate of energy absorption in graphite was constant. This finding supports the view that the results of earlier experiments, in which a monitoring calorimeter was not used, canbe compared on the basis of the measured thermal neutron dose although an accurate absolute energy yield cannot be obtained from these particular results. RESULTS IRRADIATION OF 14co + C O ~ MIXTURES Samples of a mixture containing 1.43 (+O-03) % of carbon-14 labelled carbon monoxide in carbon dioxide of normal isotopic composition at about 30 cm pressure were irradiated in experimental hole E3/2 in BEPO for periods ranging from 4 to 64 h. In fig. 1 is shown a plot of log (1 - F ) against time, corrected to a thermal I 1 I 2 0 40 6 0 - time, h FIG.1 .-Illustrating the first-order character of the exchange reaction ; 0, mean of 5 measurements ; x , bulb packed with silica wool. neutron flux of 1.09 x 1012~~ cm-2 see-1. P is the fraction of the carbon-14 which is transferred from carbon monoxide to carbon dioxide. The straight line in this plot shows that the reaction obeys a first-order rate law up to at least 35 % conversion. Included in this figure is the result of an experiment in which the bulb was packed with about 1 g of silica wool which had been heated to about 1000°C in oxygen for 8 h before the bulb was filled with gas. The rate of the reaction in this experi- ment is in reasonable agreement with the rate obtained in unpacked bulbs, showing that the reaction is predominantly homogeneous under tlie conditions used in these experiments.The value of Q, the time taken to transfer half the carbon-14 from CO to C02, is 98.4 h for this mixture. In table 1 are given the results of three experiments in which samples of a mixture containing 0.82 % of W O in C02 were irradiated in E3/2 for 16 h. It can be seen from table 1 that the proportion of carbon-14 transferred from CO to C02 in a given time is independent of pressure. The rate of the exchange reaction was38 also measured for a mixture containing 5.65 % of 14CO in CO2. The value of Q was 418 h. In fig. 2, it is shown that 14 is proportional to the % CO in the mixture. EXCHANGE BETWEEN CARBON MONOXIDE AND CARBON DIOXIDE TABLE 1.-THF, EFFECT OF PRESSURE ON THE RATE OF REACTION total gas, mmole pressure, cm 0.289 49-8 0.184 30-5 0.1 10 18.9 Over the range of conditions used is converted to C02 and vice versa, is exchange reaction, % CO out % reaction M, h 0.82 16.0 63.6 0-84 15.7 64.6 0.83 16-1 63.4 in these experiments the rate R at which CO given by the standard expression for a simple [CO][CO,] 0.693 CC0l-f- CC02l T' R = The small isotope effect reported by Stranks 4 has been ignored in this calculation since its inclusion would have a much smaller effect on the result than the uncer- tainties in the measurements.This expression only applies if there is no change I 4 % co FIG. 2.-The relation between % CO and the half-life of the exchange reaction. in the overall composition of the gas during the experiment.For mixtures con- taining only CO and C02 there is no significant change in the composition of the gas during irradiation. Since 14 is proportional to [CO]/([CO]+[CO2]), R is directly proportional to the concentration of C02 and independent of the concen- tration of CO. This is consistent with the assumption that the rate-controlling step in the exchange reaction is the rate of decomposition of C02 following ab- sorption of energy in the C02. The G-value for the exchange reaction has been determined for reactor irradiation in three sets of experiments the results of which are given in table 2.D. A . DOMINEY AND T . F. PALMER 39 Set A was carried out with a mixture containing 1.43 % of 14CO in COz and set B with a mixture containing 1.21 % of W O prepared in a mercury-free apparatus.The mean of 5 determinations of the ratio between thermal neutron dose rate and the rate of energy absorption in graphite as measured by a calorimeter at the time of the experiments in set A was 2.01 (k0.03) x 10-12 mW g-1 per n cm-2 sec-1. TABLE 2.-G-VALUE OF THE EXCHANGE REACTION FOR REACTOR IRRADIATION thermal neutron R molecules rate of energy time, h dose rate, n cm-2 min-1 mmole-1 absorption, eV min-1 C sec-1 x 10-12 x 10-14 mmole-1 x 10-16 expt. % CO A1 A2 A3 A4 B1 B2 B3 B4 c 1 c 2 c 3 c 4 c 5 1 *43 1 -43 1 -43 1 -43 1.21 1.21 1-21 1.21 1 *43 1 -43 1 *43 1 -43 1 043 20 12 15.1 8 5.1 8.1 12.4 16.1 15-8 16 64 16 16 1.088 1-061 0-990 1 *090 1.038 0-992 1 -022 0.992 0.988 0.996 1.027 1 -090 1.100 9.9 10.39 10.85 8.93 11.70 10-96 9.88 9.95 9.43 10.81 10.29 11.85 12.86 3-62 3-58 3.38 3.58 3-60 3.35 3.36 3.34 3.35 3-37 3.47 3.69 3-75 average 2-7 2.9 3.2 2-5 3.3 3.3 2-9 3.0 2.8 3-2 3-0 3.2 3.4 3.0 (f0-3) The mean of 7 measurements of this ratio during set B was 2.07 (k0-07) x 10-12.The average value of this ratio, 2-05 x 10-12, was used to calculate the rates of energy absorption from the measured thermal neutron dose rates for the third set of experiments, set C, which had been carried out without a monitoring calorimeter. The average G value of the exchange reaction for reactor irradiation is 3.0 (k 0.3). The G value of the exchange reaction for y-irradiation has been determined in 2 sets of experiments using 1.21 and 1.43 % of 14CO in COz. The results of these experiments are given in tabfe 3.TABLE 3.-G-VALUE OF THE EXCHANGE REACI'ION FOR y IRRADIATION % CO total dose, Mrad G 1.21 10 3.6 1-21 15 4.2 1.21 17.5 4.3 1.21 20 4.8 1.21 12.5 4.5 1.21 22.5 4.7 1 *43 12.5 3.8 1-43 20 3.9 1 *43 10 3.5 1 -43 10 3.7 1.43 20 4.3 1 4 3 10 4.7 average 4-2 (f0.5) EFFECT OF OXYGEN A series of mixtures of 1-21 % of W O in CO;! containing 1500 p.p.m. of oxygen was irradiated in experimental hole E313 in BEPO for different periods of time. The results which are given in fig. 3 show that both WO2 formation and oxygen40 EXCHANGE BETWEEN CARBON MONOXIDE AND CARBON DIOXIDE disappearance are proportional to the energy absorbed in the CO2 for irradiation times up to at least 16 h. The rate of formation of 14CO2 corresponds to a G value of 5.0 (k0.2) based on energy absorption in C02 alone.The G value for the dose (eV molecule-1) FIG. 3.-The formation of W02 in the presence of oxygen. 0, % conversion of 14CO to 14CO2 ; x , concentration of oxygen. 1 I I I 2 % 0 2 FIG. 4.-The dependence of the rate of 14C02 formation on oxygen concentration. exchange in the absence of oxygen is 3.0 (50.3). Therefore the increased rate of WO2 production in the presence of oxygen corresponds to a G value of 2.0 (k 0.5). The rate of oxygen disappearance corresponds to a G value of 1.2 (A 0-2), i.e., ap- proximately half the G value for the increased rate of 14C02 production. ThisD . A . DOMINEY AND T. F . PALMER 41 suggests that the additional 14CO2 is produced by oxidation of W O by the additional molecular oxygen. After 16 h irradiation (0.06 eV/molecule) approximately half of the oxygen initially present (i.e., -700 p.p.m.) has been consumed but the rate of formation of W02 is unchanged.This suggests that the rate of production of W02 in the presence of oxygen is independent of the concentration of oxygen. This conclusion is supported by the results of experiments shown in fig. 4 in which the % of the total carbon-14 present in CO;! after irradiation for 16 h in experimental hole E3/3 of a mixture containing 1.43 % of W O in COa is plotted against the initial percentage of oxygen in the mixture. For mixtures containing between about 700p.p.m. and 1-8 % of oxygen the rate of reaction is independent of oxygen concentration. In cases where the initial concentration of oxygen was less than 700 p.p.m.the ir- radiation time was long enough for all the oxygen to be consumed. The measured value of the amount of exchange in these cases is consistent with the conclusion that until the oxygen is consumed the reaction proceeds at a rate which is independent of oxygen concentration after which the rate falls to that normally found in the absence of added molecular oxygen. EFFECT OF NITROGEN DIOXIDE In fig. 5 are shown the results of experiments in which mixtures of 1.43 % of 14CO in CO2 containing up to 8 % of NO2 were irradiated for 64 h in experimental hole E3/3. The rate of production of 14CO2 is inhibited by the presence of NO2 though the reaction still proceeds slowly even in the presence of 8 % of N02. DISCUSSION G-VALUE OF THE EXCHANGE REACTION There appears to be a significant difference between the G values of the exchange reaction for reactor irradiation and y-radiation although the maximum scatter in the two measurements overlap.The maximum scatter in each set of measurements is about 217 %. Some of this scatter is undoubtedly due to uncertainties in experimental measurements of amount of exchange and of energy absorption rates. Furthermore, the presence of trace quantities of oxygen which have a marked effect on the rate of production of 14CO2 cannot be excluded. That the difference between the calculated G values for the two radiation sources is real can be demonstrated by a comparison of the results for pairs of silica bulbs, filled under identical con- ditions with gas of the same composition, and irradiated one in BEPO and the other in the T.I.G.pond. These results are given in table 4. TABLE 4.-cOMPARISON OF G VALUES FOR BEPO AND THE T.1.G. POND G for BEPO G for T.I.G. pond ratio, GT.I.G./GBEpo 2.7 3.5 1 *3 2.9 3.9 1-3 3.2 3.8 1-2 "5.0 "6.9 1.4 * These mixtures contained 1500 p.p.m. of oxygen. A similar difference was previously reported 5 between G values for CO produc- tion by irradiation of CO2+N02 mixtures. In those experiments the gas mixtures irradiated in BEPO were not accompanied by a monitoring calorimeter, the energy absorption rates being calculated from the measured thermal neutron flux and the42 EXCHANGE BETWEEN CARBON MONOXIDE AND CARBON DIOXIDE best data then available relating this flux to the energy absorption rates previously measured calorimetrically.It now seems probable that during the period of the three experiments with CO2+ NO2 mixtures which were carried out in experimental hole E3/3, the ratio between the thermal neutron flux and the energy absorption rate in graphite was 2.05 x 10-12 mW g-1 per n/cm2 sec (which was the value deter- mined during the period of the experiments on the exchange reaction) and the re- calcuIated values of Gco are 3.1, 2.6 and 2.6, average 2-8 (+O-3). These values are still significantly below the reported value of 3.5 (k0.2) for y-radiation. The ratio, 3512.8 = 1-3, is close to the ratios given in table 4. % NO2 FIG. 5.-The effect of NO2 on the rate of formation of WO2. Both the rate of exchange of carbon-14 between 14CO and CO;! and the rate of formation of CO from mixtures of C02 and NO2 are dependent on the rate of absorption of energy in the C02 and it seems likely that the rate-controlling step in both reactions is the initial breakdown of the C02 molecule.For a particular radiation source the difference between the G value for the exchange reaction and that for CO production from mixtures of C02 and NO2 is probably not significant. MECHANISM OF THE EXCHANGE REACTION A simple mechanism for the exchange of carbon-14 between CO and COz involves the two steps C02-+CO + 0 kl (1) 14co + 0 - 4 4 ~ 0 ~ k2 (2) For the dose rate at which irradiations were carried out in BEPO, kl is 3 x 10-8 sec-1. At 80°C, the average temperature at which these irradiations were carried out, k2 is 4-4 x 10-17 cm3 molecule-1 sec-1.12 The alternative to reaction (2) as a fate for oxygen atoms is reaction (3).O+O+M*02+M k3 (3)D. A. DOMINEY AND T. F. PALMER 43 k3 is taken as 0.89 x 10-32 cm6 molecule-2 sec-1.13 By a stationary-state treatment of these three reactions for a mixture of 1-43 % of CO in CO:! at a total pressure of 30 cm, the stationary state concentration of oxygen atoms is about 1 x 1011 atoms cm-3. Thus, under these conditions it might be expected that the rate of reaction (2) would be about 700 times faster than that of reaction (3). This mechan- ism leads to the prediction that the rate of the exchange process will be reduced in the presence of oxygen scavengers. However, the results of experiments in which NO2 or oxygen were added as potential oxygen atom scavengers do not support this simple mechanism.NO;! scavenges oxygen atoms by reaction (4), the rate constant of N02+O+NO+ 0 2 k4 (4) which is 3.5 x 10-12 cm3 molecule-1 sec-1 at 8OOC.14 By a stationary-state treatment of reactions (l), (2) and (4), where R is the rate of formation of W 0 2 . In fig. 6, [C02]/R is plotted against [NO,]/[CO]. If the relationship is assumed to be linear, the value of the ratio k4/k2 is about 20. The true value of this ratio is about 105. Thus, NO2 is much less efficient as an inhibitor than is expected on the basis of a simple atomic mechanism. Oxygen atoms react with oxygen to form ozone k6 is 0.4 x 10-33 cm6 molecule-2 sec-1 15 and the rates of reactions (6) and (2) will be equal under our experimental conditions when the molecular oxygen concentration is about 200 p.p.m.However, it has been demonstrated that oxygen accelerates rather than inhibits the rate of formation of W02. The simple mechanism involving oxygen atoms cannot explain the results of the experi- ments with added oxygen-atom scavengers. However, the rate constants quoted are for the reactions of oxygen atoms in the ground state. No information is available on the rates of reactions involving excited oxygen 0 2 4 - 0 + M-+03+M k6 (6) atoms and a mechanism based on such species FIG. 6.-Test of the kinetic analysis of the rate of formation of WO2 in the might be devised. presence of NO2. A very small amount of oxygen increases the rate of formation of WOz to a maxi- mum of about twice the rate in the absence of oxygen.Though oxygen is consumed in the reaction the rate of formation of 14C02 is independent of the concentration of oxygen. Therefore the reaction whereby CO is oxidized to C02 does not occur as a result of energy absorbed directly by oxygen. If the energy required to bring about the reaction is initially absorbed by CO alone, the G value of the reaction would need to be about 150 suggesting a chain reaction. Alter- natively, energy initially absorbed by C02 may be subsequently transferred to oxygen.44 EXCHANGE BETWEEN CARBON MONOXIDE AND CARBON DIOXIDE Of the possible mechanisms whereby energy absorbed by CO2 can bring about activ- ation of oxygen, the reaction, co; + ope; + coz, (71 is known to be efficient at low pressure in a mass spectrometer.16 CO,' ions are produced during radiolysis of CO2. The energy necessary to pro- duce an ion pair in COz is 32-9 eV.17 At the dose rate used in these experiments (2.6 x 1013 eV cm-3 sec-1) the maximum rate of ion pair formation is 8 x 1011 cm-3 sec-1.The rate constant for the recombination of CO; and thermal electrons is probably 10-6-10-7 cm3 molecule-1 sec-1, 18 (8) When the rate of formation of CO$ equals its rate of destruction CO,' + e-+(COd)*CO + 0. k8[COl][e] = 8 x 10'' per cm3/sec, and hence the concentration of both CO,' and electrons (assumed equal) is about 2 x 109 cm-3. The rate of reaction (7) will exceed that of reaction (8) when Since the rate constant for an ion-molecule reaction such as reaction (7) is about 10-10 cm3 molecule-1 sec-1918 this will occur when the concentration of oxygen is greater than 1-10 p.p.m.It is suggested that either 0; itself or an active species produced as a result of its neutralization is capable of oxidizing CO rapidly to C02. O$ could be neutral- ized by an electron or by an 0, ion. The neutralization of 02 by an electron leads only to the production of oxygen atoms which cannot explain the increased rate of W02 production in the presence of oxygen since these atoms will react with oxygen much faster than with CO. The neutralization of 02 by 02 might be expected to lead to the production of a number of active species 19 and it is reason- able to assume that one of these is capabIe of oxidizing CO to C02. The inhibiting action of NO2 can be explained in terms of this mechanism. It must interfere with the oxidation either by competing with CO for the active oxygen species or by competing with 0 2 for the precursor of 02, the CO? ion.The ioniza- tion potential20 of NO2 is below that of both C02 and 0 2 and therefore charge transfer from either Cog or O$ to NO2 is energetically possible, though the cross- sections of such reactions are not known. RADIOLYSIS OF co2 C02 is stable under ionizing radiation for the conditions used in the present work. The steady state concentration of CO is about 5 p.p.m.21 The fact that exchange of carbon-14 occurs between 14CO and C02 under ionizing radiation without change of chemical composition shows that this stability is only apparent, C02 being reformed from the products of the initial decomposition. The proposed mechanism for the exchange reaction in the presence of oxygen suggests an alter- native to the explanation of Harteck and Dondes 22 for the stability of C02.The initial radiolysis of COa will produce both C0iF ions and excited states of C02. Sub- sequent reactions of these species lead to the formation of CO and 0 2 : O+ 0 +M-+O, + M.D . A . DOMINEY AND T. F . PALMER 45 It has been shown above that almost complete charge transfer from CO,’ to oxygen would be expected to occur at an oxygen concentration of about 10 p.p.m. : Therefore above this level the concentration of the active oxygen species produced as a result of neutralization of 0,’ will be independent of oxygen concentration. It is suggested that CO will be oxidized to C02 by this active species at a sufficiently high rate to explain the very small steady state concentration of CO in CO2 under radiation.21 The authors wish to thank Mr. J. Wright for his interest and most helpful dis- cussions, Dr. J. K. Linacre for many stimulating discussions on dosimetry and his colleagues for carrying out the calorimetry measurements ; also the U.K.A.E.A. for Research Fellowships. 1 Brandner and Urey, J. Chem. Physics, 1945, 13, 351. 2 Norris and Ruben, J. Chem. Physics, 1950, 18, 1595. 3 Hayakawa, Bull. Chem. SOC. Japan, 1953,26, 165. 4 Stranks, Proc. 2nd U.N. Conj: Peaceful Uses of Atomic Energy, A/CONF. 15/P/403, 1955. 5 Anderson, Best and Dominey, J. Chem. SOC., 1962, 3498. 6 Stranks, J. Sci. Instr., 1956, 33, 1. 7 Dominey and Danby, J. Chem. SOC., 1962,4656. 8 Clarke, Price and Rogers, Harwell Res. Group Report, AERE-R. 3665, 1961. 9 Linacre, unpublished work. 10 Anderson and Waite, Harwell Res. Groip Report, AERE-C/R. 2253, 1960. 11 Hine and Brownell, Rodiation Dosimetry (Academic Press, New York, 1956)) chap. 1. 12 Mahan and Solo, J. Chem. Physics, 1962,37, 2669. 13 Morgan, Elias and Schiff, J. Chem. Physics, 1960,33,930. 14 Kistiakowsky and Volpi, J. Chem. Physics, 1957, 27, 1141. 15 Benson and Axworthy, J. Chem. Physics, 1957, 26, 1718. 16 Lind, Radiatioiz Chemistry of Gases (ACS Monograph, Rienhold, New York, 1961), p. 196. 17 Whyte, Radiation Res., 1963, 18, 265. 18 Hart and Platzman, Mechanisms in Radiobiology (Academic Press, New York, 1961), chap. 2. 19 Burton and Magee, J. Amer. Chem. SOC., 1951, 73, 523. 20 Collin, Nature, 1962, 196, 373. 21 Davidge and Marsh, Harwell Res. Group Report, AERE-R. 3706, 1961. 22 Harteck and Dondes, J. Chem. Physics, 1955, 23, 902. Frost, Mak and McDowell, Can. J. Chem., 1962, 40, 1064.

ISSN:0366-9033    

DOI:10.1039/DF9633600035

出版商:RSC    

年代:1963

数据来源: RSC

摘要:

7-Ray-induced Oxidation of Carbon Monoxide : Evidence for an Ionic Chain-reaction BY P. G. CLAY, G. R. A. JOHNSON AND J. M. WARMAN School of Chemistry, The University, Newcastle-upon-Tyne, 1 Received 12th June, 1963 Irradiation of CO+O2 mixtures with Co-60 prays results in the formation of C02 by a chain- process, yields as high as G(C02).-80(10 being observed under certain conditions. The reaction is inhibited by low concentrations of Hg vapour, Xe, C02 and CH4. The yield of C02 has been measured as a function of total-dose, dose-rate, gas pressure and C0/02 ratio. A reaction mechanism is proposed, which is consistent with many of the experimental results ; this involves as intermediates ionic species, some of which are in excited states. Lind and Bardwell 1 showed that when pure CO was irradiated with Rn a-rays, C02 was formed together with a solid product, probably a carbon suboxide polymer.When CO+O2 mixtures were irradiated, C02 was the only product and the yields of this were higher than in the absence of 02, the maximum, from a 2 CO : 1 0 2 mixture, being G(C02) - 24 (M/N = 8.4). In a recent investigation of the Co-60 y-radiolysis of pure CO an initial G(C02) = 2.3f0.3 was obtained.2 In agreement with Lind and Bardwell, a marked effect of 0 2 was observed. The G(C02) values from CO+O2 mixtures suggested, that a chain-process was involved. We report here a detailed investigation of the mechanism of this reaction. EXPERIMENTAL CO was prepared from A.R. formic acid by reaction with acetic anhydride in the presence of concentrated sulphuric acid, the reagents being de-aerated before reaction.The gas was purified by distillation. No impurities could be detected in the final product by mass- spectrometric analysis; the C02 present was less than 10-4 mole %. 0 2 was prepared by heating previously de-aerated KMn04 and was distilled several times. Ar and CH4 (British Oxygen Gases) were obtained from cylinders, being condensed at -196°C and purified by distillation. Ne, Kr and Xe (British Oxygen Gases, research grade) were used without further purification. A CO-60 y-ray source of about 700 curies was used. The irradiation vessels (Pyrex, 400 ml) were fitted with a stopcock and a ground-glass joint for attachment to the vacuum line. 7 he irradiations were carried out at 20 f 1°C. For dosimetry, the nitrous-oxide gas-phase dosimeter was used, an initial G(NzWN2) = 125 being assumed.3 The dose-rate in an equholar CO+O2 mixture was -2x lO14eV/ mm Hg min in the 400 ml vessel.This was calculated from the measured dose in N20 assuming the energy absorbed per unit volume to be directly proportional to the electron density of the gas. The validity of this assumption under our experimental conditions was confirmed by comparing the ionization currents in N20, and in C0+02 mixtures, using an ionization chamber previously described2 It was also established that, for each of these gases, the dose-rate was directly proportional to the gas pressure. Before use, the irradiation vessels were washed with nitric acid and then several times with 3-times distilled water, dried, pumped to below 10-4 mm Hg and pre-irradiated for at least 24 h.In order to obtain reproducible results it was essential to exclude Hg vapour from the vessels. Before each experiment, the vessels were pumped to below 10-4 mm Hg, 46P. G . CLAY, G . R. A. JOHNSON AND J . M. WARMAN 47 via a special spiral trap at - 183"C, for at least 15 h. (The trap was constructed of a 4.5 m length of 4-5 mm Pyrex tubing bent into a spiral of 6 loops such that when the trap was introduced into a Dewar, each loop was half immersed in the coolant and half at room temperature. This design of trap was found to be the most efficient in removing traces of condensable vapours from a gas.) Gas mixtures were prepared either in the irradiation vessel, or in a glass bulb prior to admitting the nlixture to the vessel, by introducing the constituents in order of increasing partial pressure, the pressure being measured with a mercury manometer.It was arranged that at least 2 traps at - 183"C, in addition to the spiral trap, were between any source of Hg vapour (manometer, diffusion pump, etc.) and the irfadiation vessel. To ensure complete mixing of the gases, the vessel was cooled to - 196°C and warmed to room temperature several times before irradiation. The composition of the gas mixtures was checked by mass-spectrometric analysis of a sample after irradiation. For C02 analysis, the irradiated gas was pumped from the vessel through the spiral trap at - 183°C and the system pumped to 10-4 mm Hg for about 10 min. The trap was then warmed to 20°C and the condensed gas transferred, via two traps at -72"C, to a gas burette by means of a l-stage difl'usion pump and a Toepler pump.After pressure-volume measurement the gas composition was determined mass-spectrometrically. For each batch of CO, a blank determination showed the C02 present before irradiation to be negligible. For 0 3 determination, a known proportion of the irradiated gas was trapped in a flask containing 20 ml of de-aerated, frozen, aqueous K1 solution (pH 7, phosphate buffer). The frozen solution was melted and shaken to ensure complete reaction of 0 3 before measuring the iodine spectrophotometrically as 13 at 350 mp. RESULTS Except where otherwise stated, the product yields referred to are the total yields obtained in a 400 ml vessel, using a dose-rate of 2.1 x 1014 eV/mm Hg min in the total gas.The rate of product formation ( R moles min-1) and the G-value are related by G = 102RN/DP, where N is Avogadro's number, D the dose-rate in eV mm-1 min-1, and P is the total gas pressure in mm. No carbon containing products other than CO2 could be detected when the oxygen present exceeded 0.1 % of the CO+O2 mixture.2 Fig. 1 shows the formation of C02 from a CO+O2 mixture (02 = 47 mole %) at a total gas pressure of 760mm Hg. Under these conditions the yield of C02 was initially linear with radiation-time, G(C02) = 195. Similar linear yield-time plots were obtained for mixtures of other compositions ( 0 2 between 1 and 98 mole %) when the total pressure was 760mm. The dependence of the initial rate of C02 and of 0 3 formation on the % O2 at a total pressure of 760 mm is shown in fig.2. At a total pressure of 60 inm, the initial G(C02) was greater than that at 760 mm. Fig. 3 shows the yields of CO2 at 60 mm ( 0 2 = 47 "/u> as a function of irradiation time : the initial G(C02) N 8100. In the presence of added Hg vapour (- 10-3 mm) G(C02) was less than 100. At a total pressure of 60 mm, the rate of C02 formation from CO+O2 mixtures decreased with increasing irradiation time, a fall-off from the initial rate being observed at conversions of less than 5 x 10-2 mole % (cf. fig. 3). Addition of low concentrations of C02 (-10-1 mole %) to the mixture prior to irradiation resulted in a marked lowering of the initial rate of C02 formation. This suggests that the observed fall-off of the rate with time may be attributed to the presence of the radiation-produced C02.The dependence of the initial rate of C02 formation on the % 0 2 at a total pressure of 60mm is shown in fig. 4. Measurement of the initial rates at this pressure was less precise than at 760 mm because of the nonlinear yield-time plots.48 OXIDATION OF CARBON MONOXIDE The initial G(C02) was markedly decreased when small concentrations of Xe or CH4 were present (table 1 ) ; Kr, up to concentrations of at least 2 mole %, had no effect (fig. 3). irradiation time (min) FIG. 1.-Irradiation of C0+02 mixtures ( 0 2 = 47 mole %) at a total pressure of 760 rnm Hg. Dependence of C02 yield on irradiation-time : 6 dose-rate = 1.63 x 1017 eV min-1 (in 400 ml) ; 0 doserate = 5-4 x 1015 eV min-1 (in 400 ml).The irradiation times shown are for the higher dose- rate; the actual times for the lower dose-rate have been divided by 30, the ratio of the dose-rates. 0 2 (mole %) FIG. 2,Trradiation of C0+02 mixtures at 760 mm Hg. Dependence of the initial rate of CO2 formation (0) and of 0 3 formation (9) on mole % 0 2 . Curve is theoretical (see discussion). Using vessels packed with small glass tubes (2 mm diam., 2.5 em long), in which the surface/volume ratio was at least 10 times that of the vessels normally used, G(C02) was identical with that in the normal vessels when the total gas-pressure was 200 rnm ( 0 2 = 47 mole %). Some results obtained at 60 mm ( 0 2 = 47 moleP . G . CLAY, G . R. A . JOHNSON AND J . M.WARMAN 49 %) in the packed vessels are shown in fig. 3. Under these conditions, the yields of C02 in the packed vessels were slightly lower than in the unpacked vessels. It / O /’ I.*- .,/” d ///,: / O ’ J‘/. &Ao t f I I FIG. 3.-Irradiation of C0+02 mixtures ( 0 2 = 47 mole %) at a total pressure of 60mm Hg. Dependence of C02 yield on irradiation time : 0 dose rate = 1-26 x 1016 eV min-1 (in 400 ml) ; 8 dose rate = 4.20 x 1014 eV min-1 (in 400 ml) ; + irradiations in packed vessel ; x 2 mole % Kr present. The irradiation times shown are for the higher dose-rate; the actual times for the lower dose- rate have been divided by 30, the ratio of the dose-rates. I I 1 I 26 40 6 0 €30 0 2 (mole %) 30 FIG. 4.-Irradiation of C0+02 mixtures at 60 mm Hg. Dependence of the initial rate of C02 formation on mole % 0 2 ; curve is theoretical (see discussion).so OXIDATION OF CARBON MONOXIDE is not certain, however, that the small difference observed lies outside the experi- mental error.TABLE DEPENDENCE OF G(co2) FROM co+02 MIXTURES ON THE PARTLAC PRESSURE OF ADDED Xe AND CH4 PCO (mm) 30 30 30 380 380 380 30 PO2 (mm) 30 30 30 380 380 380 30 G(C0z) 8100 3600 4 0 195 87 41 4 0 A detailed investigation of the dependence of C02 yield on total pressure was carried out for a given CO+O2 mixture ( 0 2 = 47 mole %) ; the initial rate of COz formation is plotted against pressure in fig. 5. A similar dependence of the initial rate on the total pressure was observed for mixtures of different compositions ( 0 2 from 1 to 98 mole %). GO iII-_-- 5 0 0 1m l j o 3 2oou z500 total pressure (mm Hg) C02 formation on total gas pressure ; curve is theoretical (see discussion).FIG. 5.-Irradiation of COfOz mixtures (02 = 47 mole %). Dependence of the initial rate of Since the dose-rate is directly proportional to the gas pressure the observed de- crease of G(C02) with pressure may have been due to a dose-rate effect, rather than to the pressure change. This possibility was excluded by studying the dependence of the rate of C02 formation on dose-rate at constant total pressure and CO/O;? ratio. At total pressures of 760 and 60 mm, G(C02) was independent of dose-rate over at least a 30-fold range (7 x 1012 to 2-1 x 1014 eV mm-1 min-I), see fig. 1 and 3. The dependence of the rate of C02 formation on the partial pressure of one component of a CO+O2 mixture for a fixed partial pressure (30 mm) of the other component is shown in fig.6. The dependence on the partial pressure of Nz, Ar or Ne of the rate of COZ formation from a mixture containing CO (30 mm) and 0 2 (30 mm) is also shown in fig. 6.P. G. CLAY, G. R. A. JOHNSON AND J . M. WARMAN 51 DISCUSSION The occurrence of a chain-mechanism in this system is indicated by the high G(C02) values. Any postulated reaction scheme must be consistent with the follow- ing experimental facts : (a) G(C02) is independent of dose-rate over at least a 30-fold range, i.e., from 7 x 1012 to 2.1 x 1014 eV mm-1 min-1 (fig. 1 and 3). (b) The initial rate of C02 formation (d[COz)/dt) is dependent upon total gas pressure and upon the ratio C 0 / 0 2 (cf.fig. 2, 4, 5 and 6). X I I I J 2 0 0 400 6 0 0 800 PX (mm Hg) (where X = 0 2 , CO, N2, Ar or Ne) FIG. 6.-Irradiation of C 0 + 0 2 mixtures. Dependence of rate of C 0 2 formation : ( e) on p 0 2 withpCO = 30 mm; (0) on p C 0 with p 0 2 = 30 mm; (+) on pN2 ( x ) on pAr and (0) on pNe with p C 0 = p 0 2 = 30 mm ; curves are theoretical (see discussion) : A for p C 0 = 30 mm and varying p 0 2 ; B for p02 = 30 mm and varying pC0 ; C for p C 0 = p 0 2 = 30 mm and varying pN2. (c) G(C02) is markedly decreased when low concentrations of Hg vapour, C02, Xe or CH4 are present (table 1); small amounts of Kr, Ne, Ar or N2 do not affect G(C02). (d) The rate of C02 formation from a given CO+O2 mixture decreases with increasing partial pressure of added N2, but is not appreciably affected by added Ar or Ne (fig.6). The primary active species, which may be produced by radiation in this system, include excited CO and 0 2 molecules, 0 and C atoms, and ions. 0; and O+ are formed from 0 2 by electron impact and a large proportion of these are produced52 OXIDATION OF CARBON MONOXIDE in excited states.4 The ions 0; and 0- can result from electron capture. With CO, electron impact gives mainly CO+, possibly in excited states.5 From investigations of the combustion of CO and, in particular of the factors which influence the second explosion limit, it has been concluded that the thermal oxidation of CO involves 0 atoms as chain carriers (cf. ref. (6)). In CO + 0 2 mixtures there is competition for 0-atoms between CO and 0 2 : At 20°C, the rate-constants of these reactions have been reported to be k(1) = 3.5 x 105 1.mole-1 ~ec-1~7 and k(I1) = 1 x 108 1.2 mole-2 sec-1,8 i.e., for a C0+02 mixture (1 : 1) at 760 mm the rate of the reaction forming 0 3 is about 13 times that of the reaction of 0 atoms with CO. In the radiolysis, the rate of 0 3 formation is always much less than the rate of CO2 formation (cf. fig. 2). We conclude, there- fore, that 0 atoms are not involved as chain-carriers in the radiation-initiated oxidation of CO. Furthermore, the results of Groth 9 show that 0 atoms, produced in CO+O2 mixtures by the photolytic decomposition of 0 2 , kv o,-+ O( ”) + O( lo), (1470 A) do not initiate a chain-oxidation of CO. From a consideration of the kinetics of the radiolytic oxidation of CO, we propose the following reaction scheme, on the basis of which it is possible to account for many of the observed features of the reaction : initiation Ei COhlVIcO’ E2 02-0; CO’+ opp1+.. . o;+co-+pl+. * . k3 k4 inhi bit i on ks CO’ + CO + M+ products 0; + O2 -+products k6 propagation k7 p1+ CO-+p, + c02 P 2 + 02-+P1+ co2 ks termination kg p1 + CO +M-+products where GO’ and 0; represent the primary species from CO and 0 2 respectively, p1 and p2 the chain-carriers, and M (third body) is CO, 0 2 or N2 but not Ne or Ar.P. G . CLAY, G . R . A. JOHNSON AND J . M. WARMAN 53 Stationary-state treatment of this reaction sequence gives the following expression for the rate of COz formation : dCCO21- APO2 c p c o -- dt pM(1 +Bp021pCO)+pM(1 +DpCOpM/p02)’ where p02, p C 0 and pM are the partial pressures in mm of 0 2 , CO and M, and A = 2k7E2/k~,, B = k6/k4, C = 2k7El/k9, D = ks/k3.Under our conditions of vessel volume and dose-rate the best fit between the theoretical plots and the experi- mental points was obtained with A = 3.9, B = 7.8, C = 4.5 and D = 9.0 x 10-3 (fig. 2, 4, 5 and 6). The observation that certain gases (Hg, Xe, C02 and CH4), when present at low concentrations, can effectively inhibit the chain is most easily explained if it is assumed that at least one of the intermediates in the chain is a positive ion which can be removed by a bimolecular reaction with the inhibitor. In view of this, and of the evidence discussed above which appears to rule out an atom-chain mechanism, we shall consider in more detail the actual reactions involved in steps (1) to (9) of the scheme given above, assuming these to be ionic reactions. However, the par- ticipation of excited, rather than ionic, species cannot be ruled out unequivocally.Of the several ionic reactions conceivable for each of the steps (1) to (9), many may be excluded on energetic grounds and by the fact that the 0 3 yields are much lower than the CO2 yields. Thus, the 0’ ion cannot be a chain carrier since pub- lished data 14 show that the reaction o+ + 0 2 3 0; + 0 is fast and its occurrence would lead to 0 3 formation in amounts comparable with the COz and, also, the propagation reactions cannot lead to 0 atom formation. Bearing these limitations in mind, we tentatively postulate the following reactions : initiation COWO+ + e 02M.’02+(411y, or higher states) + e ( 2 4 co+ + 023co; ( 3 4 Oz(411,, or higher states)+CO+CO: ( 4 4 inhibition CO++CO+M-+C201 +M Of(’II,, or higher states)+O,+O,+ + 0 ( 6 4 co; +co+co2+(2n,?)+co, ( 7 4 propagation termination Co,’(2n,)+ o~--+o~+(~II,)+ co2 CO$+CO+M-+CO;?+fCO; + M The primary ion formation (reactions (la) and (2a)) i s assumed to be followed by the ion-molecule reactions (3a) and (4a) leading to an excited ion, CO;*.It is assumed that reaction (3a) involves the ground state CO+ (A.P. = 14 eV) ; the reactant in reaction (4a) is assumed to be an excited O;, which can be the O:4IIu (A.P. = 16-3 eV) and/or higher states, since initiation by the ground state 0; cannot be reconciled with the efficient termination by COz and by CH4.54 OXIDATION OF CARBON MONOXIDE There is no independent evidence available for reactions (3a) and (4a) ; however, a CO; ion has been detected as a product from CO;! in the mass-spectrometer.10 The propagation steps involve the Of exchange reaction (7a) and the charge- exchange (8a). The formation of an excited CO; in reaction (7a), and therefore of an excited 0: in reaction (Sa) is energetically feasible, the net processes consisting of reactions (3a) and (7a) and of reactions (4a) and (7a) being at least 2 eV exothermic.The proposed reactions are consistent with the observed kinetics only if we assume that the species produced in reaction (Sa) cannot be removed by reaction (6a). We suggest that, in the absence of substances which can terminate the chain when present at low concentrations such as Hg, Xe, CO;! and CH4, inhibition can occur by reactions (5a) and (6a).Reaction (5a) has been observed in the mass-spectro- meter with CO as a third body.11 The bimolecular reaction (6a) has been observed in the mass-specfrometer,10~ 12 where the reactant is the 02+21Ju (A.P. = 17.3 ev) and/or higher states. Jt is reasonable to assume, therefore, that the species produced in reaction @a), which, on kinetic grounds, cannot be removed by reaction (6a) is the 0,’(411U). For the chain-termination step we assume reaction (9a). The theoretical curves in fig. 6 were calculated assuming that the efficiencies of COY O2 and Nz as third-bodies in reactions (5a) and (9a) were in the ratio 1 : 1 : 0.25. Inhibition by Hg, C02, Xe, and CH4 presumably involves reaction with one or more of the species responsible for chain-initiation or propagation.For example, the charge-exchange reactions, CO+ +Xe+CO+Xe+ co+ +CO,+CO+COf, have been reported to occur with high efficiency.13 Such inhibition reactions would be in competition with reactions which occur in the absence of these inhibitors. Competition of this type is consistent with the observed dependence of G(C02) on the concentration of Xe (table 1) and was also demonstrated by the dependence of the rate of COz formation on CO;! concentration at Werent total pressures. The final fate of the positive ions must involve neutralization, probably by the negative ions 0- and 0, ; however, the ion-neutralization processes are unimportant in C02 formation compared with the propagation steps.Also, since the yield of COz is independent of dose-rate, chain-termination apparently does not involve ion-neutralization. The reactions occurring in this system lead to a complex situation, any inter- pretation of which must be speculative in the present state of our knowledge of reactions between ions and neutral molecules. The reaction scheme outlined, while being in agreement with many of the observed features of the system, does not fit the results in all respects, e.g., there is a discrepancy between the theoretical and observed rates of C02 formation at low 0 2 concentration (cf. fig. 2). Furthermore, the role of added rare gases is not fully understood. For example, Ar+ ions and Ne+ ions apparently cannot initiate chains even though their appearance potentials lie above the ionization potential of CO. Also, it might have been expected that at sufficiently high Ar concentrations, the postulated intermediates, 0,”(4nU) and C0,’(2IIu) would be removed by charge-exchange. Possibly, however, the pro- pagation reactions are sufficiently fast to compete favourabfy with the charge- exchange process. We thank Prof. J. J. Weiss for his help and encouragement. The mass-spectro- metric analyses were carried out by Mr. P. Kelly. We are indebted to A.E.R.E., Harwell, for financial support.P . G. CLAY, G . R . A. JOHNSON AND J . M. WARMAN 55 1 Lind and Bardwell, J. Amer. Chem. SOC., 1925,47,2685. 2 Clay, Johnson and Warman, to be published. 3 Johnson, J. lnorg. NucZ. Chem., 1962,24,461. 4 Frost and McDowell, J. Amer. Chem. SOC., 1958,80,6183. 5 Kaneko, J. Physic. Soc., Japan, 1961, 16, 1587. 6 Lewis and Von Elbe, Combustion, FZames and Explosions of Gases (New York and London, 7 Avramenko and Kolesnikova, Izvest. Akad. Nauk. S.S.S.R. Otdei. Khim. Nauk., 1959, 1562. 8 Kaufman, Progress in Reaction Kinetics (Pergamon Press, London, 1961), p. 20. 9 Froth, Z. physik. Chem. B, 1937, 37, 315. 10 Cermak and Herman, J. Chim. Physique, 1960, 57, 717. 11 Munson, Field and Franklin, J. Chem. Physics, 1962, 37, 1790. 12 Dbng and Cottin, J. Chim. Physique, 1960,57, 557. 13 Rudolph and Melton, unpublished results, quoted by Rudolph and Lind, J. Chem. Physics, 14 Dickinson and Sayers, Proc. Physic. Soc., 1960, 76, 137. 1961), 2nd ed., p. 71. 1960,32, 586 ; 33,705.

ISSN:0366-9033    

DOI:10.1039/DF9633600046

出版商:RSC    

年代:1963

数据来源: RSC

摘要:

Neutral Fragments From Hydrocarbons Under Electron Impact BY D. BECK Physics Laboratory, University of Freiburg, Germany Received 4th June, 1963 In the sequence of steps which have to be considered in a radiation chemical experiment, this paper contributes some information about the physicochemical stage of events immediately after the sample molecule has been excited by a primary particle and then starts to react spontaneously or as a result of collisions. The results may be useful in judging the nature and products of the primary reactions of a few hydrocarbons and their relative importance. The conditions are, however, different from those generally encountered in radiation chemical investigations. There- fore a detailed description of the technique and its implications is given.EXPERIMENTAL TECHNIQUE Fig. 1 shows the principal components of the apparatus and their arrangement. It comprises the collision chamber in which a beam of electrons at a fixed energy of 200 eV is crossed by a beam of gas molecules. The gas is introduced through a multichannel aperture Collision Chamber - --Grid To Ion Upfics of FIG. 1 .-Sketch of experimental arrangement. to improve the directivity of the beam in the forward direction. The collision chamber is open on two sides to let the gas molecules travel through it. After a few cm they hit a liquid- nitrogen-cooled surface and are thus effectively pumped away. In the collision chamber, neutral and ionic fragments are formed. The latter are removed through a large opening in 56D. BECK 57 the upward direction.The neutral species are free to go into any direction. A fraction of them enters a second electron beam. This fraction is determined largely by the solid angle subtended by the entrance opening of the ion source as viewed from the collision chamber. The second electron beam has variable energy and serves to ionize the mixture of fragments and gas molecules which have not been hit in the ion source of a conventional mass spectro- meter with a 60" deflection magnet. (i) In spite of the geometrical arrangement a fraction of the parent molecules enters the ion source. The concentration ratio of fragments to parent molecules was not better than 10-4 to 10-3 in the ion source. Therefore the fragmentation of the parent molecules into ionized products in the second analyzing electron beam, i.e.the ordinary mass spectrum of the parent gas, occurs with overwhelming predominance and completely masks the signal due to ionization of neutral fragments. This can be substantially improved by chopping the electron beam in the collision chamber-and consequently the flux of fragments into the ion source- and by using a lock-in amplifier for fragment detection. Then, the signal on a particular mass at the collector of the mass spectrometer will be composed of a large d.c. current (due to parent fragmentation in the ion source) superimposed on a small a.c. component (due to the neutral fragments of interest) ; only the noise of the d.c. current interferes with the measure- ment of the fragment signal. Band widths of 0.1-0.02 clsec had to be used to achieve an accuracy of 5-30 % for different fragments.(ii) The ff ux of molecules through the collision chamber gives rise to a number density of particles corresponding to pressures between 10-6 to 10-5 mm Hg. (iii) The time elapsing between the formation of a fragment in the collision chamber and its ionization and removal from the ion source is 10-4-10-5 sec. (iv) Great care was taken to keep all charged particles present in the collision chamber from entering the ion source. For this purpose the grid between the two chambers (see fig, 1) separates two regions of oppositely directed electric fields. Hence, the neutral fragments on their way into the ion source spend most of their life in these fields of several 100 V/cm. The essential operating features are : SIGNIFICANCE OF DATA The experiment yields knowledge of the nature of fragments present in the ion source as determined by their mass number and ionization potential : from this the nature of the frag- ments formedin the collision chamber may be deduced.Moreover, the relative concentrations of the fragments-again in the ion source-an be measured ; from this one may infer their relative rates of production in the collision chamber. NATURE OF FRAGMENTS The fragment species found in the ion source are believed to be essentially the ones produced by the electron collision except for a possible radiative loss of excitation energy either spontaneously due to the long life-time of the fragment, or induced by the electric fields. The statement concerns gas or wall collisions of a fragment; while the former certainly can be excluded, the latter may occur. The following experimental evidence appears to confirm that wall collisions are ineffective enough to leave the chemical nature of the frag- ments unaltered : (a) The detection of rare gas atoms excited to a metastable state is possible (fig.2). The cross-sections of these optically forbidden transitions are known to be about 10-18 cm2,l whereas their deexcitation probability per wall collision is high with a lower limit at 0.2 to 0.5.2 A substantial loss of excited atoms by wall collisions would have reduced their concentration below the detection limit of the instrument. An estimate of the magnitude of the number of wall collisions gives between zero and ten.From this it follows for the much more frequent fragmentation processes of a molecule that a small, but measurable, fraction of fragments should be detected without having undergone a wall collision. (b) The pressure dependence of a variety of fragments from hydrocarbon molecules was found to be linear as is the case for molecular hydrogen and the methyl radical from propane, and for atomic hydrogen and the ethyl radical from butane (fig. 3 and 4).58 NEUTRAL FRAGMENTS FROM HYDROCARBONS (c) A sensitive proof was possible on the recombination of methyl radicals from propane : no ethane could be detected, which means that the fraction of methyl radicals which recom- bined at the walls was less than 1 % of those found. ’uncorrected I P from MePostablo Ne E e u .r( (4 Ld U .m c) 5 LO .CI 8 0 20 0 O t ‘ 8 13 16 ’’ “uncorrected IP from Metastable A (b) FIG.2.-(a) AP on mass 20 from neon; (b) AP on mass 40 from argon. pressure in storage bulb in mm Hg FIG. 3.-Pressure dependence of masses 2 and 15 from propane.D. BECK 59 (d) The balances of complementary fragments provide further indication that wall collisions cannot be very efficient in destroying fragment species or in forming new ones. RATES OF FRAGMENT PRODUCTION To gain quantitative insight the measured concentrations of fragments in the ion source must be related to their rates of production in the collision chamber. This was done using the relation rate-concentration/ drnass i.e., assuming essentially that the accommodation coefficient of the translational motion is near unity.Hence, the majority of fragments-which have undergone one or more wall collisions-will have speeds corresponding closely to the temperature of the ion source. 0.1 0.2 0.3 0.5 0-6 0.7 pressure in storage bulb in mm Hg FIG. 4.Pressure dependence of masses 1 and 29 from butane. Some justification for this is derived from recent work on the translational accommodation on metal surfaces2 This treatment neglects a possible small fraction of detected fragments without wall collisions for which the above relation will not hold at all. The second assunip- tion is that the experimental arrangement does not discriminate against different initial kinetic energies in the process of formation of a fragment. The validity of this assumption was checked by experiment ; the gas was introduced in such a way that no direction of parent molecules in the collision chamber was preferred. These experiments suffered from a substantial decrease in signal-to-noise ratio, but still gave a little better than order-of-magni- tude information.No difference in the two ways of introducing the gas was found for fragments heavier than ethane, but a slight diminution of 10 to 30 % for masses below ethane, and rather marked effects of a factor of 1.4 to 4 for molecular v and atomic hydrogen was observed. In all cases the effect was in the expected direction, i.e., an in- creased collection efficiency of small masses for the directional inlet. The data given below are corrected for this effect, but nevertheless the quantitative results are not as reliable as the qualitative ones, particularly the data on atomic and molecular hydrogen.RESULTS Fig. 5, 6 and 7 give the results for propane, butane, and benzene respectively. In each figure the lower part gives the more frequent ionic fragments which con- stitute the ordinary mass spectrum, and the upper the neutral fragments. The accuracy in the upper half varies from fragment to fragment and on average may be near 20 %, errors of 50 % in single cases not being excluded. The scales in the upper and the lower half are roughly correct relative to each other except, possibly, for benzene. These graphs, therefore, give some idea of the complete cracking pattern of the respective molecule under electron impact at 200 eV.60 NEUTRAL FRAGMENTS FROM HYDROCARBONS DISCUSSION It is difficult to find literature data for comparison with the above results, con- sequently, it is necessary to focus the discussion on whether the results are con- sistent.The reasoning is similar in all three cases so propane will occasionally be taken as representative. NEUTRAL MOLECULE DECOMPOSITIONS The upper half in fig. 5, 6 and 7, called the neutral spectrum, is not comple- mentary to the lower line in the sense that complementary partners in the two lines add up to give a parent ion. Thus, the neutral spectrum appears to comprise not only the neutral fragments of the breakdown of a parent ion, but also products from the fragmentation of excited neutral parent molecules. Neutral Fragrnen t s I 2 ” 15 16 +e26 28 ’I 38 LO 12 11 t, 1 2 Ionic Fragments 15 16 26 20 38 LO 42 14 mass number FIG.5.--Cracking pattern of propane. NATURE AND ORIGIN OF NEUTRAL FRAGMENTS From mass numbers and ionization potentials, the majority of neutral fragments are assumed stable molecules. For ion as well as neutral molecule decompositions this is in accord with recent theory and experiments.43 5 The only radicals formed in appreciable amounts are CH3 and H for propane and benzene ; and CH3, H and C2H5 for butane. For propane also the allyl, methylene and, perhaps, the vinyl radicals may be detected. Fig. 8 shows typical ionization efficiency curves on masses 14 and 27. Both of them exhibit marked changes in slope indicating two super- imposed processes. In both cases the higher energy process may be identified: for mass 14, the threshold of methylene production from the methyl radical whichD.BECK 61 is produced in large amounts; for mass 27, the ionization of acetylene appearing on mass 27 due to the natural abundance of carbon 13. For mass 14 the lower threshold may be determined (with higher sensitivity than in fig. 8) to be 10.7+0.5 eV which agrees reasonably with an optical value of 10-4 eV found by Herzberg,6 but disagrees with an older mass spectrometric value of 1 1 -9 by Hipple and Stevenson. For mass 27, the threshold of the lower energy process-possibly ionization of C2H3 -is covered by background noise. These radical species which are formed from propane with an abundance of less than 5 % of H and CH3 deserve mention for two Neulral Fragments 100 c Ionic Fragments m R f c nnmhpr F r c h -PrarGino nattprn nf n-hiitanp ti\ tn inTlir9te thclt the r r a r t i n m nrrttem nf f i m C; ;a h x r nn morrna rnmnlote and (ii) because H, CH2, CH3 and C2H3 are just those radicals which were postulated by Rosenstock and collaborators in determining possible reaction paths of excited propane ions.4 Their conclusion was that the niethyl radical and atomic hydrogen should be produced in great abundance while CH2 and C2H3 take part in possible, but not very probable, reaction paths.On the other hand, it is not difficult to see from fig. 5 that radical production from neutral propane molecules is certainly not frequent. Hence, except for very small amounts of ally1 radicals, all radicals appear to stem from ion breakdown, while from the excited neutral molecule predominantly smaller stable molecules are produced. This result is not in contradiction to energetic considerations.Similar reasoning may be extended to butane (fig. 6) where it readily explains the large methyl production as well as the appearance of a new member, the ethyl radical, which is the partner of one of the dominant ionic fragments, mass 29. In the benzene62 NEUTRAL FRAGMENTS FROM HYDROCARBONS spectrum (fig. 7) it accounts for the high ratio of atomic to molecular hydrogen, for the missing phenyl radical and, possibly, is the explanation of the comparatively large propargyl production, if this is due to a symmetric break-up of the benzene .- 3 1 Neutral Fragments I 2 15 26 28 38 I 0 L8 90 52 63 71 76 78 mass number FIG.7.4racking pattern of benzene. 8 9 I -- I - I -~ I---- - 10 l', .p. iz 13 I I 13 15 17 19 v AP of 13C12CH2 .T. AP from CH3 I FIG. 8.-AP from propane. ion. On the other hand, some ally1 production from butane, large enough to be included in fig. 6, and the formation of the propargyl radical may again be indica- tions that the dominant feature, radical production stems from ion decay, is an D versimplification.D . BECK 63 In table 1 the radicals found from some hydrocarbons are listed together with their ionization potentials and their abundance from the respective parent. “ Large ” means an abundance of more than 50 % of the average of the five dominant molecular fragments, “ small ” means the radical was detected and its abundance estimated to be 10 % or less of the average five dominant molecular fragments; ‘‘ moderate ” is used in the remaining cases.TABLE 1 ionization pot. 13.6 f0.2 13.5 f0.2 10.7 f0-5 11.0 f0-5 9-8 f O - 1 10.0 f0.l 9.8 f0-1 10.0 f0.2 9.4 f0-2 8.7 f 0.1 8-8 10.1 8.2 fO.2 8.7 10.5 8.0 f0.1 8.2 f0-1 8.2 f0.1 5.2 &0.4 9.8 f0.1 9.5 50.1 9.35 rt0.l 9.9 fa.5 abundance moderate large small small large large moderate moderate small small moderate large small moderate small small small small small small ? COMPLETE CRACKING PATTERN AND NEUTRAL DECAY SCHEME The results regarding the origin of radicals may be used to relate the upper and For propane, e.g., the theory of mass spectra 4 gives the lower line in fig. 5 and 6. a reaction sequence, C3Hg-+C2Hf + CH3-+C,Hl +H2+CH,, as the major source for methyl production.Consequently, one must add up the relative intensities on masses 27 and 29 in the mass spectrum and equate it to the intensity of the methyl radical in the neutral spectrum. In this way the common scale for the mass and neutral spectra in fig. 5 and 6 was found. For benzene the origin of methyl production is not certain. The scaling in this case is derived from the gas density modulation discussed below. The results obtained so far may be expressed as decay schemes for the neutral propane and butane molecules, analogous to the corresponding schemes for i ons.464 NEUTRAL FRAGMENTS FROM HYDROCARBONS NEUTRAL DECAY SCHEMES \ *--. \\ -**. C2H4 -)?2H2. \C3H6+&H4 c2 B 6+c2&*c2H2 ---- propane butane As indicated by the ally1 and propargyl formation from propane and butane, the schemes are not complete.They are inferred only from the observed presence or absence of a fragment; also they do not distinguish between decay paths leading to identical final products. Together with the corresponding scheme for the decomposing ion they provide, however, the possibility of setting up balances, e.g., the ion and the neutral decay scheme of propane each yields one path producing a neutral methane molecule, the complementary fragments being C2H4, C2H2, C2H;, C2Ha apart from H2. Adding up their relative intensities, a production of 118 methane molecules is sug- gested ; 100 methane molecules are actually found. Similarly, one calculates 180 hydrogen molecules which should be compared with 308 found; and 26 hydrogen atoms with 17 found.For butane, the balances give for methane 93 calculated to 105 found, and for molecular hydrogen 410 calculated to 470 found. From these numbers one may judge the overall consistency of the data and their interpretation. RELATIVE ABUNDANCE OF NEUTRAL MOLECULE DECAY Neglecting double ionization, the common scale in fig. 5 and 6 permits deter- mination of the ratio y of neutral molecule decompositions per ion produced. One finds y = 1.1 for butane. This ratio may also be measured directly in the following way. As the detector is sensitive enough to detect fragments it is naturally sensitive enough to detect the diminution of the Aux of parent molecules into the ion source which is caused by molecules being used up for fragmentation.This modulation of the gas density in the ion source is a measure of the sum of decay processes of parent molecules and parent ions. Consequently, comparing the ratio of the gas density modulation with the total ionization for the molecule considered with the same ratio for a rare gas gives the desired knowledge, because the gas density modulation for a rare gas is due to ionization only. This consideration allows a comparison between the final result obtained via a number of assumptions from the spectra with a directly measured quantity for which the values 1.1 f0.2 for propane and 1.1+!:: for butane were found. For benzene, this experiment gives, not very accurately, but definitely different from the aliphatic hydrocarbons, a ratio between 5 and 30 %.One remark may be in order ; Ehrhardt and Erbse 7 measured the inelastic scatter- ing of electrons from wheptane in a crossed-beam apparatus at the electron energy of the present experiment. Their results seem to indicate that the overwhelming majority of electrons suffers energy losses above the ionization limit. They surmise that a high percentage of the neutral-molecule decay-processes originates from excitation above the ionization limit. An experiment on neutral fragments from n-hexane indicates values for y slightly increasing with chain length. These results appear to be direct evidence for the superexcited states recently postulated by Platzmann. 8 y = 1.0 for propane ;D. BECK 65 EXCITATION ENERGIES Originally it was considered an advantage that the method should give excita- tion energies of molecular fragments formed by electron impact. Actually, how- ever, no excitation energy in any fragment was detected within the limits of 0.1 to 0-2 eV, except for H2 from butane, benzene, and possibly propane. In the former cases, the ionization threshold was found to be about 0.7 to 0.8 eV lower than that of H2 in its ground state. In the introduction, some evidence was presented concerning the number of wall collisions which probably occur. The conclusion was that, at least for the more abundant fragments, there should be a detectable fraction with no wall collision. Unless this argument is wrong, the present result seems to be in contradiction to the idea of internal conversion as embodied, e.g., in the statistical theory of mass spectra. 1 Bates, Atomic and Molecular Processes (Academic Press, N.Y., 1962). 2 Dorrestein, Physica, 1942, 9, 443. 3 e.g., Hurlbut, J. Appl. Physics, 1957, 28, 844. 4Rosenstock et al., Tech. Rep. no. I1 (Utah University, 1952). 5 Okabe and McNesby, J. Chem. Physics, 1962,37, 1340. 6 Herzberg, Can. J. Physics, 1961, 39, 151 1. 7 Ehrhardt and Erbse, 2. Physik, 1963,172,210. 8 Platmann, Rad. Res., 1962, 3, 17. Also Field and Franklin, Electron Impact Phenomena (Academic Press, N.Y., 1957).

ISSN:0366-9033    

DOI:10.1039/DF9633600056

出版商:RSC    

年代:1963

数据来源: RSC

摘要:

Gas-Phase Radiolysis of Propane Effect of Pressure and Added Inert Gases BY P. AUSLOOS, (MRs.) S. G. LIAS AND (MRs.) I. B. SANDOVAL National Bureau of Standards, Washington 25, D.C. Received 31st May, 1963 The direct and inert-gas radiolysis, as well as the vacuum ultra-violet photolysis of CD3CH2CD3, CH3CD2CH3 and C3&+ C3Ds mixtures have been investigated in the presence of radical scavengers. The major conclusions are : (a) at atmospheric pressures, neutral propane decomposition contributes to the observed products although to a lesser extent than parent ion decompositions ; (b) a vari- ation in pressure has a pronounced effect on the fragmentation of the parent ion ; (c) the hydrogen atoms in the ethyl ion are randomized in the direct as well as in the inert-gas sensitized radiolysis ; (d) rearrangement in the parent ion is of minor importance in the radiolysis; (e) although the formation of propylene is related to the propyi ion, it is not necessarily produced by the neutral- ization of this ion.In studies on the direct 2 and inert gas sensitized 8 radiolysis of propane, the im- portance of ion-molecule reactions has been established. Also, investigations on the radiolysis of C3D8 in the presence of non-deuterated compounds 10, 13 have demonstrated that it is feasible to obtain reliable relative rate constants of ionic reactions occurring at atmospheric pressures. The role of neutral excited molecule deconipositions, however, has not been settled. Also, relatively little is known about the effect of pressure on the fragmentation of the parent ion.The present study, which includes mainly experiments on CD3CH2CD3 and C3Hg+C3Dg mixtures was initiated in the hope that some of these questions could be answered in a more precise way. EXPERIMENTAL MATERIALS CH3CD2CH3, CD3CH2CD3 and C3D8 were obtained from Merck, Sharp and Dohme of Canada, Limited. The mass spectrum of propane-dg showed 3 % C3D7H. The purity of propane-d2 has been described earlier.2 The C13 corrected mass spectrum of propane-& obtained at 70 Ve, given in part in table 1, agrees well with that of Condon 6 except that the sample obtained from Merck contains only 1-5 % propaiie-dT. From the isotopic analysis of the methane fraction produced in the pyrolysis of the propaned6 sample, it could be deduced that it consisted of 94 % CD3CH2CD3 and 6 % CD2HCH2CD3.IRRADIATION PROCEDURE The experimental technique was essentially the same as that described previously. Experiments above 2 atm pressure were carried out in a stainless steel cylindrical cell with a volume of 10 c1-113 (1.1 cm int. diam.). The temperature of the stainless steel reaction cell was maintained at 95°C during irradiation. All other experiments were carried out in cylindrical Pyrex vessels with a volume of 500 cm3 (15 cm in height). The National Bureau of Standards 50,000 Curie C060 y-ray source was used. The dosimetry was carried out by measuring the hydrogen produced in the radiolysis of ethylene at one atmosphere (GH~ = 1-28), and independently, by measuring the saturation ion current in a cell siinilar to that of Back 66P .AUSLOOS, S. G. LIAS A N D I . B . SANDOVAL 67 et al.3 Taking W = 24 eV, the energy absorbed by propane was 1019 eV M-1 sec-1. No correction factor (cp. Toi et aZ.14) has been used for the experiments performed at high pressures. Consequently, the G values obtained at densities above 0.02g/cm3 may be as much as 30 % too low. In most radiolysis experiments, conversions based on the production of hydrogen were somewhat less than 0.1 %. TABLE 1.-MAss SPECTRAL PATTERN OF CD3CHzCD3 27 28 29 30 31 32 33 34 70 eV 31.1 61.1 117.3 218.4 76.3 438.3 26.2 20.3 13.0 eV 0-6 1-6 3.7 72.8 17-0 66.9 3.4 5-8 46 23.8 1.5 47 17.9 13.9 48 20.9 14.5 49 125.4 118.8 50 100.0 100.0 51 1 4 1.5 consolidated 21-101, 50 pamp The photolysis experiments were carried out using an air-cooled electrodeless discharge lamp to excite the resonance lines of xenon and krypton.The lamp contained a cold finger which was cooled to - 80 or - 195°C in order to eliminate the emission bands of water. A spherical reaction vessel with a volume of 1 1. was used in all experiments. Conversions in the photolysis experiments varied between 0.05 and 0.1 %. RESULTS AND DISCUSSION Only a fraction of the results are presented in this paper. Tables containing most of the data can be obtained from the authors upon request. All experiments reported in this study have been carried out in the presence of free radical scavengers, usually NO. CONTRIBUTION OF NEUTRAL PROPANE DECOMPOSITION In earlier studies on the gas-phase radiolysis of hydrocarbons, good quantitative agreement has often been reported between the observed products and those cal- culated from the 70 eV mass spectrometric cracking pattern.From this, it was often concluded that the neutral excited molecule decompositions play a negligible role. However, any quantitative agreement is necessarily fortuitous, not only because the 10-6 sec cracking pattern does not exactly represent the fragmentation of the parent ion at elevated pressures, but also because not all ionic reactions oc- curring at atmospheric pressures are well understood at present. In order to evaluate the importance of neutral excited propane in the radiolytic system, certain aspects of the neutral propane and propane ion decompositions are discussed in some detail. NEUTRAL PROPANE DECOMPOSITION CD3CH2CD3 and CH3CD2CH3 were photolyzed at 1470 and 1237 in the presence of 2 % NO.A few typical results are given in table 2. In addition to the68 RADIOLYSJS OF PROPANE TABLE ~.-PHOTOLYSIS OF CD3CH2CD3, CH3CD2CH3 AND C3H8-f- C3D8 distributions exciting lines D2 HD H2 CD4 CD3H CH3D C H 4 I. C~H~+C~DS+NO Xe 31.9 7.6 60.5 40-0 2.7 1-2 56.1 (1 : 1 : 0.04) Kr 30.9 21-8 47-3 52.4 0.8 0-8 45.9 74.8 25-2 (1 : 0.02) Kr 26.0 29.9 44.1 - - 84.6 15.4 11. CH3CD2CH3 + NO Xe 45-9 23.9 30.2 - I CZD3H C3D6 CZDzH2 GDsH 111. CD3CH2CD3 + NO Xe 8-9 23.6 674 9.8 90.2 1.83 1-76 (1 : 0.02) Kr 19-2 52.2 28.6 7.7 92.3 0.81 0.46 hydrogen/methane = 19.0 (Xe lines), 3.2 (Kx lines) observations reported before,ll the following points are of interest to the present study. (i) H2 and CD3H are, within experimental error, equal to C3D6 and C2D3H, respectively, thus indicating that the diradicals produced in the primary processes CD3CH2CD3 +h+CD3CCD3 + H2 +CD3CH+ CD3H rearrange into propylene and ethylene respectively.(ii) Elimination of methylene from propane occurs at both wavelengths : CD3CH2CD3 + h=CD2 + CD~CHZD, since in the photolysis of C3D8 + C3H8 +NO mixtures, ethane consisted entireIy of C2D6 and C2H6 while, in the photolysis of CD3CHzCD3 +NO mixtures, CD3CH2D was the only ethane produced. (iii) In the photolysis of CD3CH2CD3+N07 the ratio CD41CD3H decreases slightly with decrease in wavelength from 0.1 1 at 1470 A to 0.08 at 1237 A. Similarly, in the photolysis of CH3CD2CH3 +NO mixtures, the ratio of process (4) to process (5) is 0.34 at 1470 A and 0.18 at 1237 (3) : CH3CD2CH34CH4 + CD2CH2 (4) CH3CD2CH34CH3D + CH3CD.(5) Hence reaction (4), which requires less energy than (5), becomes relatively more important at the longer wavelength as noted before.11 (iv) There is a drastic change in the relative product distribution between the two wavelength regions. At 1470A, the elimination of " molecuIar " hydrogen prevails, while at 1237A, dissociations of the type CD~CH~CD~+~V+CD~+CH~CD~+D (6) (7) -+CD3CHCD2 + H + D predominate, the first process being the most important. DECOMPOSITION OF THE PARENT ION The mass spectral cracking pattern of propane-& is unique in that the ratio of process (9) to (8) : CD,CH2CDz-,CD3H+ C,D,H' (8) CD,CH,CDi +CD4 + C2D2H2+ (9)P . AUSLOOS, S . G. LIAS AND I .B. SANDOVAL 69 can readily be deduced. Mass 31 can unambiguously be ascribed to (8) except for the correction due to C3DsH3 impurity. Then, assuming that the ethylene ions formed by both mechanisms make the same total contribution to the cracking patterns of CD3CH2CD3 and C3H8, the ratio of process (9) to (8) can be calculated to be approximately 2.3. At 13.5 eV, a value of approximately 4.5 can be deduced for this ratio. Comparison of these two values with the less accurate estimate of 4.4f0.7 and > 7.5 which has been deduced 2 from the cracking patterns of CH3CDzCH3 at 70 and 13 eV respectively, shows that there is an intramolecular isotope effect favouring the H-atom transfer. Furthermore, (a) an isotope effect of a comparable magnitude exists in the photolysis, and (b) the elimination of CH4 and CD4 from CH3CD2CH3 and CD3CH2CD3, respectively, is favoured at low energies in both the neutral propane molecule and the parent ion.The values for the ratio CH4/CH3D and CD4/CD3H, obtained in the photo- lysis, are more than an order of magnitude smaller than those obtained for the propane ion, perhaps because the excess energy available in the neutral excited propane molecule is considerably higher than in the propane ion. EVALUATION OF RADIOLYSIS DATA Because most of the evidence for neutral excited molecule decomposition in the radiolysis of propane will be based on the formation of molecular methane in CD3CH2CD3, it should be first noted that, in the direct as well as in the inert- gas sensitized radiolysis of C3D8 + C3H8 + NO mixtures, the methane fraction con- sists mainly of CD4 and CH4.Below 1 atm, CD3H and CH3D constitute about 5 and 3 %, respectively, of the total methane fraction. Above 1 atm, there is a gradual increase in the partially labelled methanes, but even at 25 atm, they amount to only 9 and 7 %, respectively, of the methane fraction. Up to 25 atm, HD is not more than 20 % of the total hydrogen fraction and is smaller than both Dz and Hz. At pressures below 1 atm, the average distribution is : Hz9 56.6 % ; HD, 15.4 % ; D2,28-0 %. Thus, in the presence of radical scaven- gers and up to 25 atm, methane is nearly exclusively formed by an elimination from propane while hydrogen is largely produced by a detachment process. (i) The ratio CD4/CD3H9 obtained in the radiolysis of CD3CH2CD3 (table 3), is considerably lower than that deduced from the mass spectrometric cracking pattern.This ratio, which decreases with increase in pressure, cannot be recon- ciled with the mass spectrometric observation that the ratio of process (9) to process (8) becomes larger at lower electron energies. In order to bring the methane dis- tri butions, determined in the low pressure experiments (table 3), into agreement with those deduced from the 70 eV cracking pattern, one has to assume that not TABLE 3.-FOWATION OF METHANE AND HYDROGEN IN THE RADIOLYSIS OF CD3CH2CD3 I. CD3CH2CD3-NO (1 : 0.03) hydrogen G values distribution ( O h CDdCD3H D2 HD H2 hydrogens CD4 CD3H 0.3 cm 1.05 - - - - 0.533 0.508 2 cm 0.90 30.9 42.5 26.6 2-01 0.412 0.457 1-9 atm 0.70 31-0 40.4 28.5 2.06 0.294 0-419 21.8 atrn 0.56 24-0 40.0 36.0 1.61 0.131 0.234 37.7 atm 0-52 20.4 39.3 40.3 1.87 0.117 0.225 11.CD3CH2CD3-Xe-NO 19.18 cm 1.32 24.5 37.7 37.9 - - - (1 : 8 : 0.03) 50-0 atrn 0.69 18.1 38-9 43.2 - - -70 RADIOLYSIS OF PROPANE more than 50 % of the observed CD3H originates from the parent ion. Because the degree of excitation of the parent ion no doubt diminishes with increase in pressure, one may expect the relative probability of process (9) to process (8) to decrease as well so that, at elevated pressures, practically none of the CD3H will be produced by elimination from the parent ion. At the same time, one may expect an increasing contribution of neutral excited molecule decomposition to the formation of CD4.Thus, it may be estimated from the results in table 3 that the G value of methane elimination from neutral propane is 0.25 _+ 0.05, independent of pressure. A similar value can be obtained from the earlier study of CH3CD2CH3 using the revised value of 0-18 for the ratio CH&HJD obtained in the photolysis at 1237 A. (ii) An increase in the pressure of CD3CH2CD3 increases the ratios C2D3H/ C2D2H2 (0-73 at 2 crn, and 1.3 at 37.7 atm) and C3D&3D5H (0-11 at 0.3 cm and 0.60 at 37 atni). These trends which approach the photochemically observed distributions at 1470h~ are, in view of a gradual decrease of the total yields, most likely due to diminishing contributions of ionic reactions with the resulting effect that neutral excited molecule decomposition shows up more clearly.Consistent with these observations are the increases with pressure of the relative yield of D2 and of the ratio C ~ H ~ D / C Z H ~ D ~ seen before 2 in the radiolysis of CH3CD2CH3. (iii) As shown before,29 10 a small fraction of the ethane is produced by a de- tachment process (G = 0.05+0.02). A process such as CD3CH,CD: +CD; + CD3CDH, (10) is apparently of negligible importance. In the mass spectrum of CD3CH2CD3, the fragment corresponding to mass 16 which may also be produced from secondary fragmentation process is a minor one (4 % of the ethyl ion at 70 eV and 0-05 % at 20eV). On the other hand, the photolysis experiments discussed above clearly show that the elimination of ethane from neutral propane is of importance. It is of interest to note that the ethane produced in the radiolysis of benzene + CD3CH2CD3 mixtures contains approximately 75 % CDJCDH~. These observations, as well as previous ones,2 lead to the conclusion that, below 1 atm, roughly 10-20 % of the overall products can be ascribed to neutral propane decompositions, Similar estimates have been obtained in the radiolysis of methane 1 and n-butane.5 It is difficult to evaluate the degree of excitation of the neutral propane molecules in the direct radiolysis.However, since the G value of D2 (G = 0.2) obtained in the radiolysis of CH3CDzCH3 represents the maximum value for the elimination of a hydrogen molecule from the middle carbon atom of propane, comparison of this value with the estimated G value of 0.25 for the elimin- ation of methane from the neutral propane molecule shows that the ratio of the two processes is similar to or lower than that obtained in the photolysis at 1237A.Thus, below 1 atm, the degree of excitation is at least as high as that of the excited molecules produced in the photolysis at 1237 A while, at pressures above 1 atm, propane molecules, excited to lower levels, apparently contribute to the products as well. It is more difficdt to estimate the contribution of neutral excited propane de- composition in the inert-gas sensitized radiolysis. However, a number of observ- ations indicate that neutral excited propane molecules are produced. For instance, in the xenon-induced decomposition the ratio CD4/CD3H, although higher than in the direct radiolysis at comparable pressures, is still considerably lower than the value of 3 deduced by Futrellg from the charge-exchange mass spectrum of CD3CH2CD3. A decrease of this ratio with an increase in pressure is also noted.The pronounced enhancement of the relative yield of H2, as compared to the directP . AUSLOOS, S . G. LIAS AND I . B . SANDOVAL 71 radiolysis, is not unexpected if one considers that, in view of the photolysis experi- ments carried out with the xenon resonance line, the excited propane formed in process (1 1) should mainly eliminate hydrogen from the middle carbon atom. Similarly, in the radiolysis of CH3CD2CH3+Xe (1 : 10) mixtures, the hydrogen fraction consists of 16 % Dz as compared to 9.7 % in the direct radiolysis carried out at comparable pressures. On the basis of these observations, it can be estimated that, in the xenon-induced radiolysis, more than 10 % of the hydrogen is formed by process (1 1).Finally, in the radiolysis of Kr+ CD3CH2CD3 (10 : 1) mixtures, a value of 0.52 at a pressure of 20 cm is obtained for the ratio CD4/CD3H which is again lower than the value of approximately one obtained by Futrell9 in the mass spectrometer. Xe'+CD,CH,CD,-+Xe+CD,CH,CD;. (1 1) EFFECT OF PRESSURE ON THE FRAGMENTATION OF THE PARENT ION In order to obtain a meaningful comparison between the yields of the frag- ment ioils produced in the radiolysis with those deduced from the mass spectral cracking pattern, it is essential to obtain some information about the effect of pressure on the products produced in the radiolytic system. The major objective of the experiments on C ~ H ~ + C ~ D S niixtures is to follow the variation in yield of the ethyl and vinyl ions with pressure.For these two ions, the hydride transfer reaction is exothermic and it will be tentatively assumed that under our conditions this constitutes the only mode of reaction. In the radiolysis of C3Hg+C3Dg mixtures in the presence of radical scavengers, it may be assumed that, after making an approximate correction for the amount of C3D7H present in the starting material, CzDsH and C2D3H are only produced by reaction of the CzDf and C2D'j ions, respectively. Because, in all experiments, C~DSH is, within experimental error, equal to C2H5D and C2D3H is equal to CZH~D, reaction (12) does not show an isotope effect and consequently, the amount of fully deuterated and nondeuterated ethane and ethylene formed in (12) should be equal to the yield of the compounds containing one H or one D atom.Any residue may be ascribed to molecular elimination processes. R+ + C3H,-+RH+ C3H; (12) TABLE 4.-FRAGMENTATION OF THE PARENT ION; EFFECT OF PRESSURE G values total pressure I. C3Hg+ C3Ds+Ar+NO 2.17 cm 13-43 cm 1.62 atni 56.6 atm (1 : 1 : 60 : 0-05) 11. C3&+C3D8+NO 3 cm (1 : 1 : 0.05) 23 cm * - 1 atm 1.7 atm 4 . 4 atm 16.0 atm 43 atm density vinyl ion dcm3 - 0.502 - 0.310 - 0.1 65 I 0.041 - 0.20 1 - 0.179 - 0.30 0.0035 - 0-0089 - 0.032 0.069 0.21 0.01 3 ethyl ion 0-815 1.04 1.68 1.71 1 -32 1-64 1 *79 1 -77 1-41 1-16 0.4 9 vinyl ion1 ethyl ion 0.61 6 0.298 0.098 0-024 0.152 0.109 0-167 - 0.064 0.026 values calculated 15 for fragmentation of propane ion at 10-10 sec collision interval.72 RADIOLYSIS OF PROPANE The effect of pressure on the fragmentation of the parent ion is demonstrated in table 4 which shows the variation of Gethyl ion and Gvinyl ion over a 2000-fold pressure range for C3H8 + C3H8 + Ar (1 : 1 : 30) mixtures.The decrease of vinyl ion yields and the increase of the ethyl ion yields with increase in pressure can be qualitatively accounted for by considering the secondary reaction C,DZ+C,D$ +D,. (13) It can be seen that, at low pressures, values for the yield ratio C2Dz/C2Di approach but do not coincide with the value of 2-8 deduced from the argon exchange mass spectrum (2 V) reported by Pettersson and Lindholm.12 On the other hand, in con- trast to the experimental results, the calculated breakdown curves for propane predict that, for a collision interval of 10-losec (corresponding to a pressure of 1 atm), no vinyl ions are produced in the argon-sensitized radiolysis.15 Never- theless, the results given in table 4 confirm at least qualitatively the conclusion of Futrell and Tiernan 8 that a change in time-scale between the mass spectrometric investigation and the radiolysis has a pronounced effect on secondary ion decom- positions.A similar trend with pressure is observed in the direct radiolysis of equimolar C3H8 + C3D8 mixtures. In this case, there is good correspondence between the G values deduced from the calculated 10-10 sec breakdown pattern of propane and the G values observed around 20cm total pressure.Also, at high densities, there is a pronounced decrease of Gc,$ although, at the critical pressure (density, 0.2 g/cm3), ethyl ions are still produced. REARRANGEMENTS IN THE ETHYL ION It has been proposed 2 that, in the radiolysis of propane +I2 mixtures, ethane is mainly formed by the exothermic hydride transfer reaction C,Hl + C,H,+C,H, +C,IH$ (14) Proof was mainly based on the observation that ethane-&, formed in the radiolysis of propane-& consisted mainly of CD2HCH2D. It was thus suggested that the hydrogen atoms in the ethyl ion undergo a rapid randomization prior to or during reaction. Because this conclusion was based on the interpretation of a complex mixture of isotopically labelled ethanes, a confirmation seemed necessary. This was achieved by studying the radiolysis of CD3CH2CD3 in the presence of an excess of cyclohexane (1 : 4).Under these conditions, the ethyl ions react mainly with cyclohexane 10 to produce to give C2D3M3 as the major isotopically labelled ethane. A mass spectrometric analysis of the ethane-& thus produced indicated that it consisted of both CH2DCD2H and CD3CH3 in a 2 to 1 ratio. This observation corroborates the conclusion reached earlier 2 that the hydrogen atoms in the ethyl ions reshuffle statistically. From the isotopic analysis of the ethanes produced in the He, Ne, Ar, and Kr sensitized radiolysis of both CH~CDZCH~ and CD3CH2CD3, it could be concluded that the hydrogen atoms are completely randomized in all cases. This may be contrasted with the observation4 that the degree of reshuffling in the sec-propyl ion produced in the gas-phase radiolysis of isobutane increases with the recom- bination energy of the inert gas.This difference may be mainly due 4 to the rela- tively higher energy requirement for randomization in the sec-propyl ion. In the xenon-sensitized radiolysis of propane, the interpretation of the ethane fraction is more complex. For instance, in the direct radiolysis and in the He, Ne,P . AUSLOOS, S. G. LIAS AND I . B. SANDOVAL 73 Kr and Ar sensitized radiolysis of CH3CDzCH3, a constant value of 2.3+0.1 is observed for the ratio C ~ D ~ H ~ / C Z D ~ H ~ , but values as low as 1.24 are obtained in the radiolysis of Xe+CH3CDzCH3 mixtures. Similarly, a value of 1.75+0*05 is obtained for the ratio CZD4HZ/C2D3H3 in the direct radiolysis of CD3CH2CD3 (from 2 cm to 37.7 atm) as well as in the He, Ne, Kr and Ar sensitized radiolysis while values as low as 1.15 are observed in the xenon-induced decomposition of CD3CH2CD3.The reason for the anomalous behaviour of the xenon mixtures is not known. The isotopic distribution of ethanes formed in the radiolysis of xenon+C3H8+C3Dg mixtures indicates that ethane, as in the other systems, is mainly formed by a hydride transfer reaction. A plausible interpretation would be that an intermediate such as XeC2H: is involved in the hydride transfer reaction : XeC,Hl +C3H8-+Xe+C,H, +C3HT (1 6) and similar species such as XeCHf have been observed in the mass spectrum of Xe+CH4 mixtures.7 It is conceivable that XeCzH: would show a stronger pre- ference for abstracting a hydride ion from the secondary carbon atom in propane than the smaller ethyl ion.At any rate, an analysis of the isotopic ethanes formed in the CH3CD2CH3 +Xe mixtures indicates that the hydrogen atoms are completely randomized in whatever ion leads to the formation of ethane. REARRANGEMENT I N THE PROPANE ION Randomization in the parent ion may also lead to the formation of CHzDCD21-P. However, on the basis of the mass spectral cracking pattern of CD3CH2CD3, it can be demonstrated that exchange of the H atoms plays a relatively minor role. Rearrangement in the parent ion would necessarily lead to the formation of the ions C2D4Hf and C2Df. However, the mass spectrum given in table 1 shows that masses 33 and 34 are of relatively minor importance.Consistent with this is the observation that, in the radiolysis of CD3CHzCD3 at pressures around 2cm, the products C2D6 and C2D5H are respectively 3.1 and 6.5 % of the total ethane fraction while, at pressures of 21 atm and above, C2D6 and C2D5H constitute only 1.5 and 3.6 % of the ethane fraction or less. In this connection, however, in addition to the formation of ethane by a hydride transfer reaction and by molecular elimination, there is an additional, although relatively minor, process which may contribute to the yield of ethane. Experiments carried out with C3H8 + C3Dg +NO mixtures indicate that the ethane44 produced in the radiolysis of C3H8+C3D8 mixtures is larger than one can expect from the amount of C3D7H present in the starting material and the yield of C2D4H2 relative to that of CzD5H gradually decreases with increase in pressure.At a pressure of 3 mm, the corrected yield of C2D4H2 is approximately 14 % of the yield of C2D5H while, at pressures above 20 atm, it is less than 1 %. A reaction such as C,DZ + C3H8+C2D4H2 + C3Hz (17) would account for these observations. This is one of the reactions which has been proposed by Field to occur in the mass spectrometer (private communication). The observation that the yield of C2D4H2 relative to that of C2D5H diminishes with increase in pressure is consistent with the fact that the yield of methane, formed by elimination from the parent ion and, consequently, also that of the corresponding ethylene ion decreases faster than the ethyl ion from 2cm to 20 atm.From our results, it cannot be decided to what extent a reaction such as (17) would contribute to the formation of CzD6 and CzD5H in the radiolysis of CD~CH~CDJ.74 RADIOLYSIS OF PROPANE NEUTRALIZATION PROCESSES No clear answer can be given as to which ions undergo neutralization or as to what the neutralization products are. The propyl ion formed in reaction (14) may be considered as the major relatively unreactive ion in the radiolysis of propane. On the other hand, propylene which is a major product may * have the propyl ion as a precursor. This is substantiated by the observations that (a) about 90 % of the propylene produced in the radiolysis of C3D8 + C3& +NO mixtures at pressures above 1 atm consists of C3D6 and C3H6 ; (17) the addition to propane of a compound such as benzene which reacts 10 with carbonium ions drastically reduces the yield of propylene although it does not quench the formation of other products such as ethylene, methane, and hydrogen,l3 and (c) the addition of 10 % cycloliexane to C3D8 quenches the formation of propylene while C3D7H appears as an important product which is consistent with the fact that sec-propyl ions react readily with cyclo- hexane.5 The above observations, however, do not constitute conclusive proof for the formation of propylene by the neutralization of the propyl ion.Recent experiments 1 on the radiolysis of methane have shown that, ethyl and propyl ions readily react with radical scavengers, higher hydrocarbons, or reaction products, even though these compounds may constitute less than 0-02 % of the total reaction mixture. In view of these findings, it is conceivable that under our experimental conditions, the propyl ion will interact with reaction products or with NO, rather than undergo neutralization. For instance, a proton transfer reaction of the type C~H; +M+c,N,+MH+. (1 8) which has been shown 5 to occur for a number of compounds (M) could account for the formation of propylene. This work was performed under the auspices of the United States Atomic Energy Commission. The authors are greatly indebted to Dr. J. Futrell for access to his data prior to publication and for several stimulating discussions related to this problem. They also wish to express their gratitude to Dr. Hideo Okabe for his expert advice on the construction of the short wavelength equipment used in this investigation. 1 Ausloos, Gorden and Lias, J. Chem. Physics, 1963, in press. 2 Ausloos and Lias, J. Chem. Physics, 1962, 36, 3163. 3 Back, Woodward and McLauchlan, CQE. J. Chent., 1962,40, 1380. 4 Borkowski and Ausloos, J. Chem. Physics, 1963,38, 36. 5 Borkowski and Ausloos, J. Chem. Physics, 1963, in press. 6 Condon, J. Amer. Chem. SOC., 1951, 73,4675. 7 Field and Franklin, J. Amer. Chem. SOC., 1961, 83,4509. 8 Futrell and Tiernan, J. Chem. Physics, 1962, 37, 1694. 9 Futrell, private communication. 10 Lias and Ausloos, J. Chem. Physics, 1962, 37, 877. 11 Okabe and McNesby, J. Chem. Physics, 1962,37, 1340. 12 Pettersson and Lindholm, Arkiv. Fysik, 1963, 24,49. 13 Sandoval and Ausloos, J. Chem. Physics, 1963, 38, 2454. 14 Toi, Peterson and Burton, Rad. Res., 1962, 17, 399. 15 Vestal, Wahrhaftig, Johnston and Futrell, J. Chem. Physics, 1963, in press.

ISSN:0366-9033    

DOI:10.1039/DF9633600066

出版商:RSC    

年代:1963

数据来源: RSC

摘要:

The Role of Unsaturated Products in the Radiolysis of Methane BY R. W. HUMMEL Wantage Research Laboratory (A.E.R.E.), Wantage, Berks. Received 10th June, 1963 The radiolysis of gaseous C& with 4 MeV electrons gives much larger yields of C2H4 than pre- viously reported, in addition to significant amounts of C2H2. The effects of scavengers, and of pressure and dose-rate changes, indicate the reaction of CH2 with C€& to form C2HZ which is either collisionally deactivated or decomposes to form (22% or C2H2. The higher yields of these products at low conversions, low pressures and high dose rates is compared with gas-discharge work. The importance of unsaturated products in irradiated saturated hydrocarbons has been emphasized by Back.1 Unsaturates are often produced but their con- centrations do not increase linearly with absorbed energy owing to the ease with which they first scavenge hydrogen atoms and subsequently react with other radicals.An equilibrium concentration is rapidly attained which is often so low that detection is difficult and special methods have to be used to estimate the " initial " yields at very low % conversions. In the past, 1-2 % conversions of the starting material weie acceptable figures, but now, largely because of gas chromatographic methods of analysis, 0.01 % conversion may be unacceptably high. Previous papers 2-5 on the radiolysis of pure methane show that sinall amounts of ethylene have been detected (G values from 0.05 to 0*12), as well as traces of iso- butene4 and isopentene.3~ 4 Acetylene and propylene have not been reported, althougb the former is a major product in gas discharge experiments,6, 7 and has recently been found 8 in methane passed at low pressures (10-2mmHg) through a beam of low-energy electrons.Ethylene, and smaller amounts of propylene, have been found3 in CH4 con- taining NO, with G(C2H4) = 0.6. Similarly, in the presence of HI and 12, much enhanced yields of C2H4 have been found 9 as well as appreciable amounts of C2H2 but these observations 9 are not strictly comparable with the others mentioned be- cause the system used comprised only 2.5 % CH4 in Ar. The information available throws little light on the part played by unsaturated products in the radiolysis of CH4 and in consequence discussion on this point has been cursory.2-5, 9 In this paper, some experimental results are given which in- dicate that a significant fraction (about one-half) of the methane decomposed during radiolysis can be detected initially as acetylene and ethylene.EXPERIMENTAL PURIFICATION OF MATERIALS Methane from three commercial sources, containing from 86 to 99-6 % C&, was purified 10 by passing it through previously heated and degassed traps containing mole- cular sieve cooled to -78°C and then into a trap cooled with liquid N2. Subsequently, He passed through a charcoal trap at -196°C was bubbled through the liquid C& (at about - 180°C) until the 0 2 and N2 contents were reduced to less than 0.004 and 0.009 7576 RADIOLYSIS OF METHANE mole %, respectively. Finally, the He was removed by several thaw-pump cycles. No more than a fifth of the CHq was wasted.Acetylene and ethylene were initially 99 % pure. The acetylene contained appreciable amounts of impurities detectable by gas chromatography. These were removed by multiple trap-to-trap distillations in vacuo ; the detection limit was about 10-4 mole %. Propylene of the same initial purity was further purified by gas chromatography to remove traces of ethane and propane. Oxygen, " 993 % or over ", was dried using liquid N2. IRRADIATION TECHNIQUE The irradiations were done with the 4 MeV electron beam from a linear accelerator at a mean dose rate of about 1 . 6 ~ 1021 eV min-1 (g CHq)-I. The spherical Pyrex bulbs, filled with the hydrocarbon to the required pressure, were either 140 or 200 cm3 in volume.To them were sealed capillary stopcocks via a 15 cm capillary neck to keep the stopcock well out of the electron beam. A stream of cold water (10-15°C) was played over the bulb during irradiation. A measurement of the pressure increase in a bulb during irradiation showed a temperature increase of 1 l0C, occurring in the first 10 sec. DOSIMETRY The yield of C2H6 from irradiated C& at atmospheric pressure was used for dosimetry. Other work in these laboratories, comparing the c2H6 yield based on the N20 dosimeter 11 and the C2H6 yield from T2-CH4 mixtures,l2 has shown that G(c2H6) = 2.00f0.06 molecules per 1OOeV (up to about 2 % conversion, atmospheric pressure, ambient tem- peratures). In most experiments a dozen or more bulbs were irradiated consecutively; two of these contained pure CH4 for dosimetric use and the energy absorbed by the other bulbs in the set were calculated from the C2H6 produced in the dosimeters.Before each experiment the bulbs were heated to 3400°C under vacuum. IOOC IOC IC ( I 1 I 10 10 N20 pressure, cm H g FIG. 1.-Nitrogen yield (arbitrary units) as a function of N20 pressure (cm Hg). Previous work11 on N20 had shown that G(N2) was constant at pressures down to 2 cm Hg. In view of the work reported here on CHq irradiated at lower pressures, N20 was reinvestigated from 80 to 0.3 cm Hg ; the results shown in fig. 1 indicate that there is little reason to suspect dosimetric complications as a possible cause of the pressure effects observed in CHq radiolyses.R. W. HUMMEL 77 ANALYTICAL METHODS Most products were estimated by gas chromatography, using thermistor and flame ionization detectors.Calibration factors were obtained using synthetic mixtures with pure CH4 where possible. The irradiated gas was expanded from the bulb into a 7 cm3 sample vessel from which the sample was subsequently flushed on to the chromatographic column by He carrier gas. Hydrogen was estimated by PVT measurements on the gas non-con- densible at 20°K (liquid H2). RESULTS The results obtained with CH4 at 80 cm Hg are given in table 1. In pure CH4 G(C2H2) decreased rapidly from 0.25 at the lowest dose (approximately 10-3 % conversion) to 0.01 above 0.4 % conversion. G(C2H4) behaved in a similar manner (see fig. 2). The yields of H2 and the saturated hydrocarbons were linear from the lowest doses.absorbed dose, eV/gm x 10-21 FIG. 2.-Variation of the G values for C2H2 (curve A) and C2H4 (curve B) with absorbed dose. In the presence of 0 2 the yields of all products were linear with dose in the region studied (10-3-1 %) conversion). The large yield of C2H4 agrees with that found by Yang and Manno 3 in the presence of NO, but there is not a correspondingly large increase in G(C2H2). G(C2H6) is very low, in agreement with other workers, and the higher hydrocarbons are not detectable. In the presence of added C2Hz there is again a lack of higher saturated hydro- carbons; perhaps the most noteworthy feature is the appearance of C3H6 with a high G value. This product was undetectable in pure CH4 or in the presence of added C2H4 or 0 2 .In C2H4 mixtures G(C3Hg) and G(n-C4Hlo) are particularly78 RADIOLYSIS OF METHANE prominent while in C3H6, G(i-C4Hlo) predominates. There is a great difference in the G values for C2H4 and C2&, between the 0 2 system and those in which saturated hydrocarbons were added. TABLE 1 .-G VALUES OF PRODUCTS IN THE PRESENCE AND ABSENCE OF 1-2 % OF ADDITIVES IN METHANE AT 8Ocm PRESSURE 5.6 2 0-3 2 0.3 2-00 0.0 0.3 5 008 0.15 2.7 - 0.06 0.65 1.5 0.1 1-0 0.0 0.7 0.0 0.0 0.0 0.0 0.0 0.0 - - 0.19 0-13 - 1 *40 1-5 1 *34 0.0 - 2.0 1 -20 0.10 2.03 2.0 0.0s At pressures below 80 cin Hg, G(C2H4) increased linearly as the pressure was decreased (see fig. 3). G(C2H4) appears to level off at about 3 at the lowest pressures, both in pure CH4 and in the presence of 1 % C3H6.On the other hand, in similar 01 I.0 10 100 pressure (cm Hg) FIG. 3.-Variation of G(C2H4) with pressure in pure CH4 (curve A, 0.16 % conversion ; curve B, 0.04 %) and in CH4 containing 1 % C3H6 (curve C , 0.01 % conversion). 1 % C3Hd mixtures, G(C&) remained the same from 80 to 10 crn Hg total pressure. A ten-fold reduction in the mean dose rate reduces G(CzH4), the effect being greater at low pressures (see fig. 4). In fig. 3 and 4 each line is drawn through points derived from experiments in which the energy absorbed per g CH4 was kept fairly constant (within 3-10 %).R. W. HUMMEL 79 DISCUSSION It is evident that the radiolytic decomposition of CH4 proceeds to a very significant extent via the primary formation of C2H2 and C2H4 and that secondary reactions involving these unsaturates lead to certain identifiable saturated hydro- carbon products.The G value for C2H4 found at 80 cm pressure in the presence of C3H6 is the most reliable value for the initial yield ; the contribution to G(CzH4) I I - L - L 0 2 4 6 8 18 mean dose rate, eV min-1 (gm CH4)-1 X 10-20 FIG. 4..-Variation of G(C2H4) with mean dose rate at various pressures. A, 0.5 cm ; B, 10 cm; C , 80 cm Hg. by direct radiolysis of C3H6 should be negligible, and in practice a tea-fold change in C3H6 concentration does not alter G(C2H4). In conjunction with G(C2H2) -0.3, which is almost certainly a minimum value, it is seen that in CH4 at 80 cm pressure G(CH4-,C2H2+C2H~))33.4; this is nearly half the value of G(-CH4) which can be calculated from the accumulated hydrocarbon products.Since G(-CH4) = 4 with respect to c2& formation then the primary chemical processes in CH4 result almost entirely in the formation of C2 products. ETHANE FORMATION From the results in table 1 at least two reactions producing CzH6 are indicated. The residual C2H6 obtained in the presence of 0 2 (or NO) must involve a species other than H atoms or CH3 radicals, which react readily with 0 2 . Two exothermic reactions involving species other than H or CH3 are possible, viz., M CH2 +CH4+C2H~-+C2H, M e CH; +CH4+C,H; '+C2H,f--)C2H6+H. (2) The relative rate constants for reaction of CH2 and CH3 with 0 2 and CH4 are such that at the concentration used here 0 2 should not interfere with the initial steps of reactions (1) and (2). (Electron capture by 0 2 may compete effectively against the neutralization step in (2).) Bell and Kistiakowsky have used 0 2 to scavenge80 RADIOLYSIS OF METHANE CH3 in the presence of CH2.Wexler and Jesse 14 have found that the contribution due to C2H; in the mass-spectrometric pattern from CH4 increases with pressure in the mm range, unlike the other C2 ions, indicating collisional stabilization and a relatively long lifetime likely to lead to neutralization. The second C2H6-producing reaction, which must account for the C2H6 whose formation is prevented by 0 2 but not entirely by the unsaturates, is M 2CH,+C,Hi+C2H6. (3) In the presence of added C2H4 or C3&, the initial reaction of these unsaturates is with H atoms to give radicals which are probably produced in amounts equivalent to CH3 (assuming CHq-+CH2+2H is negligible).The rate constants for C2H5 and ~ s o - C ~ H ~ combination with CH3 are similar to that of reaction (3), so that half the CH3 radicals should be scavenged. Thus, with added C2H4, G(C3Hs) = 2-0 and with added C3H6, G(i'C4HIO) = 2.0, both results indicating G(CH3) (scavengeable) = 2.0 and consequently a reduction in G(CzH6) by 1.0. This should give G(C2H6) = 1.0 in the presence of C2H4 or C3H6, but in both cases higher values are observed. The C2H6 yield in the C;H4 system can be corrected for C ~ H S disproportionation using the observed G(n-C4Hlo) ; this gives a corrected G(C2H6) of 1.2, still 20 % high and possibly due to a third C2H6 source, such as which presumably requires 7-8 eV excitation energy.ETHYLENE FORMATION At least two reactions are required by the results given in table 1, one of which is stopped completeIy by 0 2 and a second which is stopped neither by 0 2 or C2H2 or C3H6. An exothermic free radical reaction which could be stopped by 0 2 is Despite its exothermicity there is little support for this reaction in previous work. However, in this work both the results obtained in the presence of additives and the increased C2H4 yields at reduced pressures indicate that this reaction may be important. 2CH,--+C2H,*--+C,H4+ H,. (4) The ion-molecule process CH; +CH4-+C2HS +Hz C2Hz +e+C2H4+H may be reduced in the presence of 0 2 due to effective competition by the latter for thermal electrons and is therefore an alternative to (4), though not pressure dependent.The second source of C2H4, by a reaction not affected by the presence of small amounts of 0 2 or added unsaturates, may be the decomposition of excited ethane molecules produced as in reaction (1) but which decompose before stabilization, This is again exothermic, and should be more important at lower pressures. CH2+CH4+C2H~-+C2H,+H,. (5) In addition, combination of CH2 radicals, M 2CH2+C2Hf+C2H4 (6) may occur with an efficiency similar to that of CH3 radical combination.15~ 16 This reaction may be the cause of the increased G value for C2H4 production at higherR. W. HUMMEL 81 dose rates (fig. 4). Mahan and Mandal,l7 in a study of the vacuum ultra-violet irradiation of CH4 at an absorbed dose rate of 1015 quanta sec-1, considered CH;! combination to be negligible in view of the rapidity of CH2 + hydrocarbon reactions.In the present work the mean dose rate in eV sec-1 g-1 was about 5 x lo4 times that used by Mahan, while the dose rate per pulse from the accelerator was another factor of 600 higher. The effective dose rate was therefore much greater than that used by Mahan and the probability of CH2 combination correspondingly higher. However, since the relative rate constants for CH2 reactions are not known, the importance of reaction (6) cannot be assessed. ACETYLENE FORMATION Comparing the maximum G value for C2H2 obtained at the lowest dose with the values found in the presence of additives, it is evident that the latter do not sufficiently protect C2H2 from secondary reactions under radiolytic conditions.In addition, other experiments with a flow system in this laboratory have yielded G(C2H2) -0.5, about twice the maximum reported here for a static system. There is evidence 73 17 that C2H2 is produced by a reaction analogous to (4). However, Bell and Kistiakowsky 13 believe that C2H2 is either stabilized by collision or dissociates to methyl radicals which then combine; they did not analyze for C2H2. This obscure situation with respect to C2H2 formation is compounded by the subsequent tendency of C2H2 to polymerize and to react rapidly with adven- titious 0 2 ; there is evidence 18 that 0 2 reacts readily with traces of C2H2 in irradiated CH4, forming CO, while the reaction of oxygen atoms with C2H2 to give ketene has been observed 19 at 20°K. The latter reaction is about 50 kcal exothermic at room temperature so that dissociation to CH2 + CO can occur.With respect to polymer- ization, some work in this laboratory has shown that a significant quantity of involatile carbonaceous material deposits on the reaction vessel wall even at a few tenths of a per cent conversion (conversions are based on the amounts of volatile products). Until further information is available little can be said except that C2H2 evidently accounts for a significant fraction of the CH4 decomposed. With regard to the modes of formation of the higher saturated hydrocarbons during the radiolysis of CH4, the addition of C2H2 gives a significant yield of C3H6, G = 0.7 ; addition of C3H6 gives G(~so-C~HIO) = 2.0. No C3H6 is observed among the radiolysis products of pure CH4, but some ~so-C~HIO is present.It appears that ~so-C~HIO is derived from C2H2, the probable steps being CzH2 + H+CzH3 C2H3 + CH3+C3H6 C3H6+ H+iso-C3H, ~ s o - C ~ H ~ + CH~-'~SO-C~H~O. It is known that the addition of H to C3H6 gives almost exclusively the iso- propyl radical.20 Similarly, from table 1 it can be seen that n-C4Hlo is formed mainly from C2W4, while C3H8 probably has both C2H4 and C3H6 as precursors, In both cases, H-atom addition is followed by further radical addition reactions. On this assumption, a value of G(H) = 6 can be estimated from the yields of C3H8 and n-C4Hlo in the presence of C2H4. However, from the C3H8 and iso-C4HI0 yields in the presence of C3H6 a much lower G(H) is found. The results reported here emphasize the importance of unsaturate formation and provide more evidence for the occurrence of CH2 and CzH3, in support of recent work.Sy 173 21 The large unsaturate yields also provoke comparison with82 RADIOLYSIS OF METHANE gas discharge work; under suitable conditions, such as low pressure, high dose rate and high flow rate, the radiolysis of CH4 may also yield CzH2, C2H4 and H2 as the principal products.I am indebted especially to Mr. D. Rush, who very ably carried out most of the experimental work. 1 Back, J. Physic. Chem., 1960,64, 124. ZLampe, J. Anzer. Chem. SOC., 1957, 79, 1055. 3 Yang and Manno, J. Amer. Chem. SOC., 1959,81,3507. 4 Mains and Newton, J. Physic Chem., 1961,65,212. 5 Maurin, J. Chim. Physique, 1962,59, 15. 6 Wiener and Burton, J. Amer. Chem. SOC., 1953,75, 5815. 7 Tickner, Can. J. Chem., 1961, 39, 87. 8 Manton and Tickner, Can. J. Chem., 1960,38, 858. 9 Meisels, Hamill and Williams, J. Physic. Clzem., 1957, 66, 1456. 10 Hummel and Hearne, Chem. and Znd., 1961, 1827. 11 Hearne and Humiel, Radiation Res., 1961, 15, 254. 12 Hearne, unpublished results. 13 Bell and Kistiakowsky, J. Amer. Chem. Soc., 1962,84, 3417. 14 Wexler and Jesse, J. Amer. Chem. SOC., 1962, 84, 3425. 15 Norrish and Porter, Disc. Farday SOC., 1947,2,97. 16Steacie, Atomic and Free Radical Reactions (Reinhold, New York, 2nd ed., 1954), vol. 2, 17 Mahan and Mandal, J. Chem. Physics, 1962, 37, 207. 18 Rowland, Koyama, Schmidt-Bleek and Umezawa, presented at the 142nd Meeting, Amer. 19 Haller and Pimentel, J. Amer. Chem. Soc., 1962, 84, 2855. 20 Moore, J. Chem. Physics, 1948,16, 916. 21 Ausloos, presented at the 144th Meeting, Amer. Chem. SOC. (March, 1963). p. 523. Chem. SOC. (Sept., 1962).

ISSN:0366-9033    

DOI:10.1039/DF9633600075

出版商:RSC    

年代:1963

数据来源: RSC

摘要:

Electron Multiplication in Argon as a Guide to Mechanism in the Radiation Chemistry of n-Butane BY E. COLLINSON, J. F. J. TODD * AND F. WILKINSON The University, Leeds Received 10th June, 1963 Chemical changes in n-butane have been induced by electron multiplication in argon. Under appropriate conditions, breakdown occurs as a result of (a) transfer of energy from argon atoms in metastable and other excited states to butane molecules, giving ions, and (b) interaction between butane molecules and electrons to give mainly excited states. Variation of the experimental con- ditions leads to a changing importance of the relative contributions of (a) or @), and the products, which have been analyzed by means of vapour-phase chromatography, may be classed as of principally ionic or non-ionic origin.The discovery that the spark breakdown potential 1 and the starting potential for the glow discharge in neon2 were reduced by small traces of argon led to the suggestion that argoil atoms were being ionized by collisions with excited neon atoms. The corresponding effect in argon has since been employed in a sensitive detector for vapour-phase chromatography.3 In this technique argon is subjected simul- taneously to ionizing radiation and an electric field of about 2000 V cm-1, the small saturation current obtained in pure argon being considerably enhanced when a trace of organic vapour is added to the system owing to the occurrence of the following sequence of processes : Ar sAr ++&- k A r * Ar* + B+Ar + B i- (or other charged species)+ 8- ( 2 4 Ar* is an excited argon atom, most probably the metastable 3p5(2P3q2)4s state lying at about 11-6 eV above the ground state, B is any gas having an ionization potential -c 11.6 eV, and which in this work is n-butane, E- is a thermal electron, and &in an electron accelerated to an energy greater than thermal. A suitable choice of con- ditions gives a high multiplication of electrons and a measurable ion current of up to 10-6A.This system promised to be a useful means of simulating some of the characteristics of radiation chemistry, and has been investigated from this point of view. EXPERIMENTAL The reaction cell (shown in fig. 1) consisted of two brass plates A separated by a disc of Perspex B in which was cut a 3 x 1 cm slot which formed the reaction zone.An ionizing source, which consisted of tritium absorbed in a thin layer of titanium deposited on copper, present address : Chem. Dept., Yale University, Conn., U.S.A. a384 ELECTRON MULTIPLICATION I N ARGON C, was mounted on one brass plate; the other plate carried the inlet and outlet tubes, and the whole unit was made vacuum-tight by rubber O-rings D. A stable d.c. potential was supplied from a 3 kV E.H.T. unit and the ion current was measured with conventional circuits and an Electronic Instruments Ltd. Vibron amplifier. The flow system (shown in fig. 2) was made of stainless steel, copper, brass, glass and Teflon (polytetrafluoroethylene). Adsorbent materials were avoided except for the O-ring vacuum seals. The whole system was mounted in an oven to facilitate gas purging at IOO"C, and the cell was operated at within a few mm of atmospheric pressure.FIG. 1.-The reaction cell. A, brass plates; By per- spex insulator ; C, tritium source; D, O-ring seals; E, gas ports. FIG. 2.-The reaction cell and analysis systems. A, Purnell 5-type injection valves ; B, mixing-bulb ; C, sampling volume ; D, stream splitter ; E, reaction cell ; F, Teflon injection valve ; G, detector ; H, gas chromatography column ; J, 2000 V unit; K, amplifier; L, mV recorder ; My to helium chromatograph unit. Butane was added to the argon in one of two ways. A convenient way of obtaining a known concentration was to make up the mixture to 2000 lb/in.2 in a small cylinder. On the other hand, the mixing-bulb technique described by Lovelock 4 was used when a range of concentration was to be studied.In the latter case a known volume of butane was injected into the argon stream by means of a modified Pumell valve.5 Analysis of the gas stream leaving the reaction cell was made by " cross injecting " a known volume into a gas chromatograph. The injection valve consisted of a Teflon key rotating in a steel barrel (A, fig, 2), but for measurement of H2 and CH4, for which absorption was of less consequence, a Purnell value was employed. Various packed columns were used, the most generally adopted being that of n-hexadecane oncelite. Argon or helium6 ionization detectors were employed. Product yields obtained were so small that complete calibration of the chromatographic system was not achieved, and although linearity between peak height and amount of product for very small peaks was proved, absolute quantitative measure- ments were unreliable.Consequently, the results have relative significance only. Measure- ments are expressed in terms of the ionic yield, MIN. My the number of product moleculesE. COLLINSON, J . F. J. TODD AND F. WILKINSON 85 collected in the sampling volume, is represented by the chromatogram peak height, and N, the number of ions corresponding to these product molecules, has been calculated from the known flow rate and ionization current. Only for unreacted butane was a satisfactory quantitative calibration achieved. The butane used was of research grade from Phillips Petroleum Co., and the argon, which was technical grade from British Oxygen Co., was dried by a trap containing a mixture of solid C02 and acetone.RESULTS AND DISCUSSION PHYSICAL ASPECTS P-particles from the tritium source release electrons from argon atoms in the cell, and these are accelerated towards the anode. They collide with argon atoms, and a proportion of these atoms is excited to metastable states. The mean free path of Ar atoms at s.t.p. is 1.1 x 10-5 cm and assuming that reaction (2a) is approximately 100 % eficient it follows that an Ar* atom will diffuse > 1/60 of the separation of the electrodes during its lifetime. Thus an electron formed in process (2a) effectively originates at the point of formation of the Ar* concerned. In this way one electron released near the cathode leads to an avalanche of electrons reaching the anode, and an exponential increase in the stationary concentration of electroils exists across the cell.The farther from the anode the initial electron is formed, the greater will be the avalanche. For this reason the tritium source is mounted on the cathode plate. Most of the ionization produced by the P-particles lies within 1 mm of the surface of the source. The factors which could influence the discharge in a particular cell are pressure, temperature, field-strength, flow-rate, and gaseous composition. Pressure and temperature affect the gas density which in turn changes the mean free path of electrons in a manner inverse to that of field-strength. These terms can thus be made to cancel out by referring to field-strength per unit gas density.As changes in ambient conditions were small their effects were ignored, and the results have been referred to field-strength only. On the assumption that the primary ionization current io is a flux of electrons leaving the cathode, the behaviour of the reaction cell should be that of a Townsend discharge, which can be expressed by the relationship,7 i = i~ exp (qy), where y-* is the average energy required to form an ion pair, and V is the potential applied to the cell. q is a function of gaseous composition, and also of field-strength. At very low voltages, no multiplication occurs and q = 0. With increasing voltage the ionization increases and q increases, tending towards a constant value. Thus at voltages in excess of the threshold for multiplication, (log i ) K V.Flow-rate would not be expected to affect the discharge unless the flow-velocity becomes comparable with the ionic drift velocities, and this was never the case. Nevertheless, there are at least two possible indirect causes of a dependence on flow-rate. Low flow rates could lead to high % conversions of reactant, thereby giving an apparent flow-rate effect. For this reason, and in order to avoid secondary reaction, conversions have been kept low wherever possible. Another possible source of dependence on flow-rate is surface contamination. If an impurity is being desorbed from the walls at a rate which is not proportional to the flow-rate then its concentration in the argon will depend on the flow-rate, and since the argon detector is quenched by permanent gas impurities,8 a dependence on flow-rate would then be expected.As the observed dependence on flow-rate disappeared86 ELECTRON MULTIPLICATION IN ARGON almost completely after the system had been heated to 100°C and flushed with argon for 12 h, this fed to the conclusion that the dependence arises mainly from the fast cause. In order to discuss the effect of gaseous composition it is necessary to consider all the possible types of process which may occur in the reaction cell. The following processes may be added to (I), (2a), (5) and (6) : Ar* + B -+Ar + (uncharged species) Ar+ + B+Ar + B+ e-+B-+B- ge\ + B4B* +E- Ar* + Ar (or wall) +Ar + Ar (or wall) B* -+products B+-,(products)* B++ B-+(products)+ B--+(products)- B+ + cathode-+B or products (products)+ + cathode+products + ionization current (products)- + anode-+products + ionization current Ar+ + cathode-+Ar + ionization current (14) (1 5 ) 8- + anodejionization current (16) (1 7) where B* is any excited state of butane having energy less than 10-8 eV.Of the processes given, (3) will be insignificant when (5) and (6) occur to any appreciable extent, (4) is unlikely (and hence (12) and (15) are also unlikely), and (17) is an unimportant process. The recurring sequence of processes (9, (6) and (2a) is es- tablished from measurements of ion-current against concentration of butane as shown in fig. 3. Amplification of the ion-current by factors of -103 may be ob- tained, the amplification being increased by increase of field strength and by con- centration of butane up to a limit of about 1 part in 1000.At concentrations greater than this the ion current diminishes, an effect which is most probably due to the increasing interruption of the ion-multiplication sequence (Za), (5) and (6), by process (7). Fig. 3 also shows that the maximum ion-current occurs at higher concentra- tions as the strength of field is increased, suggesting that higher concentrations of butane are necessary to cause reaction (7) to suppress the ion-multiplication sequence when the strength of field is increased. Under conditions such that the ion-multiplication sequence is occurring, the most important sources of products are likely to be reactions (2) and (7). The charged species from reaction (2a) are likely to be those ions which are formed predominantly at an energy of 11.6 eV.With butane these are C3H;, C4Hio, C3Hi and C&Ii in the approximate ratio 63 : 17 : 9 : 8.9 Thus, whereas the contribution of excited states to the products in this work is likely to be similar to that of con- ventional radiation-chemical studies, the contribution of ions is likeIy to be slightly different in view of the much larger variety of ions produced in the mass spectro- meter (and presumably in radiation-cheinicaf studies) at electron energies of greater than 20eV. It has from time to time been assumed that, at concentrations of substrate less than that corresponding to the concentration giving maximum ion- ization current, the contribution of process (7) is insignificant.3~ 10 Our results show that this is not necessarily the case and that much depends on the strength of the electric field applied.E.COLLINSON, J . F. 3. TODD AND F. WILKINSON I I I t 10 iGO I,OOO l0,OOO concentration of butane (p.p.m. argon) FIG. X-Varia’iion of ionization current with concentration of n-butane in argon. A, 5700 ; B, 4800 ; C , 3900 ; D, 3150 ; E, 2300 ; Vcm-1; - - - - current corresponding to sparking. FIG. 4.-Gas chromatogram of products from n-butane. 1, injection of sample ; 2, pressure peak ; 3, C2H2 ; 4, C2H4 ; 5, C2H6 ; 6, C3H6; 7, C3H8 ; 8, C3H4; 9, iso-C4HlO (impurity) ; 10, C4H8-l ; 11, n-C4Hlo ( x 1/300) ; 12, trans-C4Hs-2 ; 13, cis-C4H8-2. Length of column 30 ft ; packing 15 % n-hexadecane on elite ; temperature 22°C ; flow-rate 60 ml/min.ELECTRON MULTIPLICATION I N ARGON CHEMICAL ASPECTS The products obtained from the passage of butane+argon mixtures through the reaction cell are hydrogen, methane, acetylene, ethylene, ethane, propylene, propane, allene, butene- 1, cis-butene-2, trans-butene-2 and higher molecular weight materials.A typical chromatogram of the products, other than hydrogen, methane, and the high molecular weight materials, is shown in fig. 4. The relative yields of the products depend markedly on the conditions of operation of the cell. EFFECT OF CHANGING FIELD-STRENGTH Fig. 5 shows that the products fall into two classes with respect to changing field-strength. Acetylene, ethane and propane show an increase of ionic yield (as do also hydrogen and methane) whereas ethylene, propylene, and 1-butene show a decrease of yield, as the field strength increases.We conclude that these two FIG. 5.-The effect of field strength on the ionic yields of the products from n-butane. Concentration of butane 200 p.p.m. Ar. A, C2H4; B, C3H6 C, C4H8-1; D, C2H2; E, C3H8; F, C2H6. classes of product stein from at least two different classes of primary species, and we suggest that those products of which the yields decrease as the strength of field increases arise mainly from excited species, whereas the others stem mainly from ionic species, or from species arising from process (2b). A mechanism which involved mainly the production of ions by processes (2), (5) and (6) would be expected to give the same amount of reaction per ion pair, however many ions were produced. On the other hand, the ionic yield of a product arising from an excited state may fall with increasing field if the result of this in- crease is to increase total ionization without giving rise to a parallel increase in the number of excited states formed.This effect seems probable since at higher field strengths the average energy of the electrons in the cell will be increased, with aE. COLLINSON, J . F. J. TODD AND F. WILKINSON 89 consequently greater overlap of the curve for energy distribution of electrons with that for probability of excitation of argon, and less overlap with the corresponding curve for butane. Application of eqn. (i) has shown that values of q-1 of several hundreds of eV per ion pair may arise. Of greater interest, however, is the average energy required to form one Ar* atom, since it may be presumed that a proportion of Ar* atoms are lost by de-excitation with ground-state argon atoms rather than by ionizing butane molecules.Furthermore, only a proportion of butane molecules to which energy is transferred from Ar* will in fact be ionized, the fraction being 0.36.11 The alternatives are represented by processes (20) and (2b). Thus the fraction of Ar* atoms reacting by process (2a) is given by the expression k2,[Ar*][B](k2,[Ar*][B] + k2b[Ar*][B] + k, [Ar* ICArJI- ' = kZ,/(kz, + k2b) = 0.36,11 but the exact value of k&2, is not known. However, the ratio of the rates for the ionization of argon in trace amounts by metastable neon atoms, and the de-excitation of the latter by ground-state neon atoms, is about 4000.12 Since the difference between the excitation level of neon and the ionization potential of argon is similar to the difference between the corresponding values for Ar* and butane, we shall assume that, with suitable allowance for differences in cross-sectional area (0) for the various species, a factor of the same order may be applied in this system.We thus write Substitution of this quantity into eqn. (ii) gives a factor which, when multiplied by q-1 leads to a value for (y*)-l, i.e., the energy expended per Ar* produced. Under conditions giving (q)-l = 388 eV, (q*)-l was found to be 46 eV. From fig. 6 it is clear that the energy usage per Ar* is by far the largest at the lowest potentials. This result is, therefore, consistent with the suggestion that there is a higher proba- bility of direct excitation of butane at the lower field-strengths.A further argument in support of there being a greater chemical contribution of direct excitation at lower field-strengths is provided by the behaviour of the ionic yield for disappearance of butane. The values obtained for (- kf/N)n-butane are inaccurate, but even allowing an error of +50 :< they can be very large, as may be seen from fig. 7. Nevertheless it is clear from fig. 7 that only at the lower field- strengths are the values of ( - k f / N ) c , ~ , , so high that they cannot readily be ex- plained on the basis of ionization alone. A chain mechanism might account for such high yields but a greater contribution of excitation at lower potentials seems more probable, and in line with the arguments given above.If the ionic yield for conversion of butane at low field-strengths is thus to be attributed mainly to excitation processes, the ionic yields of products which result from such processes would be expected to show a similar dependence on field-strength. The three unsaturated products ethylene, propylene, and butene-1 do show this dependence, as exemplified by the results for ethylene in fig. 8. It could be argued that the effect of a decrease of concentration on the ionic yields of these products should be similar to an increase of field-strength, but the scatter of the experimental points does not justify drawing specific conclusions from the observed effect of concentration on the yields.90 ELECTRON MULTIPLICATION I N ARGON There is less certain evidence to support the view that products for which the ionic yields do not decrease with increasing field strength arise principally from ions.The arguments given earlier suggested that at higher applied voltages the contribution of ionic processes should increase, but this does not imply an increase X 120 CI- 80 \X X ----X--X-X-X-X-- x x' c W 60 - f 1 I I I 4 ? ~ ~ ~ I 2 0 0 1400 1600 !8OC 2000 volts x , 120 parts of n-butane per million of argon. FIG. 6.--(,,3)-1 as a function of potential applied to the reaction cell. A, 1280 ; 0 , 5 5 5 ; T7,ZSO ; 4 tL 80- +* x 0 $4 4a 3 k v) Y 6 0 8 3 'i5 0 40- E 1 2 0 - d cu 0 0 FIG. - L - L L I 1 I I parts of n-butane of the ionic yield with increasing field, as was found experimentally (fig.9). Similar results to those for ethane were found for propane and acetylene, and in all cases there exists a marked parallelism between the ionic yield and the ion current as functions of the field, which lends general support to the existence of a correlation between formation of ions and these products.E. COLLINSON, J . F. J. TODD AND F. WILKINSON 91 A possible reason for the increasing ionic yield of the paraffins with increasing field is hydrogenation of the olefinic products as the % conversion of the reactant increases. However, an increase in the propane or ethane yield is not compensated by an equivalent decrease in the yield of propylene or ethylene. volts FIG. 8.-The dependence of ionic yield of ethylene on applied potential for varying concentrations of n-butane.0,2000 ; A, 1280; 0,555; V, 280; x , 120 parts of butane per million of argon. v) I I I I I I I - I 5 Ls 1003 1200 l4GO 1600 I800 zoo0 volts FIG. 9.-The dependence of ionic yield of ethane on applied potential for varying concentrations of n-bu tane . 0,2000 ; A , 1280 ; fly 555 ; V, 280 ; x , 120 parts of butane per million of argon. Other possibilities for the increasing ionic yields of some products as the field in- creases are : (i) increasing production of different primary ions at higher fields, (ii) space charge effects, and (iii) effects arising from small concentrations of im- purity in the gas stream. An indication of the participation of higher excited levels of argon is provided by the fact that methane (ionization potential 13-1 eV) causes a small increase in ion current when added in small amounts to the argon stream passing through the reaction cell with a high field applied.This effect may be enhanced by the existence near the cathode of regions of high field introduced by space charge. Numerical calculation based on the relation log icc V, and suitable values for electron and ion drift velocities, indicate that fields of up to twice the mean field over the cell may be established in this way. Nevertheless, the changes which occur in the mass spectrum of n-butane as the energy of the ionizing electrons is92 ELECTRON MULTIPLICATION IN ARGON increased from 11.6 to 15.8 eV 9 are not such as to readily explain the increasing ionic yields found on the basis of a change in the nature of process (24. At this stage it is not possible to say whether or not changes in the nature of process (2b) could account for the results.EFFECT OF OXYGEN The results reported above were obtained with gas mixtures which contained about 20 p.p.m. of oxygen. Owing to the difficulty of removing oxygen from the argon and from the walls of the various parts of the apparatus, we have not yet obtained results in complete absence of oxygen, but the effect of changing concentra- tion of oxygen has been examined. Two series of experiments were performed, one in which the reaction mixture contained a trace of added oxygen and another in which there was about five times as much oxygen as butane. In all cases, the most important high-molecular weight product which was formed was found, by mass spectral analysis, to be methyl ethyl ketone.The ionic yields of ethane and propane were significantly depressed by oxygen (and could be virtually eliminated at high concentrations of Oz), whereas those of hydrogen, methane, ethylene and propylene were much less affected, and that of the ketone was increased, as was also the yield for the disappearance of butane. It is clear that oxygen interferes with the processes occurring in the cell, though the effect produced by small added amounts of oxygen as compared with the effect of large concentrations suggests that the adventitious oxygen present did not alter the course of reaction appreciably. The comparatively small effect of oxygen on the yields of ethylene and propylene suggests that these arise from a " molecular " decomposition of an excited butane molecule.On the other hand, oxygen reduces the yields of ethane and propane, which on previous arguments are thought to stem from ions, but the ionic yield of the ketone formed falls with increasing field, as for those products which are thought to stem from excited states. This result leads to the conclusion that the ketone is most probably formed via excited oxygen molecules or oxygen atoms, formed in processes (20) or (21) or (22) : 0 2 + &;+ 0 + 0 - 0 2 + &e;-+ 0 + 0 + & - for which the energies required are 4 1-62,13 4.7 to 6*3,14 and 8.44 eV,ls respectively. Oxygen would thus be expected to reduce the mean energy of the electrons in the system, which will have the same effect as a reduction of field on the products thought to arise from ionization.RELATION OF THE RESULTS TO THOSE OBTAINED FOR BUTANE BY THE USE OF OTHER TECHNIQUES In order to compare the present results with published work, it is necessary to estimate relative ionic yields. To convert the readings of the detector to these quantities, sensitivity factors must be employed for each of the products, since these have not all been calibrated. Factors were deduced from the results obtained by Yamane 16 for a static system, and all the yields have been normalized to a yield of ethylene = 100. A feature of the argon system is that the relative yields of pro- ducts vary according to the operating conditions applied to the cell. In consequence, it is useful to select data giving the closest fit with published results and to compareE. COLLINSON, J .F. J. TODD AND F. WILKINSON 93 the conditions under which the two corresponding sets of results were obtained. Relevant information is presented in table 1. The relative yields calculated from the mass spectrum correspond most closely with those for run 1 of mixture VI, i.e., at a potential of 2100 V, and though there is a discrepancy in the ratio of C2 products/C3 products, this may result from an error in the assumed cracking pattern, since the abundance of the fragment of mass 43 varies rapidly over the range of energy 11-12 eV. The ratio propylene/propane is nevertheless almost the same in both systems. The theoretical yields from the breakdown of ions are thus most closely followed by the yields of products from TABLE 1.-YIELD OF PRODUCTS FROM THE BREAKDOWN OF Il-BUTANE, RELATIVE TO ETHYLENE = 100 products H 2 CH4 reaction systems xenon resonance 2000 140 photolysis 18 calculation * from mass spectro- 2,493 405 metric data cr-radiolysis 17 643 85.7 electron multiplication mixture VI, run 1 (120 p.p.m., mixture VI, run 8 ,, (120 p.p.m., mixture 111, run 5 ,, (555 p.p.m., mixture 111, run 6 ,, (555 p.p.m., no data 2100 v) 1600 v) 1280 V) 1250 V) C2H2 200 - 37.1 44 25 18 14 c2H4 100 100 100 100 100 100 100 C2& 400 1007 93 999 420 152 90 c3H6 140 54 13 8 9 13 13 60 537 25 107 51 26 12 total = 600 total = 805 5 nodata 4 Y Y 5 Y 3 * These figures have been calculated on the basis of the mass spectrum of butane for 11.6 V electrons 9 by the method of Futrell.19 electron multiplication under conditions in which ionization is expected to be im- portant.In contrast, the relative yields from the xenon resonance photolysis of butane correspond most closely with those for run 6 of mixture 111, and would probably correspond even more closely with results obtained at a lower applied potential. Ethylene is an exception to this generalization since it apparently is formed in greater yield by the electron multiplication decomposition. However, the degree of conversion in the xenon photolysis was high, and this may have led to diminished yields of some unsaturated products. Apart from the yields of ethylene, the results of the photolysis thus correspond most closely with those results of electron multiplication in which excitation is expected to be predominant.The yields of ethane and propylene from the a-radiolysis of gaseous butane compare well with the results of mixture 111, run 6 (1250 V) whilst the ratio of propylene to propane is similar to that of run 5 (1280 V). Apparently the radiolysis corresponds most closely to an intermediate region of applied potential (fig. 8) where both ionization and excitation are thought to be important processes.94 ELECTRON MULTIPLICATION IN ARGON We thank Prof. F. S . Dainton for his advice and encouragement and for pro- viding facilities for the work within his department. The work was financed under contract by the National Physical Laboratory, and our thanks are due to the Director for permission to publish. J. F. J. T. would also like to acknowledge receipt of a maintenance grant from the Department of Scientific and Industrial Research. 1 Penning, Naturwiss., 1927, 15, 818. 2 Penning and Addink, Physica, 1934,1, 1007. 3 Lovelock, J. Chromatography, 1958, 1, 35. 4 Lovelock, Gas Chromatography, ed. Scott (Butterworths, London, 1960), p. 26. 5 Pratt and Purnell, Ind. Eng. Chem. (Anal.), 1960, 32, 1213. 6 Berry, Gas Chromatography, ed. van Swaay (Butterworths, London, 1962), p. 321. 7 see, e.g., Loeb, Busic Processes of Gaseous EZectronics (University of California Press, Berkeley, 8 ref. (4), p. 24. 9 Omura, Bull. Chem. Sac. Japan, 1961,34, 1227. 10 Littlewood, Gas Chrornatogruphy (Academic Press, New York, 1962). 11 Platzman, J. Physique Rud., 1960,21, 853. 12 Jesse and Sadauskis, Physic. Rev., 1955,100, 1755. 13 Massey and Burhop, Electronic and Ionic Impact Phenomena (Oxford University Press, Oxford, 14 Field and Franklin, Electron Impact Phenomena (Academic Press, New York, 1957), p. 40. 15 Lassettre, Radiation Res., 1959, suppl. 1, 530. 16 Yamane, J. Physic. SOC. Japan, 1960,15, 1076. 17 Back and Miller, Trans. Faraday SOC., 1959, 55,911. 18 Sauer and Dorfman, J. Chem. Physics, 1961,35,497. 19 Futrell, J. Amer. Chem. Soc., 1959, 81, 5921. 2nd ed., 1959, p. 665. 1952), p. 258.

ISSN:0366-9033    

DOI:10.1039/DF9633600083

出版商:RSC    

年代:1963

数据来源: RSC

摘要:

Determination of the Number of Separated Ion Pairs Produced in the Irradiation of a Liquid BY A. 0. ALLEN AND ANDIUES HUMMEL Chemistry Dept., Brookhaven National Laboratory, Upton, L.I., New York, U.S.A. Received 4th June, 1963 The degree of ionization of irradiated hexane in the absence of an applied field has been studied by means of determinations of the electrical conductivity in extremely low fields at known radiation intensities, and of the ionic mobilities. The number of separated ion pairs formed by 1.5 MeV X-rays is 0.09 per 1OOeV absorbed by the hexane, with a probable uncertainty of 50 %. This low value indicates that practically all sub-excitation electrons are captured by their parent positive ions before they can escape into the bulk of the liquid. The electrical conductivity of irradiated liquids has been much studied.The curve of current against voltage bends over with increasing voltage as in an irradi- ated gas, but instead of approaching a saturation value, the current in liquid continues to increase until breakdown occurs at very high voltages. Most interest has been in the high-voltage region in which the applied field is apparently interfering with the normal process of initial recombination. Mohler and Taylor 1 studied the curve for carbon disulphide and extrapolated the data from moderately high field strength to infinite voltage, obtaining a total ion yield approximately equal to that determined from the saturation current in carbon disulphide vapour. Of perhaps more interest is the number of ions set free by irradiation of a liquid in the absence of an applied field.This number does not appear to have been determined. Its value should offer a clue to the important question of how many subexcitation electrons are able to escape primary recombination with positive ions in the liquid. The present paper presents data on this point for liquid hexane irradiated with hard X-rays generated at 1500 kV. PRINCIPLE OF THE METHOD The method essentially makes use of the limiting conductivity of the irradiated liquid at zero voltage, i.e., the initial slope of the current-voltage curve, which is linear as long as the current is so low that the number of ions going to the elec- trodes is very small compared to the number which are formed by the irradiation and disappear by general volume recombination.Under these conditions, the conductivity is equal to the ionic concentration multiplied by the mobility, whence icL/VS = uC, where C is the number of ions of either sign per cubic centimetre, u = zc+ + 1 ~ - is the sum of the mobilities of the positive and negative ions, ic the current in the conductivity cell expressed in ions/sec (=A x 6.24 x 1018), V the applied voltage, L the distance between the parallel electrodes and S their area. At steady state the rate of ion generation equals the rate of ion disappearance by volume recombination : IG/100 = kC2, (2) 9596 NUMBER OF SEPARATED ION PAIRS where I is the dose rate in eV cm-3, G = lOO/W is the number of ions of either sign formed per 100eV of energy dissipated in the liquid by the X-rays, and k is the ion recombination coefficient in cm3/ion-sec.From (1) and (2), we find G kizL2 100 I u v2s2' -=- (3) To evaluate k/u, the rate of decrease of the current is determined when the irradi- ation is suddenly interrupted ; then - dC/dt = kC2. Integrating and substituting from (1) : 1 1 kL - - - + - t , - i, i," uVS (4) where i: is the steady-state current. the target current it in the X-ray generator, to which it is proportional : Experimentally, the dose rate is monitored by I = ai,. ( 5 ) Determination of the ion yield G then requires four separate determinations: (i) a dosimetry measurement to determine a ; (ii) a determination of the drift mobility of the ions, us and u- ; (iii) a determination of ic as a function of I and V-if the as- sumptions are correct, ic should be exactly proportional to Y and Z* ; (iv) determin- ation of the rate of current decay when the X-ray beam is suddenly interrupted -if the assumptions are correct, this rate should be exactly second-order.EXPERIMENTAL ION MOBfLITIES Hexane is distilled into a glass cell, the upper end of which consists of a thin plate of beryllium, 2-2cmdiam., cemented to the glass. Parallel to this is a platinum electrode, which is split into a central circle, 1-5 cm diam., and a 0.25-cm guard ring. A metal bellows, sealed to the glass, allows the interplate distance to be varied between 0.5 and 3 cm. The cell is placed with the beryllium plate beneath the beryllium window of a Picker X-ray machine, which is run at its lowest feasible peak voltage, about 6 kV.Ionization of the hexane is thus confined to a region within about 2 mm of the beryllium plate. In operation, the X-rays are turned on for a brief time. Then actuation of a relay switches off the X-rays ; applies a potential, which may be adjusted from 100 to 2000 V positive or negative, to the beryllium plate ; and simultaneously triggers an oscilloscope which is arranged to respond to the current passing between the beryllium electrode and the lower, grounded, electrode. Fig. 1 shows typical oscilloscope tracings of the current as a function of time. Theoretically, one expects an initial surge, composed of a charging current. due to the capacitance of the cell, and an ion current generated as the ions of opposite sign to the charge on the beryllium are drawn to that electrode.The surge quickly dies away as all these ions disappear by neutralization at the electrode or by recombination. A layer of ions of the other sign will be left, and will slowly drift in the field toward the lower electrode. A constant drift current should then appear, which should begin to decrease and tail off to zero as the ions reach the electrode? The quantity measured is the average drift time required for the ions to reach the lower electrode ; the mobility is equal to the distance from the ionized region to the lower electrode, divided by the average drift time and by the field strength. In practice, the surge quickly dies down as expected, but the drift current is not constant ; it rises with time to a maximum betore tailing off.We do not understand the reason for this maximum. The average drift time is taken as the time from application of the voltage to the time of the last inflection point. These times are reproducible to f5 %; and they are within this same precision inversely proportional to the applied field, when the interelectrode distance is held constant. Since we do not understand the cause of the maximum in the current, the absolute significance of these drift times might be questioned. However, we haveFIG. 1 .-Typical oscilloscope traces of current against time in ion mobility measurement. Positive ions, L = 1.1 cm. Top, V = 1000, time-scale (left to right), 0.5 sec per division ; time to inflection [To face page 96. point, 2-6 sec. Below, V = 500, 1 sec per division, time to inflection point, 5.0 sec.A .0. ALLEN A N D A . HUMMEL 97 also changed the interplate spacing, and calculated the mobility by dividing the diflerence in the interplate spacing by the diflerence in the times to reach the inflection point, when the field strength in V/cm was the same. This difference method would seen1 to reduce the absolute uncertainties involved. The difference method, however, gave the same results for the mobility (within f5 %) as the use of the absolute time to attain the inflection point. CONDUCTIVITY MEASUREMENTS The radiation source was a Van de Graaff electron accelerator ; the X-rays were generated at 1500 kV on a gold target. Most of the conductivity data were obtained either with the same cell used for the mobility determination, or with another parallel-plate cell with adjust- able interplate distance, the electrodes being of aluminium.Currents through the cell, which were generally 10-13-10-12 A, were read by a Cary vibrating reed electrometer, using a calibrated lOlo-ohm resistor. The electrometer was shielded from radiation by lead bricks. The output of the electrometer was read on a millivoltmeter, a recorder or an oscilloscope. Either of the latter two could be used to follow the decay of the cell current with time when the radiation was interrupted, while the voltage across the cell was maintained. Instantaneous interruption of the electron beam in our generator is accomplished by application of a small potential to an appropriate point within the electron gun, which stops the beam at its source.Dosimetry was done in three ways. (i) Ampoules of ferrous sulphate were placed within the cell, and the oxidation rate determined as a function of target current. This method was feasible only for intensities orders of magnitude higher than those used in most of the hexane work. (ii) The cell with A1 electrodes, filled with air, was used as an extrapola- tion ionization chamber. The saturation ionization current per unit volume was determined at constant target current for different interplate distances, and extrapolation to zero distance provided a measure of the rate of energy absorption in aluminium. (iii) A portable X-ray Dosemeter by Electronic Instruments Ltd., fitted with a 350-cm3 ionization chamber, read the radiation intensity directly in r/sec when the chamber was placed with its centre as near as possible to the point ordinarily occupied by the centre of the conductivity cell.These methods agreed within 3 % (under the assumption that air, water and aluminium absorb energy from these X-rays proportionally to their electron densities), and showed that the dose rate and target current are proportional over five orders of magnitude. The dose rate to hexane in the cell was calculated to be 8.0~ 1016 eV cm-3 sec-1 A-1 of target current. PURIFICATION OF HEXANE After preliminary purification, the hexane was distilled from sodium or sodium-potassium alloy in vacuo into a bulb fitted with metal electrodes. A potential of about 1000V was applied for some hours to get rid of residual conductive impurities by electrolysis.The hexane was then distilled into the conductivity cell. If the resistance of the hexane was still thought too low, it was poured back into the electrolysis bulb for further treatment. The specific resistances of the samples used lay in the range 1015-1017 ohm cm, when measured at 300 V/cm. Vapour-phase chromatographic analysis of the hexane preparations used here showed a small peak corresponding to the presence of about 0.3 % of methylcyclopentane. One preparation, called F2, was found, after sonie electrical measurements were made, to give a faint yellow colour to sulphuric acid, indicating the presence of unsaturation. The chro- matogram was not noticeably changed, however, by extracting the unsaturates with H2SO4. The unsaturated material was probably mostly hexene-2, which appears at the same point as methylcyclopentane in chromatography, so that its peak would be covered by that of the larger impurity.It must have been present to an extent less than 0.03 %, otherwise the size of the methylcyclopentane peak would have been noticeably decreased by its extraction. A later preparation, called F3, was properly treated with sulphuric acid before distillation to remove unsaturates. Research-grade hexane (called RG) was also used ; like F3, it apparently contained some methylcyclopentane but no unsaturates. Preparations F2 and F3 gave very different results for the ionic mobility, despite the fact that the concentlation of unsatur- ates present in one but not the other was very small. D98 NUMBER OF SEPARATED ION PAIRS RESULTS AND DISCUSSION MOBILITIES Table 1 shows results of our mobility determinations, together with values froin the literature.Our values for the purer preparations were the same, whether the material was air-saturated or vacuum-distilled. The negative ions in this experi- ment move tremendous distances on the molecular scale, and their charge should shortly attach itself to any trace of oxygen remaining in the evacuated liquid; so it seems probable that in both cases we are dealing with negative oxygen ions. The TABLE IONIC MOBILITIES IN HEXANE AT ROOM TEMPERATURE ref. hexane air mobilities (cm2/V sec) x 104 preparation present U- u+ RG no 7.6 3.8 present work F3 Yes 7.5 4.0 present work F2 Yes 1.8 3-3 present work 3Q - no 10 - - ? 9-2 5.8 4 - ? 13 4.1 2 - no 12.5 - 5Q a negative ions generated by a pulse of ultra-violet light incident on the cathode.presence of a trace of unsaturation (preparation F2) causes a remarkable decrease in the negative ion mobility. Presumably 0; attaches to hexene, forming a large, sluggish ion. Literature values for u- are not in good agreement but all are slightly higher than ours. It is possible that their preparations were purer. The nature of the negative ion in hexane has been much argued; because of the difficulties of getting rid of the last traces of oxygen, it may be an oxygen ion in all cases. For our calculation of G, we use our own determination, and take u = u+ + u- as 1-14 x 10-3. CONDUCTIVITY Fig. 2 is a typical plot of cell current against applied voltage, under constant radiation intensity.The current varies linearly with voltage, passing smoothly through zero. The current at zero appiied voltage is caused by irradiation of the lead-in wires, and varies in magnitude and sign when the cell is removed and re- placed. Fig. 3 is a typical plot of the slopes of such conductivity lines against the target current (which is proportional to the radiation intensity), on a log-log scale. A line of theoretical slope 3 is drawn through the points. A line of slightly greater slope, say 0.54, would be a better fit, but there is no doubt that, for any particular cell filling, the conductivity is at least very nearly proportional to the square root of the radiation intensity. For the determination of the yield of ions, the important quantity is just this proportionality constant, or its square, which we may express as u2/it, where IC = icL/VS and it is the target current.This ratio unfortunately has so far proved to be not very reproducible, even for different fillings of the cell from the same source. Values for apparently good preparations ranged from 1.4 x 10-21 to 2.9 x 10-21 Q-2 cm-a/A. We suppose the variations to arise from variable traces of impurity, perhaps arising in part from radiolysis as well as im- perfect cleaning of the cell. Since impurities tend to lower u and hence ii, the higher values are perhaps more reliable; but some weight should be given to the more numerous lower values. Further work is required on comparison of carefully purifiedA .0. ALLEN A N D A. HUMMEL 99 samples in thoroughly cleaned cells. We tentatively take ~ 2 / & as 2.2 x 10-21 with a probable error of 0.7 x 10-21. 4 2 0 m H 0, 2 -2 - 4 -6 -0.5 0 t 0.5 V FIG. 2.-Cell current against applied voltage, hexane, 1500 kV X-rays, dose rate 4.5 x 1010 eV cm-3 sec-1, L = 0.131 cm. m r( E: X t I I I 1 I11111 I I I I l l l l l I I I 1 1 1 1 1 1 2 4 5 2 4 6 2 4 it, A FIG. 3.-Conductivity of irradiated hexane against target current in X-ray generator. DECAY CONSTANT klu If the decay of the cell current (after the radiation is interrupted but voltage across the cell maintained) is due to second-order ion recombination, then a plot of l/ic100 NUMBER OF SEPARATED ION PAIRS against time should be a straight line. We have made many such plots, of which fig.4 is typical. The points are read from a recorder tracing of the current. Values of k/u in Vcm are obtained from the slopes of such lines by multiplying by the 0 10 2 0 30 40 t (sec) 5 0 FIG. 4.-Reciprocal cell current against time after interruption of radiation, V = 5.0, L = 0.35 cm. applied voltage and dividing by the cell constant L/S and by 6.24 x 1018. The resulting figures are not in very good agreement, even for successive readings taken with the same cell. Typical values, taken from the better runs, are 1-31, 1-43, 1-47 x 10-6 V ern. There is a simple theory6 which should give an accurate value of k/u for solvents of low dielectric constant. Ions should combine whenever they approach within a critical distance rc, at which their mutual Coulomb energy equals the energy of thermal agitation kT.Then re = e2/ckT, where e is the electronic charge and E the dielectric constant, 1-89 for hexane. In our case, rc = 260& which is so large that the actual size, shape and character of the ions can have little effect on the interaction. Then the rate constant k = 471rC(D+ + D-), where D+ and D- are the diffusion coefficients of the re- acting ions. There is also the general relation D/u = kT/e, Combining, we have klu = 4nelc. This quantity, ex- pressed in V cm, is equal to 0.96 x 10-6. Our measured values are around 50% higher than the theoretical value. It is difficult to see why the true ion recombination coefficient should differ appreciably from the theoretical value. The ion concentration in our experiments is of the order of only 109 6111-3 or less, so that " ionic strength " effects can be of little importance.Possibly the experimental values are vitiated by polarization effects. As decay proceeds while current continues to pass, a layer next to the elec- trodes is depleted of ions more rapidly than the bulk. The electrical potential thus might fall appreciably more rapidly across these layers than across the rest of the solution; and the field acting on the bulk of the ions would be less than V/L, to an extent which should increase with time. Then the ratio of the measured current to the ion concentration should decrease with time, and the current should decay more rapidly than the ion concentration. Provisionally, we assume that the true value of Jc/u is close to the theoretical, 0.96 x 10-6 V cm.THE ION YIELD Putting the above numbers into eqn. (3), we find G = 0.09, with a probable error of perhaps 50 %. If the experimental value of 1-4 x 10-6 for klu were used, G would be increased to 0.13. While this work was under way, a note by Freeman 7 appeared, giving a valueA . 0. ALLEN AND A. HUMMEL 101 of G for hexane of 0.2. This was calculated from an extrapolation to zero field of the conductivity observed at very high fields, a sort of extrapolated saturation current, the physical significance of which appears to us very dubious. Similar methods were used in two papers 8 9 9 presented at a recent meeting; their results for hexane, calculated in terms of G, were 0-09 for X-rays of various energies and 0.07 for tritium 0-rays.The present method, though difficult to apply with pre- cision, seems to us to be in principle the correct one. The number of ions escaping initial recombination in liquid hexane is about 4 % of the number formed in gases, and presumably also formed transiently in the liquid. Most or all of the observed charge separation may be attributed to the higher-energy delta rays. The beginning of a delta-ray track must have one excess positive charge, the end must have one excess negative. If the two ends are separ- ated by more than rC = 260& then a separated pair of ions must result. It appears that, apart from this delta-ray effect, almost none of the sub-excitation electrons produced by ionization escape recombination, or become thermalized at a distance greater than 260A from a positive ion.This result may seem surprising since it is difficult to imagine a mechanism for the rapid thermalization of electrons in hexane below the molecular vibrational energy level of around 0.1 eV. The mechan- ism of Frohlich and Platzman 10 should be of little importance in a non-polar liquid like hexane. It seems more likely that the great majority of the electrons are never completely thermalized, but are rapidly reduced, by loss of energy to molecular vibrations,ll to some epithermal energy E (of about 0.2 eV) while still at a distance from the positive ion small compared to e2/&E. They must then, as pointed out by Samuel and Magee,l2 return to the positive ion and neutralize it, the energy of neutralization being dissipated in the form of molecular fragmentation and of heat. This research was performed under the auspices of the U.S. Atomic Energy Commission. 1 Mohler and Taylor, J. Res. Nat. Bur. Stand., 1934, 13, 659. Taylor, J. Res. Nat. Bur. Stand., 2 Gzowski and Terlecki, Acta Phys. Polon., 1959, 18, 191. 3 Chong and Inuishi, Tech. Reports Osaka Univ., 1960, 10, 545. 4 Gzowski, Z. physik. Chem., 1962,221, 288. 5 Le Blanc, J. Chem. Physics, 1959, 30, 1443. 6 Debye, Trans. Electrochem. Soc., 1942, 82, 265. 7 Freeman, J. Chem. Physics, 1963, 38, 1022. 8 Adamczewski and Januszitis, Conf. Electronic Processes in Dielectric Liquids (The Institute of Physics and The Physical Society, University of Durham, 23-25 April, 1963), paper no. 6.3. 9 Gzowski and Chybicki, (Conf. Electronic Processes in Dielectric Liquids (The Institute of Physics and The Physical Society, University of Durham, 23-25 April, 1963), paper no. 6.4. 1936, 17, 557. 10 Frohlich and Platzman, Physic. Rev., 1953, 92, 11 52. 11 Chen and Magee, J. Chem. Physics, 1962,36, 1407. 12 Samuel and Magee, J. Chem. Physics, 1953,21, 1080.

ISSN:0366-9033    

DOI:10.1039/DF9633600095

出版商:RSC    

年代:1963

数据来源: RSC