摘要:

GENERAL DISCUSSIONS OFTHE FARADAY SOCIETYDate190719071910191 1191219131913191319141914191519161916191719171917191819181918191819191919192019201920192019211921192119211922192219231923192319231923192419241924192419241925192519261926192719272927SubjectOsmotic PressureHydrates in SolutionThe Constitution of WaterHigh Temperature WorkMagnetic Properties of AlloysColloids and their ViscosityThe Corrosion of Iron and SteelThe Passivity of MetalsOptical Rotary PowerThe Hardening of MetalsThe Transformation of Pure IronMethods and Appliances for the Attainment of High Temperatures in aRefractory MaterialsTraining and Work of the Chemical EngineerOsmotic PressurePyrometers and PyrometryThe Setting of Cements and PlastersElectrical FurnacesCo-ordination of Scientific PublicationThe Occlusion of Gases by MetalsThe Present Position of the Theory of IonizationThe Examination of Materials by X-RaysThe Microscope : Its Design, Construction and ApplicationsBasic Slags : Their Production and Utilization in AgriculturePhysics and Chemistry of ColloidsElectrodeposition and ElectroplatingCapillarityThe Failure of Metals under Internal and Prolonged StressPhysico-Chemical Problems Relating to the SoilCatalysis with special reference to Newer Theories of Chemical ActionSome Properties of Powders with special reference to Grading byThe Generation and Utilization of ColdAlloys Resistant to CorrosionThe Physical Chemistry of the Photographic ProcessThe Electronic Theory of ValencyElectrode Reactions and EquilibriaAtmospheric Corrosion.First ReportInvestigation on Oppau Ammonium Sulphate-NitrateFluxes and Slags in Metal Melting and WorkingPhysical and Physico-Chemical Problems relating to Textile FibresThe Physical Chemistry of Igneous Rock FormationBase Exchange in SoilsThe Physical Chemistry of Steel-Making ProcessesPhotochemical Reactions in Liquids and GasesExplosive Reactions in Gaseous MediaPhysical Phenomena at Interfaces, with special reference to MolecularAtmospheric Corrosion. Second ReportThe Theory of Strong ElectrolytesCobcsion and Related ProblemsLaboratoryElutriationOrientationVolumeTrans. 33678999101011121213131314141414151516161616171717171818191919191920202020202121222223232Date1928192919291929193019301931193219321933193319341934193519351936193619371937139819381939193919401941194119421943194419451945194619461947194719471947194819481949194919491950195019501950195119511952195219521953195319541954GENERAL DISCUSSIONS OF THE FARADAY SOCIETYSubjectHomogeneous CatalysisCrystal Structure and Chemical ConstitutionAtmospheric Corrosion of Metals.Third ReportMolecular Spectra and Molecular StructureOptical Rotatory PowerColloid Science Applied to BiologyPhotochemical ProcessesThe Adsorption of Gases by SolidsThe Colloid Aspects of Textile MaterialsLiquid Crystals and Anisotropic MeltsFree RadicalsDipole MomentsColloidal ElectrolytesThe Structure of Metallic Coatings, Films and SurfacesThe Phenomena of Polymerization and CondensationDisperse Systems in Gases : Dust, Smoke and FogStructure and Molecular Forces in (a) Pure Liquids, and (b) SolutionsThe Properties and Functions of Membranes, Natural and ArtificialReaction KineticsChemical Reactions Involvhg SolidsLuminescenceHydrocarbon Chemistry35The Hydrogen Bond 36The Oil-Water Interface 37The Mechanism and Chemical Kinetics of Organic Reactions in LiquidSystems 37The Structure and Reactionsof RubberModes of Drug Action 39Molecular Weight and Molecular Weight Distribution in High Polymers.(Joint Meeting with the Plastics Group, Society of Chemical Lndustry) 40The Application of Infra-red Spectra to Chemical Problem 41Oxidation 42Dielectrics 42 ASwelling and Shrinking 42 BElectrode Processes Disc.1Surface Chemistry. (Jointly with the Socikt6 de Chimie Physique atColloidal Electrolytes and SolutionsThe Interaction of Water andporous MaterialsThe Electrical Double Layer (owing to the outbreak of war the meetingwas abandoned, but the papers were printed in the Trmactiuns)38The Labile Molecule 2Bordeaux.) Published by Butterworth Scientific Publications, Ltd.Trans. 43Disc. 34Lip-Proteins 6Heterogeneous Catalysis 8Physico-chemical Properties and Behaviour of Nuclear AcidsSpectroscopy and Molecular Structure and Optical Methods of In-vestigating Cell Structure Disc.Electrical Double Layer Trans.47Hydrocarbons Disc. 10The Physical Chemistry of Process MetallurgyCrystal Growth 5Chromatographic Analysis 7Trans. 46The Size and Shape Factor in Colloidal Systems 11Radiation Chemistry 12The Physical Chemistry of Proteins 13The Reactivity of Free Radicals 14The Equilibrium Properties of Solutions of Non-Electrolytes 15The Physical Chemistry of Dyeing and Tanning 16The Study of Fast Reactions 17Coagulation and Flocculation 18Volume2425252526262728292930303132323333343435353GBNBRAL DISCUSSIONS OF THE FARADAY SOCIETYDate1955195519561956195719571958195819591959I96019601961196119621962196319631964Subject VolumeMicrowave and Radio-Frequency Spectroscopy 19Physical Chemistry of Enzymes 20Membrane Phenomena 21Physical Chemistry of Processes at High Pressures 22Molecular Mechanism of Rate.Processes in Solids 23Interactions in Ionic Solutions 24Configurations and Interactions of Macromolecules and Liquid Crystals 25Ions of the Transition Elements 26Energy Transfer with special reference to Biological Systems 27Crystal Imperfections and the Chemical Reactivity of Solids 28Oxidation-Reduction Reactions in Ionizing Solvents 29The Physical Chemistry of Aerosols 30Radiation Effects in Inorganic Solids 31The Structure and Properties of Ionic Melts 32Inelastic Collisions of Atoms and Simple Molecules 33High Resolution Nuclear Magnetic Resonance 34The Structure of Electronically-Excited Species in the Gas-Phase 35Fundamental Processes in Radiation Chemistry 36Chemical Reactions in the Atmosphere 37For current availability of Discussionvolumes, see back cover

ISSN:0366-9033    

DOI:10.1039/DF964370X001

出版商:RSC    

年代:1964

数据来源: RSC

摘要:

I. REACTIONS AND PHOTOCHEMISTRY OF ATOMS AND MOLECULES Introduction to Chemical Aeronomy BY MARCEL NICOLET Centre National de Recherches de l'Espace, 3 Avenue Circulaire, Bruxelles 18 Received 12th March, 1964 Thirty-three years ago, Chapman 1 demonstrated that the dissociation of molecular oxygen is important above 100 km, and, therefore, that the photochemistry of atmospheric oxygen 2 is an important problem in aeronomy. However, the treatment of oxygen dissociation at high altitude must be examined 3 by studying the departure from photochemical equilibrium conditions. In fact, any detailed investigation requires a knowledge of aeronomic conditions in the various atmospheric regions. Theoretical studies are simplified by dividing the atmosphere into two parts : the homosphere, in which the composition of the principal constituents (N2 li 78 %, O2 N 21 % and Ar N 1 %) remains constant and the heterosphere, in which the dissocia- tion of oxygen and diffusion affect the air composition.If the temperature distribu- tion is introduced as the second aeronomic parameter, the homosphere is subdivided in three regions : the lowest region is the troposphere where the temperature decreases with height up to the tropopause. The tropopause has a temperature of about 220°K in the polar regions at a height of some 8 km and about 190°K at the equator for an altitude of about 17 km. The stratosphere is essentially that region where the tem- perature increases or at least does not decrease, with altitude. It extends from the tropopause to an altitude of about 50 km where the temperature reaches a peak of about 273°K.The third region belonging to the homosphere is the mesosphere situated between the stratopause (50+5 km) and the mesopause (85+5 km) where the temperature reaches a minimum as low as 160- 170°K. Above the mesopause, there is an increase of the temperature which has its largest gradient (up to 20" km-1) near 150 km. In this region, the thermosphere, the problems of the chemosphere change in that ionization must be considered along with dissociation. The vertical distribution of atomic oxygen makes it possible to understand the different roles of the three regions of the homosphere and the heterosphere. In the thermosphere atomic oxygen is more abundant than molecular oxygen above about 120 km.In the mesosphere, atomic oxygen is more abundant than ozone for an atmosphere illuminated by the sun. The day and night-time conditions are different. In the stratosphere, atomic oxygen is always less abundant than ozone, the presence of which depends on photochemical reactions. Finally, ozone is a pzrmanent element of the air mass in the troposphere and is subject to variations associated with advective and dynamical transport. After considering the principal constituents, it is necessary for chemical aeronomy to introduce the minor constituents. An inert gas such as helium (ratio 5-24 x 10-6 per volume) which has no chemical importance is, however, a tracer for atmospheric diffusion processes since it is not a minor constituent in the upper thermosphere.Several of the minor constituents observed at ground level (table 1) can play a role in 78 INTRODUCTION TO CHEMICAL AERONOMY the chemosphere. No systematic study of their abundance has been made at high altitudes and their behaviour is known from infra-red spectroscopic identifications or from chemical analysis at low levels. molecule TABLE MO MOLECULAR CONTENT OF MINOR CONSTITUENTS ratio by volume 3~ 10-3 10-5 to 10-2 10-7 to 10-8 2 . 5 ~ 10-7 5~ 10-7 5 x 10-8 to 2 x 10-7 1 . 5 ~ 10-6 5 x 10-10 to 2 x 10-8 remarks Mixed in the troposphere, small variation. Variable ; dissociation in mesosphere. Variable ; peak in stratosphere. Mixed in troposphere ; dissociation in stratosphere. Mixed in troposphere ; dissociation in stratosphere. Mixed in troposphere ; dissociation in thermosphere. Variable ; industrial.Variable ; industrial ; chemical origin in meso- sphere and thermosphere. The photochemistry of atmospheric water vapour was studied in considerable detail by Bates and Nicolet 4 after Meinel 5 discovered that the vibrational rotational bands of the hydroxyl radicle OH appear in the airglow with a total energy6 of about 3 ergs cm-2 sec-1. Such an emission arouses interest in the photochemistry of hydro- gen+oxygen compounds 7, not only of water vapour but also of methane and perhydroxyl radicles. The attack of methane by atomic oxygen was studied by Bates and Witherspoon 8 and there is little doubt that the concentration of methane in the mesosphere depends on collision processes involving atomic oxygen. The enteric fermentation corresponds to a production 9 of CH4 of at least 1010 molecules cm-2 sec-1.The chemical oxidation of methane in the stratosphere leads to a production of hydrogen, the accumulation of which is limited by the possibility of the upward transport into the mesosphere. Unlike oxygen, nitrogen is extremely difficult to dissociate and it is so stable that it remains in the molecular form up to very great altitude. This low degree of dis- sociation was suggested 10 in studies of nitric oxide which was considered to be an important ionic constituent of the terrestrial ionosphere. The photochemistry of tropospheric nitrous oxide has been investigated by Bates and Witherspoon 8 who indicated that this molecule is not a member of the main photochemical family of nitrogen oxides which were studied by Bates 11 and Nicolet 12, 13.Free nitrogen atoms in an oxygen atmosphere make possible a large number of reactions which are now studied in the laboratory. A difficulty in giving a systematic account of the chemical aeronomy has been the grevious lack of reliable basic data. Our knowledge concerning the experimental rate coefficients has increased rapidly, however, in recent years and systematic accounts can be found in several review papers presented at the symposium an aeronomy held in Berkeley in August 1963 : Three-body reactions by Barth ;14 reactions involving nitrogen and oxygen by Schiff 15 and aeronomic reactions involving hydrogen by Kaufman.16 Much progress must still be made in the elucidation of chemical re- actions for a complete application to aeronomy. It cannot be overemphasized that laboratory investigations under controlled conditions are of fundamental importance for a useful interpretation of space observations.SOLAR RADIATION AND ITS ABSORPTION A knowledge of the radiation available for dissociation in the atmosphere is required before conclusions can be reached regarding the relative importance of aeronomic processes. The principal gases in the thermosphere, molecular nitrogen,M. NICOLET 9 atomic oxygen and molecular oxygen, limit the penetration of solar radiation into the heterosphere at wavelengths iz < 796 A, A < 910 A and iz < 1025 A, respectively. Since absorption cross-sections are not less than 10-18 cm2 between 1000 A and 100 A, the solar radiation in this spectral range is absorbed above 100 km and ionizes N2, 0 and 0 2 .The total number of solar photons available at the top of the earth’s atmosphere is not greater than 2 x 1011 photons cm-2 sec-1, and corresponds to the number of ionizing processes in the E and P ionospheric layers. Molecules such as NOa, HzO, 0 3 , NzO, CH4 and OH can be neglected in the study of the ionization of the atmosphere since their ionization potential is greater than that of molecular oxygen. No radiation will be available to ionize these molecules below 100 km where they are subject to dissociation processes. At 1750 A (see fig. 1) where the Schumann-Runge continuum of molecular oxygen begins, the total number of photons available at the top of the earth’s atmosphere is 17 about 2 x 1012 photons cm-2 sec-1 and consequently this value also represents the total number of oxygen molecules which is dissociated in a vertical column of the atmosphere for iz < 1750 A.wavelength (A) FIG. 1. At 2420 A where the Herzberg continuum of 0 2 begins, the total number of solar photons is about 2.7 x 1014 photons cm-2 sec-1 and such a value corresponds to the maximum number of oxygen molecules which could be dissociated per cm2 sec-1 in a vertical column of the earth’s atmosphere. Thus the number of photodissociation processes of molecular oxygen D(O2) in the earth’s atmosphere is The maximum value which can be reached depends on ozone absorption which needs to be taken into account in estimating the dissociation rate in the Herzberg continuum.Ozone shows an important absorption which begins near 3500 A, extends below 3000 r f with rapidly increasing cross-section to a maximum at about 2550 A, and is still important in the Herzberg continuum, iz <2400 A. Due to the presence of O3 molecules, it will be shown that the rate of dissociation of 0 2 is strongly affected below the stratopause. Under these conditions, the dissociation of molecular oxygen must be studied before determining the behaviour of other constituents. 2 x lOlz< D(o2)<2.7 x 1014 cm-2 sec-1. (1)10 INTRODUCTION TO CHEMICAL AERONOMY The photodissociation process, 02+hv(;l5 1750A)+O(3P)+0(1D), (2) J,,(AI 1750 A) = 5 x sec” (3) leads to a photodissociation rate coefficient JoZ of at zero optical depth. The continuum of the Schumann-Runge system has an absorption cross-section varying from about 2 x 10-19 cm2 at the threshold with a peak not less than 10-17 cm2 between 1500 A and 1400 A.The penetration of solar radiation into the atmosphere is limited where the total content of 0 2 molecules is between 1019 cm-2 and 1017 cm-2. For example, a vertical column of 2 x 1018 0 2 molecules cm-2 (about 100 km) leads to photodissociation rate, Jo2(z- 100 km) = 2 x sec-l. (4) Thus the life-time of an oxygen molecule in the sun’s radiation field is very long at heights where photochemical equilibrium conditions should be applied if transport process are ignored. The penetration of solar radiation to greater depths than 1019 molecules cm-2 occurs only in various “ windows ” between 1225 A and 1 100 A.The most important “ window ” is situated at 1216 A of the important solar radiation, Lyman-a, for which the unit optical depth corresponds to a cross-section of the order of 10-20cm2. Since the number of photons available in Lyman-a is between 2 and 4 x 1011 photons cm-2 sec-1, the dissociation rate coefficient is ~ , , ( ~ y - a ) = 2 to 4 x 1 0 - ~ sec-l. ( 5 ) For an overhead sun, the dissociation rate coefficient at 75 km becomes Since the concentrations of molecular oxygen at 75 km 402) N 2 x 1014 cm-3, the minimum dissociation rate is still 2 x 105 cm-3 sec-1. A difficulty occurs, however, in the determination of oxygen dissociation in the mesosphere. This difficulty comes from the impossibility of obtaining a sufficiently accurate value for aeronomic purposes.The dissociation rate in the Herzberg continuum, particularly near 2000 A where the Schumann-Runge bands occur, is not sufficiently precise;18 we cannot enter into details here and an approximate distribution has been deduced (fig. 2). A plot of Jo, against height between 50 km and 100 km referring to an overhead sun indicates that .lo2 decreases by about a factor of 100 in this 50 km height interval. The effect of the Schumann-Runge continuum is apparent in the thermosphere and the decrease of Jo, in the mesosphere is related to the absorption in the Herzberg continuum. The photolysis of ozone is due to the absorption in the ultra-violet and the visible. It is customary to take absorption cross-sections as dissociation cross-sections and to adopt average numerical values for aeronomic purposes.It should be pointed out, however, that errors of about 10 % still seem to occur in recent data.19~ 20921 In applying average values, the total rate coefficient .loz at zero optical depth is about J , ~ = sec-l, (7) J,,(visible) = 3.5 x sec-l. (8) while the visible part of the spectrum leads to onlyM. NICOLET 11 It can be assumed that ozone and molecular oxygen are the two molecules which absorb solar radiation between 3000A and lOOOA, i.e., that the other constituents constitute a negligible role. Ide dissociation rate coefficient (sec-1) FIG. 2. I I I I I wavelength (A) FIG. 3. Photodissociation of water vapour, which begins around 2400 A, is related to the absorption of molecular oxygen. The absorption cross-sections is less than 10-20 cm2 at 1900 A (see fig.3) and increases rapidly in the spectral range of the12 INTRODUCTION TO CHEMICAL AERONOMY Schumann-Runge bands system. It reaches about 10-18 cm2 near 1800 A. Thus the photo-dissociation of H20 in the mesosphere is related to the complicated structure of the absorption spectrum of 0 2 . At the present time, only a rough estimate can be obtained. The total dissociation rate coefficient at zeroth optical depth JH,O is about which becomes about 10-6 sec-1 near the mesopause level after absorption in Schumann-Runge bands. It is still about 10-6 sec-1 in the middle mesosphere due to the effect of solar Lyman-a. With such low values for the dissociation rate coefficients it is difficult to consider photochemical equilibrium conditions for water vapour in the mesosphere.The life time of water vapour in the solar radiation field is sufficiently long to lead to departures from photoequilibrium conditions. Fig. 3 shows also the absorption cross-section of C02. Its photodissociation in the mesosphere is related to the effect of Lyman-a which leads to the following value of Jco,, Methane also has an absorption spectrum in the region of the Schumann-Runge continuum of 0 2 and its direct photodissociation in the mesosphere depends on the penetration of Lyman-a. At zero optical depth, it is Even if collisions involving atomic oxygen are effective as a loss process for methane in the mesosphere, photodissociation by Lyman-a at 75 km is sufficient to reduce its initial concentration to 50 % in about 3 days of 12 h.J~~~ = 10- sec- I , (9) Jco,(Lyman-a) = 3 x sec-l. (10) J,,,(Lyman-a) = 5 x sec-l. (11) PURE OXYGEN ATMOSPHERE The photochemistry of an oxygen atmosphere has been studied by a number of investigators (see, for example, ref. (4), (22), (23)) since Chapman 1 gave the complete set of equations. We follow here the analysis made by Bates and Nicolet.4 The dissociation of oxygen obtained by photodissociation O2 + hv(A < 2420 A)+O + 0; coefficient J02, (12) (1 3) (14) (1 5) (16) is followed by the three-body recombination, and by The bimolecular process occurs, therefore, and of course the photolysis of ozone is considered, The equations governing the rate of change of the concentrations n(Oz), n(0) and n(03) are dn(O2))ldt + n(02)Jz + k~n(M)n(Oz)n(O) = kln(M)n2(0) + 2k3n(O3)n(O) + n(03)J3, dn(O)/dt +2kln(M)n2(0) + k2n(M)n(O~)n(O) + k3n(03)n(O) = 2n(02)J2 +n(03)J3, O+O+M-+02+M+118 kcal; coefficient kl, 0 + 0 2 + M-+O3 + M + 24 kcal ; coefficient k2.0 + 0 3 -+ 0 2 + 0 2 + 94 kcal ; coefficient k3, O3 + hv-+O, +O; coefficient Jo3. (17) (1 8) (19) dn(03))ldt + n(03)J3 + k3n(O)n(O3) = k~n(M)n(O2)n(O>.M. NICOLET 13 The conditions for the simultaneous variation of n(0) and n(O3) can be con- veniently written At sufficiently high altitudes, i.e., in the thermosphere, (20) becomes, n(03) <n(O), For the day equilibrium of ozone in the mesosphere, In the stratosphere, the day-time conditions become, n(O) <n(03), indicating that the equilibrium conditions for ozone, is reached depending on t being the time measured from an initial time to.The time t increases with lower heights since the dissociation rate coefficient J 2 decreases more rapidly than 4 0 2 ) increases. Departure from photochemical equilibrium conditions takes several days below 40 km. Any variation in the ozone content of the stratosphere modifies the value of J2 and affects the vertical distribution of ozone. If numerical values of the various parameters are considered, it is evident that (1) the ratio J2/kl is important in the thermosphere ; (2) the ratio k~J2/k3J3 is important in the stratosphere. In the mesosphere all parameters are involved, since the day- time equilibrium conditions are and dn(O)/dt + dn(03)dt + 2kln(M)n2(0) +2k3n(O3)n(O) = 2n(02)J2. (20) dn(O)/dt +2kln(M)n2(00) = 2n(02)J2.(21) (22) dn(03)ldt N 2n(02)J2, (23) dn(O)/dt + 2kln(M)n2(03) + 2k3n(O)n(O) = 2n(02)J2. nW3) = (k2lk3)n(M)n2(02)(J21J3), nt(O3) = nt,(O3) + 2n(Oz)J24 (25) n(03)/n(O) = [k2n(M)n(O2) - k3n(03)1/J3, (26) in which the terms k3 n(X) can be neglected. If there is no theoretical obstacle in discussing the ozone atomic oxygen problem, numerical results differ by a large factor. The difficulties mentioned concerning the photodissociation rate coefficients are not important compared with the inaccuracies in the chemical rate coefficients. The rate coefficient of the three-body reaction (13) of oxygen atoms 14 can be taken as, for aeronomic purposes, kl = (3 k 1) x 10-33 cm6 sec-1, (284 corresponding to the values assumed by Bates-Nicolet 4 k3 = 5 x 10-34Ti cm6 sec-1.The values of k2 and k3 are not yet certain. Using the laboratory values of Eucken and Patat,24 Bates and Nicolet 4 adopted and However, recent investigations such as Benson and Axworthy,2s Zaslowsky 26 and Kaufman 27 lead to with an undetermined activation energy. k2 = 5 x lO-36Pcm6 sec-1, k3 = 1.5 x lO-llT* exp (- 3000/T) cm3 sec-1. kz = (5 k2.5) x 10-34 cm6 sec-1 (29) (30) (31)14 INTRODUCTION TO CHEMICAL AERONOMY As regards the value of k3, an exact evaluation is difficult.15 The uncertainty is illustrated by the values deduced from laboratory measurements, namely,15 and k3a = 5 x 10-11 exp (- 3000/T) cm3 sec-1, k3b = 7 x 10-12 exp (- 1600/T) cm3 sec-1. If H20 is an impurity in the measurement, k3b could be the result of the bimolecular reaction between ozone and atomic hydrogen and k3c;c should be the exact rate coeffici- ent.It is clear that only approximate numerical solutions can be derived as the uncertainties in the coefficients are too great. The results obtained with (28a), (31) and (32a) or (32b) are given in fig. 4 and exhibit the same general features found by concentration (CM-3) FIG. 4. earlier workers. But the absolute values are essentially different. An activation energy of 6 kcal for the bimolecular process O3+0 with a relatively small ratio kl/k2 5 6 leads to large concentrations of ozone and atomic oxygen in the mesosphere. Also the limit set for night-time conditions by or by (33) (34) leads to an ozone concentration in the mesosphere much larger at night than during the day for k3b.Consequently, there is a need for careful experimental work on ozone reactions. It must be stressed that extremely precise data are required for the analysis of aeronomic conditions in the mesosphere, in which it is possible to study photochemical and chemical processes without additional effects such as advective and dynamical transport and without a practical inff uence of solar activity. Without a perfect knowledge of the ozone+atomic oxygen behaviour in a pure oxygen mesosphere it becomes difficult to study departures from photochemical equilibrium conditions in the stratosphere and thermosphere. Finally, the introduction of other minor constituents necessary in the study of the terrestrial atmosphere cannot be made if the idealized atmosphere is not properly defined.M.NICOLET 15 ORIGIN OF A HYDROGEN OXYGEN ATMOSPHERE A hydrogen oxygen atmosphere is very complicated. Photoaction on water vapour in the mesosphere and oxidation of methane in the stratosphere are im- portant processes leading to the production of hydrogen atoms.7~ 8 Bates and Nicolet 4 made the first attempt to estimate the various aeronomic processes 13 years ago and are now again considering (with the new experimental data16) the very complicated situation resulting from chemical actions and atmospheric mixing effects. It has been shown 28 that there is a continuous escape of atomic hydrogen atoms at exospheric levels corresponding to a diffusion flow FD(H) at the 100 km level of F~(H)loo = 2.5 x 107 cm-2 sec-1. (35) This must correspond to a total loss of about lO7H20 molecules cm-2sec-1 or 6 x 106 CH4 molecules cm-2 sec-1.Under mixing conditions, the diffusion flow 29 of methane with concentration n(CH4) = 1.5 x 10-6 n(M) is FD(CH~) = 7 x 106 cm-2 sec-1, (36) which must be compared with a production rate 9 of CH4 of at least 1010 molecules cm-2 sec-1. Thus, the escape flow is always supported by the diffusion flow of CH4 and is a small fraction of its total production. Atomic oxygen attacks methane through CH4 + 0 + CH2 + H20 + 30 kcal (37) with an activation energy of 7-8 kcal.30~ 31 Adopting a(CH4, 0) = 2 x 10-11 exp (-4OOO/T) = 4 x 10-12 exp (- 3600/T) cm3 sec-1 (38) for the rate coefficient of (37), it apears that the life-time of CHq in the mesosphere is relatively short.8 As the re-formation of CH4 is a very slow process, its concentration in the mesosphere must be very small.There is no diffusion or atmospheric mixing process able to maintain an adequate vertical flow of methane. It is almost certain that its fractional abundance begins to fall off well below the stratopause. With a rate coefficient of the order of 10-19 cm3 sec-1 for (37), adapted to tropopause condi- tions corresponding to about 5 x 1012 CH4 molecules cm-3, it can be seen that a 10 km layer with about 107 oxygen atoms cm-3 will lead to a production of about 107 H20 molecules cm-2 sec-1. Judging from this value it seems probable that, in the stratosphere, the fractional concentration of CH4 is affected by its transformation into H20, and consequently the formation of H20 depends on the methane exchange between the troposphere and the stratosphere.The tropospheric mixing time is short enough to lead to a uniform vertical distribution of CH4,32 and its injection rate into the stratosphere should be known with precision in order to determine the actual production of stratospheric H20. With uniform transport due to diffusion, its fractional abundance would be about 3 x 10-6. However, since the life-times T(CH~) at 30 km and 40 km are of the order and this implies that the exchange between troposphere and stratosphere is controlled by " turbulent " processes rather by diffusion. Consequently, an abundance than of H2O greater than 3 x 10-6 can result from methane oxidation in the stratosphere.16 INTRODUCTION TO CHEMICAL AERONOMY REACTIONS OF ATOMIC HYDROGEN The products of dissociation of H20 in the mesosphere give rise to a complicated series of chemical processes.More than 30 processes are involved 4 and we retain here the more important processes that a detailed study of the situation suggests. The principal reactions are listed below. Those involving hydrogen atoms are : H+ 0 2 + M-+HO2 + M + 46 kcal, a1 = 3.3 x 10-33 exp (SOOlT) cm6 sec-1 (41) (42) with a rate coefficient according to Clyne and Trush 33 of showing a negative temperature coefficient, and for which the rate coefficient a2 16 is very large, H + 03+OH + 0 2 + 77 kcal, a2 = 1-5 x 10-12T* cm3 sec-1. (43) This reaction was introduced by Bates and Nicolet 4 in 1950 to explain the observed airglow emission of the hydroxyl radicle OH up to the vibrational quantum number 9 (75.2 kcal) but not up to v" = 10 (81 kcal).The same reaction, leading to HO2, is less important and instead of the three-body association between OH and 0, the bimolecular process is noted as also having a high rate coefficient with practically no activation energy :I6 Since 02(X3Xg),=4 = 17.4 kcal only the first 3 vibrational levels of 0 2 are involved. The reaction of HO2 with 0 is also a fast process,l6 HOz + O-OH + 0 2 + 55 kcal, OH+O-+H+02+16.6 kcal (45) a5 = 3 x lO-12T* cm3 sec-1. (46) (47) with a rate coefficient a7 which may reach a7 = 1 -5 x 1 O - l W cm3 sec-1. Assuming as a first approximation that only the preceding reactions are involved, the following ratios corresponding to chemical equilibrium conditions are obtained : and Since atomic oxygen is present in the mesosphere with concentrations not less than lOlocm-3 during the day, (49) and (50) are representative of day-time conditions. At the stratopause level aln(M)n(O2)-5 sec-1 is the most important term in eqn.(49) and (50), and, therefore, n(HOz)>n(H) and n(CH)>n(H). Atomic hydro- gen becomes important only in the mesosphere (and above). The rapid decrease of the term aln(M)n(O2) with height compared with that of bimolecular processes leads to the approximation, somewhere above the stratopause, This indicates, since n(O3) < n(0) in the middle mesosphere and thermosphere, that the hydroxyl radicle OH must have an upward sharp decline. Under the same conditions, i.e., a2n(03) > aln(M)n(O2), we have n(0H) > n(HO2), which indicates that hydrogen is in atomic form to such a degree as to play an important role in meso- spheric processes.n(OH)/n(H) = (aln(M)n(O2) + a2n(03))@n(O), (49) n(H02)/n(H) = aln(M)n(02)/a7n(O). (50) n(OH)/n(H) = n(03)/240)* (51)M. NICOLET 17 If atomic hydrogen is sufficiently abundant in the mesosphere, eqn. (20) must be dn(0)Jdt + dn(O&dt + 2kln(M)n2(0) +2k3n(O3)n(O) + 2a2n(03)n(H) = 2n(02)J2. Again, in order to derive any numerical value, it is necessary to know first the exact numerical expressions in a pure oxygen atmosphere. In any case, comparing the numerical values of k3 and a2, it is clear that for atomic hydrogen concentrations in the mesosphere greater than 107 cm-3 Since n(H) is of the order of 107 cm-3 at 100 km 28, 34 and increases downwards in the lower thermosphere, it is certain that the ozone + oxygen equilibrium in the mesosphere is affected by atomic hydrogen which acts as a catalyst.As a example, let us assume a concentration of n(H) = 3 x 108 cm-3 at 80 km, namely, a normal value if atomic hydrogen is in mixing between 80 km and 100 km. The ozone concentration which is about 109 cm-3 in a pure oxygen atmosphere (see fig. 4) decreases to 108 cm-3 and the atomic oxygen concentration decreases from 7 x 1011 cm-3 to 1011 cm-3. It is clear, therefore, that the mesospheric behaviour of a hydrogen + oxygen atmosphere is completely different from a pure oxygen atmo- sphere. modified by adding another term, (52) k3n(O) < azn(H). (53) REACTIONS I N A HYDROGEN + OXYGEN ATMOSPHERE Numerous secondary processes, involving the destruction of atomic hydrogen, hydroxyl and perhydroxyl radicles occur in the mesosphere.Among the data re- quired for a complete discussion are the rate coefficients of the various reactions of all hydrogen + oxygen compounds.16 Recent publications on the subject were also used as main sources 35-38 since a number of investigators have studied these re- actions under various aspects. The source of hydrogen atoms (free or combined) is the photodissociation of H20 (and H202). The final loss processes which are directly related to hydrogen atoms are not important compared with collision processes where OH and HO;? are involved. This is due to the fact that at the stratopause and in the lower meso- sphere n(H)<n(OH)<n(H02) and also that the reactions are not very rapid.The main final reactions are as follows : between two hydroxyl radicles, OH + OH+H20 + 0 + 17 kcal, which is discussed by Kaufman 16 and has a reaction rate (54) between hydroxyl and perhydroxyl radicles, for which a high rate coefficient is suggested 16 and is taken as OH + H02-+H20 + 0 2 + 72 kcal, between two perhydroxyl radicles HO2 + HOpH202 + 0 2 + 42 kcal, (58) with a rate coefficient of the same order as a16, i.e.,18 INTRODUCTION TO CHEMICAL AERONOMY Among the loss processes of atomic hydrogen which could be added to the reactions just described, we may consider the three-body reactions : with rate coefficient a21 of The exothermic bimolecular process with rate coefficient, H+H+M+Hz+M+ 103.2 kcal, (60) a21 = 3 x 10-32 cm6 sec-1.(61) H+OH+Hz+O+ 1.9 kcal (62) (63) a22 = 2 x lO-W’”* exp (- 3400/T) cm3 sec-1, can be compared with the endothermic process, with rate coefficient Hz+O-)OH+H- 1.9 kcal a24 = 5 x lO-13P exp (-aOO/T) cm3 sec-1. (65) Such a process is important in the thermosphere since it leads to the final dissociation of molecular hydrogen. Finally, the reaction with perhydroxyl radicles H+H02+H2+02+57 kcal (66) (67) with a rate coefficient a23 = 5 x 10-1W exp (- lOOO/T) cm3 sec-1 should be a normal production process of molecular hydrogen. The expression for complete equilibrium is written as follows : n(H20)JH,0+ n(H202)JH,0,+a2,n(0)n(H2) = a16n2(0H)+a17n(0H)n(H02)+ a2,n2(H02) + azln(M)n2(H) + a,,n(H)n(OH) + a2,zt(H)n(H02). (68) which can be applied in the mesosphere for daytime conditions.The vertical distribution of the production function depends on the values of n(H2O) and J - , o for which exact values are not available. Because of absorption due to molecular oxygen, JH,O is a very sensitive function of the optical depth of molecular oxygen. Various estimates can be made, but it is not possible to discuss here all aspects which must be considered for a complete discussion. The omission of reactions involving atomic hydrogen at sufficiently low altitudes (above the stratopause) where n(H) must decrease rapidly according to (49) leads to a simple way of considering the result of night-time conditions. The differential equation is simply, see (68), - [dn(OH)/dt + d(HO~)/dt] = al&(OH) + ~2~n2(HO2) + al7n(0H)n(H02).(69) Since (55), (57) and (58) show that a16 = a27I+a17, the relevant solution to (69) is, t being the time measured from to which is sunset, The nocturnal decay is important in the lower mesosphere since for a16 = 2-5 x 10-12 cm3 sec-1 and t = 4 x 104 sec the initial concentration is reduced to less 107 cm-3. Thus, even if the aeronomic problem considered in this section is idealized, it does indicate that the whole mesosphere is a transition region in which the freeM. NICOLET 19 hydrogen atoms are formed and diffuse upwards. Water vapour diffuses from the stratosphere into the dissociation region with an equivalent current which can be furnished by methane in the troposphere. If gentle mixing winds occur and carry up H2O at a greater rate, then there will be regions of abnormal specific humidity which will persist for a certain period of time.But the final process must be a down- ward transport of water vapour. The reactions (54), (56) and (58) obviously leads to H20 since hydrogen peroxide is destroyed (see, e.g., ref. (35, 39, 40)) by the following chemical processes : with a rate coefficient of H + H20pOH + H20 + 69 kcal, (71) a29 = 5 x 10-12Ti exp (- 3000/T) cm3 sec-1; (72) OH + H202-+H02 + H20 + 30 kcal, (73) a30 = 1.5 x 10-1W cm3 sec-1, (74) O+H202-+02+H20+86 kcal (75) (76) with a rate coefficient which may be and with a rate coefficient a31 = 1-5 x 10-13Ta exp (-2OOO/T) cm3 sec-1, The exact role of hydrogen peroxide near the stratopause level must be re- determined from the solution of the differential equation dn(H202)/dt+ n(H202)[J~,o~+ a29n(H)+ a3on(OH)+ a3dO)l = 2a27n2(H02)* (77) An approximate value for JH~o, is 10-4 sec-1, and almost equilibrium day-time conditions can be imposed.Molecular hydrogen should exist in the mesosphere since its dissociation pro- bability is small in this region. In addition to processes (62) and (67) leading to the formation of H2, and to (65) leading to the loss of H2, we may consider also with a rate coefficient a19 of The general equation being OH+H2-)HzO+H+15 kcal (784 a19 = 0.5 x lO-llT+ exp (- 3000/T), &(Hz)/dt + n(H2)[a24n(O) + aign(OH)] = n(H)[a22n(OH) + a23n(H02)1 +n(H20)J~,-o. (79) This leads only to equilibrium conditions when a2#(0) is sufficiently large, i.e., in the thermosphere where the temperature is high.1 Chapman, Phil. Mag., 1930,10, 369. 2 Chapman, Report Progr. Physics, 1943, 9,92. 3 Nicolet and Mange, J. Geophys. Res., 1954, 59, 15. 4 Bates and Nicolet, J. Geophys. Res., 1950, 55, 301. 5 Meinel, Astropliys. J., 1950, 111, 207 and 433. 6 Chamberlain and Smith, J. Geophys. Res., 1959, 64, 611. 7 Bates and Nicolet, Compt. Rend., 1950, 230, 1943 ; Pub. Astronom. SOC. Pacific, 1950, 62, 106. 8 Bates and Witherspoon, Month. Notices Roy. Astro. Soc., 1952, 112, 101. 9 Hutchinson, chap. 8, in Kuiper (ed.), The Solar System, vol. II, The Earth as a Planet (The University of Chicago Press, Chicago, 1954.)20 INTRODUCTION TO CHEMICAL AERONOMY 10 Nicolet, Inst. Mktior. Belg., Mkmoires, 1945, 19, 124. 11 Bates, Ann. Giophys., 1952, 8, 194. 12 Nicolet, J. Atm. Terr. Physics, 1955, 7, 152. 13 Nicolet, Aeronomic Chemical Reactions in Physics and Medicine of the Atmosphere and Space (John Wiley and Sons, New York, 1960). 14 Barth, Ann. Gkophys., 1964,20, 182. 15Schiff, Ann. Giophys., 1964,20, 115. 16 Kaufman, Ann. Giophys., 1964,20,106. 17 Detwiler, Garret, Purcell and Tousey, Ann. Gkophys., 1961, 17,9. 18 Ditchburn and Young, J. Atm. Terr. Physics, 1962,24, 127. 19 Vigroux, Ann. Physique, 1953, 8, 709. 20 Inn and Tanaka, Y., J. Opt. SOC. Amer., 1953,43, 870. 21 Hearn, Proc. Physic. SOC., 1961, 78, 932. 22 Diitsch, in Chemical Reactions in the Lower and Upper Atmosphere, (Interscience Publishers, 23 Paetzold in Chemical Reactions in the Lower and Upper Atmosphere (Interscience Publishers, 24 Eucken and Patat, 2. physik. Chem., 33B, 1936, 459. 25Benson and Axworthy, J. Chem. Physics, 1957, 26, 1718. 26 Zaslowsky, Urbach, Leighton, Wnuk and Wojtowicz, J. Amer. Chem. SOC., 1960, 82, 2682. 27 Kaufman, this Discussion. 28 Kockarts and Nicolet, Ann. Gkophys., 1962, 18, 269. 29 Nicolet, Me'm. SOC. Roy. Sci. LiGge, 1962, 7, 1960. 30 Steacie in Atomic and Free Radical Reactions, (Reinhold Publishing Corporation, New York, 31 Wong and Potter, J. Chem. Physics, 1963, 39, 221 1. 32 Goldberg and Miiller, Astrophys. J., 1953, 118, 397. 33 Clyne and Thrush, Proc. Roy. SOC., 275A, 1963, 559. 34 Bates and Patterson, Planet. Sp. Sci., 1961, 5, 257. 35 Foner and Hudson in Adu. Chem. Series no. 36 (American Chemical Society, Washington D.C., 36 Knox, Ann. Reports Progr. Chem., 1962, 59, 18. 37 Minkoff and Tipper, Chemistry of Combustion Reactions (Butterworths, London, 1963). 38 9th Symp. Combustion (Academic Press, New York, 1963). 39 Baldwin and Mayor, Trans. Faraday SOC., 1960,56,103. 40 Foner and Hudson, J. Chem. Physics, 1962,36,2681. New York, 1961), p. 167. New York, 1961), p. 181. 1954), p. 601. 1962), p. 36.

ISSN:0366-9033    

DOI:10.1039/DF9643700007

出版商:RSC    

年代:1964

数据来源: RSC

摘要:

Chemical Reactions Contributing to the Nightglow BY D. R. BAITS Dept. of Applied Mathematics, Queen’s University, Belfast Received 8th January, 1964 Recent work on the chemical reactions contributing to the nightglow is briefly surveyed. The view is expressed that in spite of difficulty in interpreting certain laboratory measurements the Chapman and the Barth mechanisms should continue to be regarded as possible sources of X 5577 ; and that three-body recombination of oxygen atoms should continue to be regarded as a possible source of the Herzberg and Atmospheric bands. The position regarding the D-doublet of sodium remains undecided. To avoid unnecessary repetition I shall, in general, confine myself to the research that has been carried out since 1960 when I last reviewed the subject.1 FORBIDDEN GREEN LINE OF ATOMIC OXYGEN Chapman’s suggestion 2 that the presence of A 5577 of 01 in the nightglow spectrum is due to was widely accepted in 1960, not because of any compelling positive evidence, but because of the failure to find a plausible alternative excitation mechanism.To account for the observed mean intensity of 250 rayleighs * it was necessary to suppose that the rate coefficient kl was at least about 2 x 10-35 cm6 sec-1. An attempt at determining kl has been made by Young and Clark 3 who measured the absolute intensity, I(5577), of the green light emitted from a 72-1. bulb con- taining a known quantity of atomic oxygen produced by Assuming that the loss of O(1S) atoms by non-radiative processes is inappreciable they deduced that kl is less than 2 x lO-36cm6 sec-1 and hence that the Chapman process (1) is not responsible for the A 5577 line of the nightglow.However, Young and Sharpless 4 stated later that such a conclusion is premature. Thus finding that where n ( 0 ) is the number density of 0 atoms they were led to believe that some deactivation process like is, in fact, important. They interpreted their measurements as indicating that kl/k4 is 2.6 x 10-22 cm3. If kl were 2 x 10-35cm6 sec-1 or more (as required by Chapman’s theory) k4 would then be at least about 1 x 10-13 cm3 sec-1. Barth and Hildebrandts have also sought to determine kl in the laboratory. With the aid of an electron paramagnetic resonance spectrometer they found n(0) to be 3 x 1015 cm-3 in a mixture of atomic and molecular oxygen (with n(02) ap- proximately 3 x 1016 cm-3) flowing through a tube of 3.5 cm diam.and 48.6 cm length. From their inability to detect A5577 and the calibrated sensitivity of their * A rayleigh corresponds to an emission rate of 106 photons (cm2 column)-1 (sec)-1. 21 0 + 0 + 0+02 + O(1S) (1) NO +N-+N2+ 0. (2) I (5577)ocn(O)2 (3) ops) + o(~P)+o(~D) + ope) (4)22 THE NIGHTGLOW equipment they deduced that the photon emission rate was less than 2 x 107 cm-3 sec-1. To obtain information on deactivation they carried out a series of sub- sidiary measurements on I(5577) from a mixture of atomic oxygen and nitrogen. They inferred from these that collisions with the walls of their tube were unim- portant compared with and that k5 is about 5 x 10-15 cm3 sec-1.Since the radiative transition probability A(1S- 10) is 1.3 sec-1 and A(1S- 3P) is much lower they concluded that O(1s)+o2-O(3P or 1D)+O2, (5) This limit is 250 times smaller than the minimum value needed for Chapmans' theory. In view of the results obtained by himself and Hildebrandt, Barth6 has put forward the hypothesis that the excitation of A 5577 in the upper atmosphere is due to the sequence O+O+M+O~(X,U)+M, (7) 02(X,U) + 0 - 0 2 + O(lS), (8) in which M is any species and (x,v) signifies that the 0 2 molecule is electronically and vibrationally excited. This two-stage mechanism does not differ fundamentally from the single stage mechanism of Chapman. It is not possible to predict with assurance which is the more effective. The internal energy of an O~(X,II) molecule may be reduced below that required for (8) by wall deactivation (rate Wzv), by the emission of a photon (transition probability Asv) or by atom-atom interchange and other collision processes which I shall simply write taking kg to be the mean rate coefficient.It may be seen that the yield of O(1S) atoms from the (7,8) sequence is O~(X,U) + M + 0 2 + M, (9) A7,8n(0)3 cm-3 sec-1, (10) where A,,* is an effective three-body rate coefficient given by k,k,n(M) W""+A""+ k,n(M,' A7,8 = If the sequence is to account for the 25577 line of the nightglow without incon- sistency with the null observation in the experiment of Barth and Hildedrandt 5 it is necessary that 1 7 3 should decrease by a factor of 250 or more in passing from conditions at an altitude of about 100 km to conditions in the tube used.Denoting the number denisties concerned by I atmos and I lab respectively the requirement may be written : 250n(M I lab){AXv+ k,n(M I atmos)) < Wxv. n(M I atmos) Since n(M I lab) and n(M I atmos) are about 3 x 1016 cm-3 and 1.5 x 1013 cm-3 respectively and since Wxv would not exceed 1 x 104 sec-1 even if' every encounter with the sides of the tube brought about deactivation, requirement (12) leads to and It is very doubtful if these inequalities are in fact satisfied. kg c 1 -3 x 10-15 cm3 sec-1, Am < 2 x 10-2 sec-1. (13) (14)D. R . BATES 23 Other hypotheses regarding the excitation of 15577 have been advanced.4~ 7-9 Some of them have been examined critically by Dalgarno.10 None are at all at- tractive.Further laboratory work on process (1) and processes (7) and (8) is desir- able. These remain the two most promising possibilities. HERZBERG AND ATMOSPHERIC BANDS OF OXYGEN It is natural to attribute the Herzberg (A3Zz-,X3Z;) and Atmospheric (blZz-’X3Zg) bands of the nightglow to oxygen atoms recombining to form electronically excited oxygen molecules 0 + 0 + M-, 0,(A3Zi) + M, (1 5) The observed intensities are much less than the intensities which would be expected to result from processes (15) and (16) so it has been suggested there must be rapid deactivation, probably by the atom-atom interchange process 02(a) + 0-0 + 02(p) (17) whiGh may remove either electronic or vibrational energy.1 Barth and Patapoff 11 have reported that they have confirmed in the laboratory that three-body recombination of oxygen atoms does indeed give rise to the Herzberg bands.However, they apparently did not determine the absolute intensity. The spectral range which they covered did not include the Atmospheric bands. Young and Sharpless 4 9 9 have also investigated the emission of the oxygen bands from an afterglow. According to them oxygen atom recombination takes place almost entirely into the ground electronic state of the molecule * so that processes (15) and (16) fail to account for the nightglow intensities (about 1.5 kilo- rayleighs in the Herzberg bands and about 15 kilorayleighs in the Atmospheric bands). It is difficult to see how the ground electronic state could be specially favoured in recombination. The theoretical work of Benson and Fueno 12 has been cited in this connection but it does nothing to make the results of Young and Sharpless understandable. The highest vibrational levels of 02(A3Z:U+) and 02(blX,+) concerned in the night- glow are the seventh and zeroth which have excitation energies with respect to the normal molecule of 4.9 eV and 1.6 eV.The excitation energy of the Herzberg bands, like that of A 5577, is so high that the only obvious mechanism for supplying it is three-body recombination of oxygen atoms. In contrast, there are many mechanisms which can provide the low excitation energy of the Atmospheric bands. However, the great intensity of the emission restricts the possibilities drastically. Perhaps first consideration should be given to the processes involving hydrogen and its oxides.Reactants pass through these processes extremely rapidly as is evident from the observed intensity of the Meinel bands which requires 139 14 that vibrationally excited hydroxyl radicles are formed at a rate of some 1012 (cm2 column)-1 (sec)-1. Catalytic recombination of oxygen atoms by hydrogen is comparable in importance to direct recombination and con- ceivably contributes to the Atmospheric bands by some process like HO2+O+OH+O2(b1X~). (18) * It will be recalled that Barth’s hypothesis on the excitation of h 5577 requires recombination into excited electronic states.24 THE NIGHTGLOW However, it is scarcely worth pursuing this at present since rocket experiments (cf. Packer 15) have shown that the distribution with altitude of the emission of the Atmospheric bands is effectively the same as that of the Herzberg bands and that of 15577 but is markedly different from that of the Meinel bands.B-DOUBLET OF ATOMIC SODIUM Chapman 16 has tentatively suggested that the B-doublet may be excited by NaO + O+Na(2P) + 02, NaH + OjNa(2P) + OH. (19) (20) Na+ 0 p N a O + 0 2 (21) and Bates and Nicolet 17 that it may be excited by The sodium compounds might perhaps be formed by and followed by or Na + 0 2 + M +Na02 + M Na02+O-+NaO+02 NaOz+H+NaH+02. As Dalgarno 10 has pointed out the objection that process (19) is endothermic is not conclusive since the NaO molecules resulting from processes (21) and (23) are probably vibrationally excited. Potter and Del Duca 18 have attempted to determine the altitudes of the emissive layers due to processes (19) and (20) from a theoretical study of the photo-chemistry of sodium and its compounds.They obtained much lower altitudes than the rocket measurements 15 indicate and concluded that neither in (19) nor (20) is the chemi- luminescent process operative. However, the uncertainties are necessarily very great since the rate coefficients of some of the processes are unknown and since the adopted distribution with altitude of n ( 0 ) and n(H) may be considerably in error. It would be unwise to abandon either of the two hypotheses under discussion because of the lack of accord between the calculated and observed altitudes. Another hypothesis on the D-doublet has been advanced by Potter and Del Duca.18 This is that vibrationally excited oxygen molecules transfer some of their energy to free sodium atoms Taking n(Na) to be 1 x 104 cm-3 through a layer 10 km thick it may be seen that the process would yield the winter intensity of some 200 rayleighs if n(02, u>l2) were 1 x 107 cm-3 and k25 were 2 x 10-9 cm3 sec-1.Potter and Del Ducca estimated the rate of formation of 02(u>12) as 5 x 105 cm-3 sec-1. At an altitude of 90 km, n(02) is about 1.3 x 1013 cm-3 and n(0) is about 1 x 1012 cm-3. Hence in order that n(O2, u >12) should be as high as re- quired, the rate coefficients for the deactivation processes and would have to satisfy the inequalities, 02(v > 12) + Na-02 + Na(2P). (25) Oz(u > 12) + 0- 0 + 02(u < 12), 02(U >, 12) 4- 02(U = 0) -+ 02(u < 12) -I- 02(U < 1 2), (26) (27) (28) k26 < 5 x 10-14 cm3 sec-1, k27<4 x 10-15 cm3 sec-1.D. R.BATES 25 These inequalities are quite severe since process (26) can take place simply by atom- atom interchange (which, contrary to the assertion of Potter and Del Duca, can change the vibrational quantum number by more than unity), and since process (27) may be in very close energy balance. In justification of their adoption of an extremely high value of k25 Potter and Del Duca recalled that the emission of the D-doublet is strongly quenched in mole- cular nitrogen, bromine and (presumably) oxygen, the rate coefficients of the col- lision processes involved being of the order 10-10 to 10-9 cm3 sec-1. The process in oxygen would be the inverse of (25), as assumed by Potter and Del Duca, if the electronic energy were converted entirely into vibrational energy.However, some is probably converted into translational energy 10 since the process apparently takes place through the crossing of potential energy surfaces.19 This makes it likely that the value assigned to kzs is much too high. This research has been sponsored by Cambridge Research Laboratories, OAR, through the European Office, Aerospace Research, United States Air Force, under Grant No. AF-EOAR 63-85. 1 Bates, Physics of the Upper Atmosphere, ed. Ratcliffe (Academic Press, New York, 1960), 2 Chapman, Proc. Roy. SOC. A, 1931, 132, 353. 3 Young and Clark, Physic. Rev. Letters, 1960, 5, 320. 4 Young and Sharpless, Reactions of Nitrogen and Oxygen Atoms with Applications to the Upper Atmosphere Nightglow (Technical Report of the Stanford Research Institute, California, 1962), chap. 4, and J. Chem. Physics. chap. 5. 5 Barth and Hildebrandt, J. Geophys. Res., 1961, 66, 985. 6 Barth, Science, 1961, 134, 1426; J. Geophys. Res., 1962, 67, 1628. 7 Krassovsky, Ann. de Geophys., 1958, 14, 395 ; Planet. Space Sci., 1963, 10, 7. 8 Young and Clark, J. Chem. Physics, 1960,32, 607. 9 Young and Sharpless, Reactions of Nitrogen and Oxygen Atoms with Applications to the Upper Atmosphere Nightglow (Technical Report of the Stanford Research Institute, California, 1962), chap. 5 ; J. Geophys. Res., 1962, 67, 3871. 10 Dalgarno, Planet. Space Sci., 1963, 10, 19. 11 Barth and Patapoff, Astrophys. J., 1962, 136, 1144. 12 Benson and Fueno, J. Chem. Physics, 1962,36,1597. 13 Chamberlain and Smith, J. Geophys. Res., 1959, 64, 611. 14 Ferguson and Parkinson, Planet. Space Sci., 1963, 11, 149. 15 Packer, Ann. de Geophys., 1961, 17, 149. 16 Chapman, Astrophys. J., 1939, 90, 309. 17 Bates and Nicolet, J, Geophys. Res., 1950, 55, 235. 18 Potter and Del Duca, J. Geophys. Res., 1960, 65, 3915. 19 Laidler, J. Chem Physics, 1942, 10, 34, 43.

ISSN:0366-9033    

DOI:10.1039/DF9643700021

出版商:RSC    

年代:1964

数据来源: RSC

摘要:

Rate Constant of the Reaction O + 2 O 2 + O 3 +O2 BY FREDERICK KAUFMAN AND JOHN R. KELSO Ballistic Research Laboratories, Aberdeen Proving Ground, Md., U.S.A. Received 1 3th January, 1964 The recombination of 0-atoms was studied in a discharge-flow system for purified oxygen whose mole fraction of total hydrogen was 10-6 or less. The absence of recombination below a pressure of about 3 mm Hg and the very slow recombination at 3-6 mm Hg indicates the presence of meta- stable, energetic species capable of dissociating 0 3 and of generating additional 0-atoms well down- stream of the discharge. Experiments in which 0 3 was added to discharged oxygen provide evidence for the presence of these metastable species. The concurrent but unrelated catalytic recombination of 0 by hydrogen impurities explains the wide range of rate constants and their disagreement with those calculated from the thermal dissociation of 0 3 .The recombination was then studied for 0-atoms produced by the rapid thermal decomposition of 0 3 at a few mm Hg. No indication for the presence of metastable species was observed, and a rate constant kl of 7.5 x 10-34 cm6 molecule-2 sec-1 was obtained, in fair agreement with the ozone data. Information on the homogeneous, three-body recombination of 0 and 0 2 is available from two sources : ozone decomposition studies and direct measurements of 0-atom recombination. The thermal decomposition of ozone was recently studied by conventional manometric methods 1 9 2 in the temperature range 7O-13O0C, and in a shock tube 3 from 420-640°C. One of the manometric studies 1 also in- cluded the re-evaluation of older experimental data.4 Moreover, all available data prior to 1960 were summarized and critically reviewed in an authoritative report 5 on the ozone+oxygen system.These studies provide values of the rate constant k!l for M+03-+0+02+M(-l), from which the desired rate constant k y for the recombination process can be calculated using the equilibrium constant.6 Where M is not 0 2 but 0 3 , Ar, or N2, conversion to k? also requires information on the relative efficiency of these gases in (- 1). Such information is available from the above papers and from a new study of the photolysis of ozone with red light.7 The second approach, i.e., the direct measurement of k y in a low-pressure discharge-flow system under steady-state condition, has given discordant results.The present work was thus prompted by the finding that increasingly rigorous purifi- cation of the oxygen led to decreasing values of k? and an increasing discrepancy with the results of ozone decomposition studies. Its experimental work consists of three parts: (i) development of methods for the quantitative determination of impurities, particularly those containing nitrogen and hydrogen ; (ii) study of 0-atom decays in a discharge-flow system using 0 2 of known and controlled purity ; (iii) study of 0-atom decays in the same flow system but without a discharge. EXPERIMENTAL DISCHARGE-FLOW SYSTEM The apparatus is similar to that described in earlier papers.8-10 The main flow tube is of precision-bore, 2.54f0.003 cm int.d i m . and about 1 m long. At its upstream end a discharge is excited by C.W. microwave radiation from a magnetron, type QK-390 26F. KAUFMAN AND J . R . KELSO 27 (Raytheon Mfg. Co.), at 2450 Mclsec, with up to 800 W power. Between the quartz discharge tube and the flow tube are two mixing inlets, one for diluent gases and the other for ozonized oxygen. Near the upstream end of the flow tube is a multi-perforated loop of Teflon tubing for the rapid admixture of NO or NO2 to the gas stream. A mechanical pump (4501./min) produces linear average flow velocities of up to 6m/sec. All flows are controlled by stainless steel needle valves and measured by the recorded pressure drop in one of various calibrated volumes by means of electrical pressure transducers. 0-atom concentrations are measured along the flow tube by the intensity of the O+NO (air afterglow) light emission at eight equidistant positions by means of a photomultiplier movable along the tube which receives the radiation through two collimating slits in a direction perpendicular to the tube. The amplified photomultiplier output is recorded by a strip-chart recorder.NO2 " titrations " of 0-atoms are carried out by measuring that flow of added NO2 which will reduce the O+ NO light intensity to a small fraction (- 1 %) of its maximum, at a point 30cm downstream of the point of addition. The surface of the flow tube is coated with aqueous H3P04 to decrease surface recombination, and the system is then evacuated to < 10-4 mm Hg with an oil diffusion pump. PURIFICATION OF 0 2 A N D MEASUREMENT OF IMPURITIES A few experiments were run with 0 2 vaporized from liquid oxygen collected from the thermal decomposition of purified KMn04.It was found possible, however, to remove hydrogenous impurities from cylinder oxygen to within the limits of the analytical methods, and cylinder oxygen was then used exclusively. Hydrogenous impurities were removed by passing the cylinder gas at 1 atni pressure through a quartz tube, 1 cm int. diam., 30 cm long, packed with quartz chips, and heated to 1100°C ; then through a column, 90 cm long, packed with zeolite molecular sieve, type 5A; and finally after expansion through the needle valve, at the lower pressure, through two glass traps filled with glass beads and glass wool and immersed in liquid nitrogen.- 8 .6 O1 -4 *2 2co 300 400 0 0 XH x lo6 FIG. 1 .-Calibration plots of 01 E (IoH, 41-4/10) against mole fraction of total added hydrogen. Curve 1 : FoZ = 3.6 cm3 S.T.P./sec ; P = 1.28 mm Hg ; H2 added ; curve 2 : FoZ = 3.6 cm3 S.T.P./sec ; P = 5.24 mm Hg ; H2 added ; curve 3 : Fo2 = 0.9 cm3 S.T.P./sec ; P = 0.58 mm Hg ; H2 (0) or CH4 (A) added. No attempt was made to remove nitrogeneous impurities, but selected cylinders were used whose discharged gas showed very little afterglow. Since N 2 is fairly efficiently con- verted to NO in such discharges,ll the intensity of the residual afterglow when no nitric oxide is added serves as a semiquantitative measure of N-impurities. The mole fraction of N 2 in selected cylinder 0 2 was thus found to range from 1 to 5 x 10-6 which is sufficiently low that the effect of the small amounts of NO on the decay of 0-atoms can be neglected.28 The amount of H-impurities was determined by focusing light from the microwave discharge on the entrance of a grating spectrometer (Leeds and Northrup, 0.75 m, Ebert, Photoelectric, Recording) and recording emitted intensities of the lower members of the Ql branch of the (0,O) band of OH, 2X++2ll, as well as of the atomic transition, at 6158.2 A of oxygen.The intensity ratio, (IOH,Q~-~/IO) = a was found to decrease from 0.28 for un- purified 0 2 to between 0 and 0.01 for purified gas. To relate a to the mole fraction of hydrogen XH, known mixtures were prepared by adding small flows of 1.2 % H 2 in purified 0 2 to larger flows of purified 0 2 .The results are shown in fig. 1 in which a is plotted against XH for several mixtures. Three conclusions can be drawn: (i) a varies linearly with XH for &<5x 10-5 as expected; (ii) the limiting slope for small XH increases some- what with pressure at constant flow ; (iii) a appears to be a single-valued function of the mole fraction of total hydrogen independent of its form. Curve 3 includes some points representing H2 additions and others representing CHq additions. Under the vigorous conditions of oxidation in the discharge, this equivalence is reasonable and suggests com- plete conversion of hydrogenous impurities to H, OH, and H 2 0 . The lower slope of curve 3 is probably due to the lower pressure and lower flow.Since most of the experi- ments reported below were carried out at pressures of 1-6 mm Hg and flows of 1-5-25 c11-13 S.T.P./sec, the observed a of 0.005-0.01 for purified 0 2 corresponds to an XH of about 0.5- 2 x 10-6 (or half that when expressed as X H ~ ) . FORMATION OF 0-ATOMS WITHOUT DISCHARGE The direct dissociation of 0 2 in a flow system requires very high temperatures. Though these could be attained either in special furnaces or on certain hot filaments, a simpler method proved to be successful. The rapid thermal decomposition of ozone at low pressure should be capable of producing enough atomic oxygen under steady-state flow conditions to permit the study of recombination reactions in the absence of interfering metastable discharge products.Therefore, a measured flow of purified 0 2 is passed thfough the annular space of a Siemens-type ozonizer across which an a.c. voltage of 10-12 kV produces 0.4-3 % 0 3 . The ozonized oxygen is then expanded into the low pressure region (1-9 mm Hg) thfough a stainless steel needle valve, and it traverses a quartz tube, 30 cm long, 2.5 cm int. diam., placed in a furnace at a temperature of 800-1 100°C. A Pyrex tube of 60 cm length connects the downstream end of the 0 3 decomposition region to the inlet of the flow tube where all measurements of 0-atoms are carried out. The connecting tube is internally coated with H3P04 in order to inhibit surface recombination. The gas stream enters downstream of the discharge tube to permit experiments in which 0 3 is added to discharged oxygen.In the " thermal " 0-atom experiments, the discharge is off and the rapid thermal decomposition of 0 3 is the only source of atomic oxygen. The slight, further pressure drop between the furnace and flow tube amounts to 0.2-3.5 mm Hg depending on pressure and total flow. At the temperature of the furnace, the residence time of the ozonized oxygen in the hot quartz tube ranged from 10 to 80 msec, and was usually equal to 10-20 half-lives of the thermal decomposition of 0 3 (in the presence of excess 02). The concentration of 0 3 was measured by light absorption measure- ments along the full length of the flow tube. The 0 3 analysis apparatus consists of a mercury lamp (Spectroline Quartz Pencil Lamp), quartz collimating and condensing lenses, light filters to isolate the 2537 A line as described by Kasha 12 or, later, a small grating monochromator (Farrand, Catalogue no.103420), quartz windows, and a photomultiplier, type 1 P-28, using the common amplifying and recording system of the grating spectrometer and the photomultipliers on the flow tube. The total optical path length is 115 cm and the minimum analyzable 0 3 pressure is about 0.5-1 x 10-4 mm Hg. In several experiments, it was shown that as the temperature of the 0 3 decomposition furnace was raised, the ozone concentration in the flow tube remained virtually unchanged until a temperature of about 500°C was reached, then decreased upon further temperature increase so that at and above 750°C, the 0 3 was completely decomposed. At the largest flow rates and at pressures above 5 mm Hg, a small amount of O3(" 5 x 10-4 mm Hg) was found to be present in the flow tube even at the highest furnace temperature. This was undoubtedly due to the recombination of 0 in the flow system.F .KAUFMAN A N D J . R . KELSO 29 When NO was added to such thermally decomposed, ozonized oxygen, a normal emission of the air afterglow was observed whose intensity was proportional to the NO concentra- tion. Small NO2 additions also brought on the O+NO glow, but larger amounts then extinguished the glow in a normal atom " titration ". Only very small concentrations of atomic oxygen could be produced in this manner, usually less than 0.5 p, but quite sufficient for the study of its decay down the tube. The small yield of atomic oxygen from ozone (usually t 2 %) is probably due to its fast surface recombination in the heated quartz tube.RESULTS DECAY OF O-ATOMS FROM DISCHARGED 0 2 Experiments were performed at several pressures and flowrates for purified 0 2 ; for unpurified 0 2 whose total mole fraction XH of hydrogen was determined simul- taneously; for purified 0 2 in the presence of a plug of glass wool between the dis- charge and flow tube and with known additions of H2; and for purified 0 2 with or without addition of 0 3 downstream. In each case, the emitted light intensity was measured at eight equidistant places along a section of the flow tube or 53 cm length. In most cases, the very small amount of NO produced from the N2 impurity in the discharge gave sufficient light intensity.Traces of additional NO were often introduced upstream of the observation section in order to show that the decay was independent of the NO concentration. With that (experimentally verified) as- sumption, the decay should be expressible as d l n I dln[O] k* dx dx 2 ) ' --=--- -- where k* is the effective first-order rate constant for the O-atom decay, u is the average flow velocity, and I the measured light intensity. The dependence of I on [O] by I = 10 [O] [NO] and the constancy of [NO] due to its rapid regeneration from NO2 have been abundantly verified in this pressure range, and together they make dl/dx equal to d[O]/dx. The disappearance of 0 should be attributable to 0 +wall (w) which makes k* = k , + 2k1[02]2 assuming (a) first-order wall recombination ; (b) ozone steady state; (c) neglect of 0 + 0 + 0 2 because of small fraction of 0 present; and (d) neglect of or correction for effects of viscous pressure drop or axial diffusion along the tube.Of these, (a) and ( d ) were shown to be satisfied, and (c) is justified because [02]/[0]>200 in most experiments. The question (b) of the 0 3 steady state is a complicated one, but it can at most lead to a range of a factor of two in the second term of k*. If a steady state is established, [ 0 3 l S s = (kl/k2)[02]2. The ratio kl/k2 is known approximately from work on the thermal decomposition and photolysis of 03. A qualitative comparison of the initial 0- atom concentration, [Ole, the expected [O&, and the change of [O], A[O] = [O]O- [O], will show whether or not steady state was approached. If either [O]O or A[O] are much smaller than the expected [O&, steady state could not obtain, and k" would be given by k* = k,+ k1[02]2.In an intermediate case, k* might increase along the tube as [Oj] builds up and approaches [O3lSs. A typical example of experimental results for highly purified 02(XH< 1 x 10-6) is shown in fig. 2. Logarithmic decays are shown for 5 experiments using a constant flow of about 3.6 cm3 s.t.p./sec, of 0 2 , but varying the pressure in 5 steps from 1.2530 REACTION 0 + 2 0 2 + 0 3 + 0 2 to 5.08 mm Hg by bleeding air directly into the pump. The corresponding flow velocities ranged from 430 cm/sec at the lowest pressure to 100 cm/sec at the highest. The most surprising result in fig.2 and confirmed many times is the total absence of any 0-atom decay at the lowest pressures. Curve 1 (1.25 mm Hg) thus shows a slight rise of I with x, curve 2 is flat at first and then drops slightly, and succeeding curves show a continuing trend of increasing initial rates of decay followed by ac- celerating rates downstream. Thus the initial k* would be slightly negative at 1.25 mrn Hg, zero at 2-18, about 0.3 sec-1 at 2.91, 0.8 sec-1 at 4.11, and 0.9 at 5.08. I I I- ---0 - 0 1 O-- I I I I I I 0 10 20 30 40 50 distance, cm FIG. 2.-Spatial decay of 0 in discharged, purified 02. Fo2 = 3-5 cm3/sec ; X,<2 x 10-6. curve 1 : P = 1-25 mm Hg; o = 430 cmlsec 2: 2-18 245 3 : 2-91 190 4 : 4-11 125 5 : 5.08 100 Moreover, the contribution of the surface recombination must be subtracted to obtain an estimate of kl, since that process is unaffected by the concurrent homo- geneous reactions.The work with thermally produced 0-atoms gave k, = 0.5 to 0.6 sec-1 as shown below. When such a kw is subtracted, kl would be negative below 4 mm Hg and very small at 4 and 5 mm Hg. The simple mechanism for the homogeneous recombination is thus inapplicable. These and other experiments described below suggest the presence in discharged oxygen of energetic, metastable species capable of re-dissociating 0 3 formed in the recombination and of producing additional, small amounts of 0 downstream.F . KAUFMAN A N D J . R. KELSO 31 Typical examples of 0-atom decays using discharged unpurified 0 2 are shown in fig. 3. The conditions of flow and pressure closely correspond to those in fig.2, but hydrogen-containing impurities were not removed. a was measured for all runs and XH can be estimated (using fig. 1) to be about 3 x 10-5. At the lower pressures, the curves are similar to those of purified 02, i.e., there is still hardly any 0-atom decay. At higher pressures, the small amount of hydrogen impurities accelerates the recombination, but the initial k* is still far too small. This oxygen traversed the long column of zeolite, but the 0 2 purification furnace was off and the 0 10 20 30 40 50 distance, cm FIG. 3.-Spatial decay of 0 in discharged, unpurified 0 2 . Fo2 = 3.5 cm3/sec ; XH-30 x 10-6. curve 1 : P = 1.26 mm Hg ; D = 440 cm/sec 2 : 2.27 240 3 : 2.83 200 4: 4.11 130 5 : 5-00 108 two traps just before the discharge were not cooled.It seems probable that meta- stable species are again responsible for the abnormally slow decay, but that hydrogen impurities recombine 0-atoms by a concurrent and independent mechanism. The recombination is then characterized by plots of In [O] against distance which are concave duwnwards or S-shaped. Morgan, Phillips and Schiff 13 successfully used Pyrex glass wool between their discharge and flow tube to quench’vibrationally excited N2 in active nitrogen. Since the metastable species in this work may be vibrationally excited 02, the same remedy was tried here. Fig. 4 shows two 0-atom decays for purified 0 2 at pressures of 2.86 and 4-92mmHg and two corresponding decays in which a mole fraction of 7 x 10-5 of H2 was added upstream of the discharge (XH = 1.4 x 10-4).Curves 1 and 3 (without H2) show that if vibrationally excited 0 2 is efficiently deactivated on the H3P04 coated glass surface then it is not the species responsible since the32 REACTION 0 + 2 0 2 + 0 3 + 0 2 0-atom decay is again abnormally slow. Curves 2 and 4 show a similar accelerating effect to that in fig. 3, but of larger magnitude, since more hydrogen is present. Direct evidence for the decomposition of 0 3 by metastable species other than 0-atoms was obtained by mixing ozonized oxygen with discharged oxygen, measur- ing the 0 +NO light intensity and titrating with N O 2 in the presence and absence of 0 3 , and measuring the total 0 3 in the flow tube in the presence and absence of the discharge. Fig. 5 shows the effect of 0 3 on the spatial variation of 0. The flat I I I I distance, cm FIG. 4.-Spatial decay of 0 in discharged, purified 0 2 with glass wool plug between discharge and flow tube. Fo2 = 3.5 cm3/sec. curve 1 : P = 2.86 mm Hg ; X,< 1 x 10-6 2: same XH = 140 x 10-6 (added H2) 3 : P=4*92mrnHg; xH<1x10-6 4: same X , = 140 x 10-6 (added H2). profile (curve 1) is a normal one for discharged, purified 0 2 , except that 1.4 cm3 S.T.P./sec of the total 0 2 flow of 4.0 cm3/sec traverse the ozonizer which is turned off and thereby bypass the discharge. When the ozonizer is turned on, curve 2 is obtained. The 0-atom concentration now increases along the flow tube and is everywhere larger than in the absence of 0 3 . NO2 titrations without 0 3 gave 3-2 p of 0, but with added 0 3 they showed 8.7 p of 0 to be present. The partial pressure of O3 along the tube with the discharge off was 18-9 p, but it decreased to 8.0 p when the discharge was turned on. This experiment clearly shows not only that the reaction o;+ 0 ~ - ) 0 ~ + 0 ~ + O (3)

ISSN:0366-9033    

DOI:10.1039/DF9643700026

出版商:RSC    

年代:1964

数据来源: RSC

摘要:

Mass Spectrometric Studies of Atom ReactionsPart 4.-Kinetics of 0 3 Formation in a Stream of Electrically Discharged 0 2BY A. MATHIAS AND H. I. SCHIFFUpper Atmosphere Chemistry Group, McGill University, Montreal, CanadaReceived 10th January, 1964A mass spectrometer was used to study the 0 3 concentration as a function of time after O2was partially dissociated by a microwave discharge. Reproducible results were obtained onlywhen the 0 2 was passed through traps cooled in liquid air, which suggested catalytic decompositionby hydrogenous impurities. Even with purified 0 2 , the 0 3 concentration reached maximum ratherthan steady-state values which would have been expected if only the reactions,were operative. The observation that more 0 3 could be destroyed than the number of 0 atomspresent suggested the occurrence of the reaction,(3)The 0 and 0: concentrations were determined by titration techniques. The ratio of [O;]/[O]was found to increase with pressure and the kinetics were consistent with the mechanism suggested.Values of the rate constant ratios were found to be k3lk2 = 1.7 and k2/k1 = 1 .2 ~ 1019 molecules,in agreement with the results of 0 3 decomposition studies. The inclusion of reaction (3) there-fore provides an explanation for the spread in the reported values of kl and permits reconciliationof the work on discharged 0 2 and on 0 3 decomposition.0; 4- 0 3 + 0 2 -k 0 2 4-0.Perhaps the most important chemical reactions which occur in the stratosphereare those which determine its ozone content.Aside from photolytic processes,these are primarilyO + ~ ~ + M - + O J + M (1)0+03+202. (2)The rate constants for these reactions are therefore of great importance in chemicalaeronomy. Unfortunately they are not known with any unanimity.Laboratory studies of these reactions are frequently performed with flow systemscontaining small percentages of atoms produced from electrically discharged 0 2 .The homogeneous recombination rates in such systems are usually equated to2k1[0][02][M]. However, such studies have produced a large spread in the reportedvalues of kl.1 These values are also appreciably lower than those which are deducedfrom studies of the thermal and photolytic decomposition of 03.2, 3A value for k:! has recently been obtained 4 from measurements of the rate ofdisappearance of O3 added to a stream of discharged 0 2 .Although it agreedreasonably well with the results of one study on the thermal decomposition of 0 3 , sit was considerably higher than the results of other such investigations.6 An evenhigher value 7 was obtained from the reaction of 0 3 with 0 atoms produced by thetitration reaction of " active " nitrogen with NO. However, the titration reactionalso produces 8 9 9 excited N2 molecules which are capable of decomposing 03.3A. MATHIAS AND H. I . SCHIFF 39The present paper reports an attempt to resolve these differences by an inde-pendent method. A mass spectrometric study was made of the 0 3 concentrationin a stream of discharged 02. If only reactions (1) and (2) were operative, a steadystate concentration would be established :from which the ratio of rate constants could be readily evaluated This relationshipwas, however, not found to be valid, which showed the occurrence of other processes.EXPERIMENTALThe mass spectrometer used was identical to the one described earlier4 except thatrhenium filaments were used throughout and the sampling hole was 15 p wide and 20 11deep.Linde U.S.P. grade 02, containing less than 0.1 % N2, was used directly since samplespurified by liquefaction gave identical results. The oxygen was passed through a 100 cmlength of quartz tubing 10 mm int. diam., over the mass spectrometer sampling " leak "and thence to the pump. The mass flow rate was controlled by a fine needle valve andmeasured with a calibrated capillary flow meter.The pressure in the system was variedat constant mass flow rate by throttling a valve located between the mass spectrometerleak and the pump. The pressure was measured just beyond the mass spectrometer leakeither with a Consolidated micromanometer or with a McLeod gauge. Although therewas a considerable pressure drop down this long length of relatively narrow tubing, theresults of these experiments indicated that the 0 3 concentration adjusted to the local pressurein the system so that the pressure at the leak was the significant parameter.The oxygen was dissociated to the extent of a few % in a cylindrical microwave cavitypowered by a Microthem unit. This cavity could be slid along the quartz tubing to alterthe distance (and hence the time) between the discharge and the mass spectrometer leak.The mass spectrometer was calibrated for 0 3 by introducing part of the effluent gasfrom a laboratory ozonizer into the flow system and measuring the mass 32 and 48 peakheights.Simultaneously, a known volume of the same gas was diverted through anaqueous 2 % KI solution, and its 0 3 content determined by conventional thiosulphatetitration.RESULTSExperiments with this arrangement gave results for the dependence of 0 3 con-centration on time which are typified by the plots of the open circles shown in fig. 1.Curves of this sort were irreproducible but all had the general characteristic of arise to a relatively flat portion followed by a further rapid increase at longer reactiontimes.However, when traps cooled by liquid air were inserted directly before thequartz tube, reproducible curves similar to the one through the closed circles wereobtained .A family of such curves at different pressures is shown in fig. 2. It will be seenthat the 0 3 concentrations reach maximum rather than steady-state values and thatthe maxima become sharper as the pressure increases.The dependence of the maximum 0 3 concentration on pressure is shown in fig. 3.The lines are drawn through two sets of experiments performed with different flowtubes and microwave generators. The linear pressure dependence is also at variancewith the quadratic relationship for the steady-state concentration based on.reactions(1) and (2). These results strongly suggest the presence of other reactive species.To test for the existence of such species, the apparatus was modified in the follow-ing manner. A section of 10 mm int. diam. Pyrex tubing, containing an inletthrough which other reactants could be added to the gas stream, was inserted be-tween the mass spectrometer leak and the quartz tube. This inlet, similar to thos40 MASS SPECTROMETRIC STUDIES OF ATOM REACTIONSdescribed previously,4~ 10 was located 25 cm from the mass spectrometer leak.Excess NO2 was added through this inlet to determine the 0-atom concentration inthe discharged 0 2 stream.4 Alternately, a relatively large flow of pure 0 3 wasintroduced through the inlet to determine the maximum amount which could bedestroyed by the discharged 0 2 .The amount of 0 3 which was decomposed wasI I I II I I01 0 2 03 0t , secFIG. 1 . 4 3 concentration as a function of time : 0, values obtained without traps cooled to 77°K ;e, values obtained with traps cooled to 77°K.always found to be greater than the 0-atom concentration, thus providing directevidence for the presence of some other species capable of decomposing 0 3 , mostlikely excited molecules, 03 :o;+ o,-+o,+ o,+ 0. (3)The concentration of excited molecules can be derived from the relation[0~]=2.(A[03]-A[NO2]) where A[NO2] and A[O3] are the amounts of NO2 and O3respectively which can be destroyed by the discharged oxygen stream. The inclusionof 3 in the expression results from reaction (3) being followed by (1) which consumesan additional O3 molecule.A number of experiments were performed to determine the concentration of0 atoms and 0: molecules at reaction times which produced the maxima in the[03]-time curves.The distance between the discharge and mass spectrometer leakwhich gave the maximum O3 concentration was first determined and the value of[031max noted. The discharge was then moved back 25 cm so that the [031maxposition now occurred at the inlet. The values of [O] and [O:] were then deter-mined as described in the previous paragraph. The results of these experimentsare shown in table 1A. MATHIAS AND H. I . SCHIFF 410:I 0.2 0.3 0.4 0.5time (sec)FIG. 2.-03 concentrations as a function of time at different pressures.The gas has been passedthrough traps cooled to 77°K.I I 42 MASS SPECTROMETRIC STUDIES OF ATOM REACTIONSTABLE 1 .---CONCENTRATIONS OF REACTANTS CORRESPONDING TO [O3]mm1021 r033rnax 101 rot1torr 10-4 torr 10-4 tom 10-3 tom0.250.620.731 -041-201 -451.851-952.303.201.581.672.766-102-123.647.3 13.8 14.8 16.6219.419.435.712.420.814.512-49.335.527-890-062.1 14.405.404.6513.412.019.918.616.2DISCUSSIONThe behaviour shown by the lower curves in fig. 1 can be understood if it isassumed that water vapour is the impurity removed by the traps. It is well knownthat water vapour is largely dissociated in an electrical discharge. The reaction ofH atoms with 0 3 ,has a rate constant 11 which is at least 103 times larger than the rate constant ofreaction (1).Thus, if one H atom is formed per H20 molecule, then in a dischargedgas stream containing a few % of 0 atoms, (4) will dominate over (1) whenever theH20 content of the original gas exceeds one part in 105. Moreover, the OHradicals formed either in the discharge or as a result of (4) will undergo the rapidreaction, 12Reactions (4) and (5) therefore constitute an H-atom catalyzed equivalent to reaction(2). The overall kinetics will be indistinguishable from (1) and (2) alone, so thatthe rate of 0-atom recombination should remain unaltered.* On the other hand,the 0 3 concentration will be lowered to a pseudo steady-state value given approx-imately by kl[0][02]2/kq[H], which can be associated with the flat portions of thecurves in fig.1. The 0 3 concentration will increase again only after the H atomsare removed by relatively slow three-body processes. Other evidence for the catalyticeffect of H atoms in 0-atom recombination has recently been reported.7If (l), (2) and (3) are the predominant reactions in dried, discharged oxygen, asteady-state 0 3 concentration will not be produced. Instead, a maximum will bereached given byH4-03-)OH+02, (4)O+OH+02+H. (5)klC0lCO2l2C031max = k2[O] + k3[0;]’This expression can be rearranged to the form* A similar argument might be invoked for nitrogeneous impurities which could yield NO inthe discharge. The catalytic 0 3 decomposition could then occur by the counterpart reactionsNO+03 +N0;!+02 (4’) and NO2f-O +N0+02 (5’). However, the rate constant for (4’) is notsignificantly larger than for (2), so that this reaction cannot be significantA .MATHIAS AND H . I . SCHIFF 43Thus, if this mechanism is operative, a plot of the left-hand side of this expressionagainst [O%]/[O] should yield a straight line. Such a plot is shown in fig. 4 for thedata of table 1. Least-mean-square treatment of the data yields the equation[02]2/[03]max = 4-02 x 102+6.98 x 102[0J.,]/[O] (torr).Fig. 3 shows that [03]max is proportional to [02] and independent of [O] or[O:]. Inspection of the expression for [03Imax shows that this result will be con-sistent with the proposed mechanism only if [O*,]/[O] is a linear function of [OZ] ;i.e.- k2 + -- k3[0Y21 = a[O4, where a is some constant. Fig. 5 shows that withinadmittedly large experimental error such a relationship seems to apply fora set of data obtained under similar conditions. However, a probably dependskl kl [OlFIG. 4.-A plot of [0#/[03] against [O,*]/[O] obtained at reaction times corresponding to maximum0 3 concentrations.on the purity of the gas and other discharge parameters, and there is no reason whyit should remain unaltered if discharge conditions are changed. This is probablywhy the data of fig. 3, obtained with two different experimental arrangements, lieon curves with different slopes. The increase of [O?]/[O] with 1021 also explainswhy the maxima in fig. 2 get narrower with increasing pressure, since the rate ofreaction (3) will then increase relative to the rate of reaction (2).The value of k2/kl = 1.2 x 1019 molecules cm-3 is in good agreement with theresults of a careful re-investigation 3 of the photolysis of 0 3 , and in satisfactoryagreement with the revised data from thermal 0 3 decomposition studies.Kaufmanand Kelso 13 have recently reported a value of 7-5 x 10-34 em6 molecule-2 sec-1 forkl from studies in flow systems uncomplicated by the presence of excited 0 2 . Com-bination of this value with the result of the present study yields a value of k2 = 9.0 44 MASS SPECTROMETRIC STUDIES OF ATOM REACTIONS10-15 cm3 molecule-1 sec-1 which is appreciably lower than the value of 2-5 x 10-14reported by Phillips and Schiff.4 However, their result may have been too high,since no particular precautions were taken to dry their oxygen.Also no correc-tions were made for possible reaction with 0:. This may not have been too seriousin their case because the same authors found 14 that the maximum 0 3 decompositionnever exceeded the 0-atom concentration by more than 10 %. This is probablybecause most of their experiments were at lower pressures where the present resultsshow that the [Oj.,]/[O] ratios are small, and probably because the ratio may alsodepend on gas purity.3020- *N - o_!o10I I I1.0 2.0 30[021 ton-FIG. 5.-The dependence on pressure of the [O,*]/[O] ratio obtained at reaction times correspondingto maximum 0 3 concentrations.From fig.4 it may be concluded that k3 = 1.7 k2. Reaction (3) may, however,be composite, since some measurements of the rates of 02 disappearance indicatedthe presence of more than one excited species. Arnold and Ogryzlo 15 have alsoreported that the concentration of 02(lAg) decreases slowly and O.#Zi) more rapidlywhen 0 3 is added to a stream of discharged 0 2 . A similar decrease in 02(1Ci)molecules was reported by Clyne et aZ.,7 although they estimate their concentrationto be less than 1 %. In the present work 0% concentrations amounting to several% were observed. Mass spectrometric studies 16, 17 of discharged 0 2 have indicatedup to 20 % of excited molecules which were tentatively identified as O@Ag). Theexcitation energy of this state is some 2 kcal less than the dissociation energy of0 3 .This is not, however, inconsistent with the deduction that k3 is of the sameorder of magnitude as k2 which is known to have an activation energy between4 and 6 kcal mole-1. Positive identification of the different species and their relativeimportance must await further study. However, the inclusion of reaction (3) doesprovide an explanation for the spread in the reported values of kl and permitsreconciliation of the work on discharged 0 2 and on 0 3 decompositionA . MATHIAS AND H . I . SCHIFF 45Acknowledgements are gratefully made for financial assistance to the U.S.A.F.Cambridge Research Laboratories and to the Defence Research Board of Canada.1 Kaufman and Kelso, Chemical Reactions in the Lower and Upper Atmosphere (Interscience,2 Campbell and Nudelman, AFOSR TN-60-502, 1960.3 Castellano and Schumacher, 2. physik. Chem., 1962, 34, 198.4 Phillips and Schiff, J. Chem. Physics, 1962, 36, 1509.5 Zaslowsky, Urbach, Leighton, Wnuk and Wojtowicz, J. Amer. Chem. SOC., 1960, 82, 2682.6 Benson and Axworthy, Ozone Chemistry and Technology (American Chemical Society, Wash-7 Clyne, Thrush and Wayne, Nature, 1963, 199, 1057.8 Phillips and Schiff, J. Chem. Physics, 1962, 36, 3283.9 Morgan, Phillips and Schiff, Disc. Firaday SOC., 1962, 33, 11 8.10 Morgan and Schiff, Can. J. Chem., 1963,41, 903.11 Phillips and Schiff, J. Chem. Physics, 1962, 37, 1233.12 Del Greco and Kaufman, Disc. Farday SOC. , 1962,33, 128.13 Kaufman and Kelso, Disc. Faraday SOC., this Discussion.14 Phillips and Schiff, J. Chem. Physics, 1962, 37, 924.15 Arnold and Ogryzlo, Abstr. 46th Con$ Chem. Inst. Canada ; Chem. in Can., 1963, 15, 54.16 Foner and Hudson, J. Chem. Physics, 1956,25,602.17 Herron and Schiff, Can. J. Chem., 1958, 36, 1159.New York, 1961), p. 255.ington, 1959), series 21, p. 388

ISSN:0366-9033    

DOI:10.1039/DF9643700038

出版商:RSC    

年代:1964

数据来源: RSC

摘要:

Reactions of 0 2 (I&) and O2 (lCg+)BY L. W. BADER AND E. A. OGRYZLODept. of Chemistry, University of British Columbia, Vancouver 8, CanadaReceived 14th January, 1964A method of obtaining measurable concentrations of 02(1Ag) and Oz(lXC,+) for kinetic studieshas been developed, and a number of reactions of these excited molecules are described. Evidenceis presented for simultaneous electronic transitions in a weakly bound complex between two 02(1Ag)molecules resulting in emission bands at 6340 and 7030A. The phenomenon is discussed in termsof the " association theory " of gases.The (0,l) 1 -58 ,u band of the (1 A, - 3Z;) system is one of the more intense featuresof the atmospheric " day glow " (the (0,O) 1.27 p band is re-absorbed by the loweratmosphere).l A number of excitation mechanisms have been suggested by VallanceJones ; 2 however, all of them depend on assumptions about the deactivation cross-sections for O@A,) which have not yet been determined.A prominent featureof the " night air glow " is the (0,l) 8645 A band of the (1Z$-3E;) system (the(0,O) 7619 band is also re-absorbed). Here again deactivation cross-sectionshave not been determined and Young and Sharpless 3 have indicated that no satis-factory excitation mechanism has yet been proposed.Both of these species are known to be present in electrically discharged oxygentogether with oxygen atoms, where the ratio of O@Z;) : 0 : 02(1Ag) : 02(1Zi) isusually about 1 : 0.1 : 0.05 : 0.0015.4 When these products are pumped out of thedischarge into a fast flow system the atom concentration is essentially unalteredsince both the gas phase and wall recombination are sl0w.5 The 02(1Zl) concen-tration is still large; however, since it is continually being formed by the recom-bination of atoms, its reactions cannot be studied conveniently in this system.Thereis, in the literature, only indirect evidence for the presence of 02(1Ag) in the dis-charge products. Foner and Hudson6 showed the presence of a species with anappearance potential about 1 eV below that of ground-state oxygen. They sug-gested that it might be 02(1Ag), in which case about 10 % of the oxygen could be inthis state. Elias, Bgryzlo and Schiff 5 presented calorimetric evidence that wasconsistent with about 10 % 02(1Ag) in the discharge products.They also describedthe following three methods that could be used to remove atoms preferentially fromthe gas stream. (i) NO2 can be added to the stream. Atoms are rapidly removedand some calorimetrically-detectable excited molecules remain. However, thepresence of NO and NO2 then complicates any kinetic studies. (ii) A coil of silverwire placed in the gas stream rapidly recombines the atoms and more slowly de-activates the excited molecules. (iii) The distillation of mercury through the dis-charge creates a mercuric oxide deposit after the discharge which removes atomsbut not excited molecules. Using these sources of excited molecules we have begunan investigation of some of their reactions.EXPERIMENTALA typical Pyrex fast-flow system5 was equipped with Edwards needle valves and a581./min vacuum pump. The discharge was maintained in air-cooled 13 mrn Pyrex tubingby a Raytheon 2450 rncfsec, 100 W generator.Two light-traps separated it from the4L. W. BADER AND E. A . OGRYZLO 47reaction-tube. The 20mm (int. diam.) reaction-tube was jacketed and insulated so thatthe temperature could be varied between 173 and 373°K. Two different spectrographscould be placed against an optical window at the end of the reaction tube. A low dis-persion (5 4.6) Hilger-Watts glass prism monochromator was equipped with a 27 c/secmechanical chopper, an RCA-7265 (or 7 102) liquid-nitrogen-cooled photomultiplier anda conventional tuned a.c. amplifier. With this instrument very weak emissions could bedetected between 3000 and 13,OOOA.A Jarell-Ash (5 6.3) grating spectrograph blazedat 7500 A with a dispersion of 20 A/mm in first order was used when greater resolution wasrequired.An isothermal calorimetric detector 5 could be moved along the reaction tube. Its cobaltsurface removes all detectable excited molecules from the gas stream. By operating thecoil at a constant temperature the total " energy content " of the gas stream could bedetermined.For most work, ordinary tank oxygen was used without further purification. Whennecessary, it was dried by passing it over P205 and through a liquid-nitrogen trap. Nitrogen-free oxygen was prepared by thermally decomposing KMnO4. Other gases were obtainedfrom the Matheson Company.Ozone was prepared with a commercial generator andstored on silica gel at dry-ice temperature.Preliminary studies indicated that under conditions where atoms were completelyremoved from the gas stream by a silver wire, the excited molecule concentration was alsogreatly reduced. This was not found to be the case when mercury was used to destroythe atoms, and hence the later method was used in all studies reported in this paper.RESULTSCurve (a), fig. 1, shows a typical example of the energy liberated to a cobaltdetector by the products of electrically discharged oxygen from which all the atomshave been removed. The presence of the slightest trace of atoms is easily detectable20 3 0 4 0distance from discharge (cm)FIG. 1 .-Curve (a), heat measured by cobalt detector in excited molecule stream ; P = 3 mm Hg ;T = 300°K.Curve (b), heat measured when a partial pressure of 0.03 mm Hg of water is addedafter the discharge.by the yellow-green NO2 emission to which the eye is very sensitive. When allthe atoms have been removed, a weak red emission becomes visible. Fig. 2 showsthe emission detectable with the Hilger-Watts spectrograph and a cooled 7102photomultiplier. The most prominent peak is at 7619A. This is undoubtedlythe (0,O) band of the (lX;-3Z;) intercombination. The band at 8645A is thenthe (0,l) transition in the same system. Photographs of these bands with the highe48 REACTIONS OF 02(1Ag) AND 02(2S;)dispersion grating spectrograph show the characteristic rotational fine structure ofthe (lZl-3S;) transition, confirming the assignment.We have been unable todetect either the (1,l) band at 7708 A or the (1,O) band at 6882 A, though thetransition probabilities are comparable to those for the (0,O) and (0,l) transitionsrespectively. This is taken as evidence for the absence of appreciable concentrationsof vibrationally excited oxygen.000II///06340 7030 7619 8645 12700A <A>FIG. 2.-Emission spectrum of electrically discharged oxygen with all atoms removed obtainedwith Hilger-Watts glass-prism spectrometer, slit 500 microns, and liquid nitrogen cooled RCA-7102 photomultiplier. Spectral sensitivity of RCA-7102 given by dotted curve.The peak at 12,700A is undoubtedly the (0,O) band of the (1Ag-3Z;) inter-combination, confirming previous mass spectrometric 6 and calorimetric 5 evidence.Since the sensitivity of the photomultiplier at 12,700 A is less than 10-2 of its maximumvalue at 8000A, the emission is fairly intense.The peaks at 6340 and 7030A cannot be attributed to any known transitionin 0 2 .They have been observed in a number of chemiluminescent reactions insolution ; 79 8 however, Khan and Kasha's 9 recent assignment of these peaks tosolvent shifted (0,O) and (0,l) (1Zl-3Zi) transitions must be in error. Fig. 3shows the 6340 A band photographed with a grating spectrograph (20 &mm dis-persion). Neon calibration lines are superimposed on it. Only a structurelessdiffuse band could be obtained. To eliminate the possibility that it is an " impurityemission ", nitrogen-free oxygen was prepared and thoroughly dried with P205.The intensities of the peaks were essentially unaltered.Furthermore the additionof nitrogen and water before the discharge, and NO?, NO, H20, N20, Ar, HeL. W. BADER AND E. A. OGRYZLO 49CO, C02, NH3 and H2 after the discharge had no significant effect on either the6340 or the 7030A peak. The 7619A peak was, however, affected by a numberof gases. Water had the most marked effect (heavy water was about half as effec-tive). A small amount of water added after the discharge completely removed the7619 and 8645A bands and left the 6340, 7030 and 12,700A bands unaltered.This observation shows that the two new bands do not involve 02(1X$).The lack of reactivity of the species responsible for this emission, the presenceof only two bands, and the large spacing between the bands allows us to eliminateother 0 2 such as C3A,, A3E; and clZ;.-rexcited states of r .r.."6217 6266 6334 6302 64021 (A)FIG.3.-The 6340A band obtained with a Jarell-Ash f 6-3 grating spectrograph, 2 0 & m dis-persion, 30 micron slit, 72 h exposure on Kodak 103 a F spectroscopic plate. Small peaks aresuperimposed neon calibration lines.A study of the relationship between the 02(1Ag) concentration and the new bandsprovided more positive evidence for their source. A direct comparison of the6340 peak and the 12,700 A peak was not possible because of the extremely variablesensitivity of the cooled photomultiplier in the 12,700 A region.However, thedetector could be used to measure the 02(1Ag) calorimetrically. Curve (b), fig. 1,illustrates the effect of water (added after the discharge) on the energy content ofthe gas stream. Spectroscopically the removal of 02(lXg) can be seen by the reduc-tion of the 7619A emission intensity to less than 1 % of its original value. The02(1Ag) concentration remains unaltered. In all of our experiments we have foundthat the 6340 A emission intensity is proportional to the square of the heat measuredby the detector. A typical set of results is given in table 1, where it is assumedthat the heat measured by the detector is due entirely to 02(1Ag).TABLE 1P = 4-4 ~llfn Hg ;T = 25°C ;0 2 flow = 160 pmoles/sec ;H20 flow = 0.5 ,u moles/sec.experiment 102(1 AJ flow (pmoles/sec) 2.5 1.7I = 6340 A emission intensity (arbitrary units) 55.0 27-5microwave power output (watts) 100 20C = 02(1Ag) concentration (moles/cm3) 3 .7 ~ 10-9 2 . 6 ~ 10-911/12 = 2-00; Ci/C2 = 142 ; (Ci/C2)2 = 2.050 REACTIONS OF 02(lAg) AND 02(1E:)In view of these observations, and the fact that twice the electronic energy of02(1Ag) is equal to 15765 cm-1 or 6343 A, we propose the following processes toaccount for the two emission bands.M2O2('A,)~O~+(O4),=, + hv(6340 A),20,(1Ag)+O:-(04),=1 +hv(7030 A). MTo determine whether the emitting species (0:) is simply a colliding pair ofmolecules or a stabilized dimer, the temperature of the reaction tube was varied.Fig. 4 shows the emission spectrum recorded with an RCA-7265 photomultiplier.16340 7 0 3 0 76 191 (A)FIG. 4.-Effect of temperature on the excited molecule emission.RCA-7265 photomultiplier,500 micron slit. Curve (a) obtained at 25°C ; curve (b) at -29°C.Because of its different spectral response it yields 7619 and 6340A peaks of com-parable height. Curve (a) was obtained at 25°C and curve (b) at -29°C. In theflow system the pressure remains constant when the temperature is changed in asmall region of the system. Assuming ideal behaviour (P = cRT), the concentra-tion of all independent species will be inversely proportional to the temperature.Hence the concentration of 02(1CB+) would be expected to increase by a factor of2981244 = 1.22 when the temperature is changed between 25 and -29°C.The ratioof the 761 9 A emission intensities at these temperatures (obtained by integrating underthe curves in fig. 4) is 1-21 which is the expected result within experimental error.The ratio of the emission intensities for the 6340A band is, however, 1.97. Since thL. W. BADER AND E. A. OGRYZLO 51emission is proportional to the square of the 02(1Ag) concentration, a factor of(1 *22)2 = 1-49 is due to the concentration change alone. The additional decrease weattribute to the dissociation energy of 0:. If we assume that the process consists ofthe following steps :K02('Ag)+ 02('Ag)+ M+O,('Ag, 'A,)+M, (3)04('Ag, ' ~ i ~ ) - + o ~ ( ~ X ; "Xi)+ hv(6340 A), (4)kwhere M is any third body, k is the emission probability defined by I = d(hv)/dt =k[04(1Ag, lAg)], I is the 6340 A band emission intensity, K is the equilibrium con-stant for the formation of dimolecular complexes (abbreviated to dimols in the restof this paper), i.e.and the dimol concentration is governed by K (i,e., that reaction (4) is the rate-controlling step) ; then since04(1Ag,lAg) = I/k and 02(1Ag) = P/RT(where P = partial pressure of 02(lAg)),Hence, on substitution into the thermodynamic relationship,In (IlT:/I2T,2) = AU(T~-TI)/RT~T~,where AU is the change in internal energy and equals the bond dissociation energyD at some average temperature T.Substituting into this equation the values forZ and T given above, one obtains - AU = D(02(lAff) -02(1Aff)) = 600 cal.K = C04(lAg, 1Ag)llCo2(1Ag)129.'.K = RZIT2/kP2.In (K1IK2) = AU(T~-TI)/RTIT~,REACTIONS WITH OZONEThe effect of the addition of 0 3 to the excited molecule stream is shown in fig. 5.Visually the effect is dramatic since the weak red emission is masked by the strongyellow-green NO;! emission which results from the reaction : 0 + NO-,NO2 + hv.It can be seen that the 02(1Zgf) concentration is lowered by more than a factor of10, suggesting that the dissociation is due to this species, i.e.02(12,')+ 0 3 + 2 0 2 ( 3 ~ , ) + 0. ( 5 )o(~P)+ 03(1A)+02(1Ag)+ 0,(~2;). (6)On the other hand, the 6340 and 7030 A bands increase when 0 3 is added. If thismeans that the 02(1Ag) concentration increases, it may be produced in the reaction,This explanation is not entirely satisfactory in view of the effect of the addition ofwater shown in fig.6, where 99 % of the Oz(lX,+) has been removed by water.Ozone is, however, still strongly dissociated. No change in the 02(1Zi) concen-tration occurs and surprisingly the 02(1 Ag) concentration (taken from the 7030peak height) seems. to have risen. Clearly, eqn. (5) and (6) are not sufficient. Itis possible, however, that these results can be explained by some energy chain thatis only initiated by excited molecules.NO2 - GLO WWhen NO2 is added to the gas stream containing 02(1E:) and 02(1Ag), theconcentrations of the excited molecules are not greatly altered even when larg52 REACTIONS OF 02(1Ag) AND 02(1Ei)I I I I I6340 7 0 3 0 7119 8645 1270A (A)FIG. 5.-Effect of ozone on emission.Curve (a), excited molecules only; curve (b), excitedmolecules + ozone.I6 340 7030 7619 8545 12700A (4FIG. 6.-Effect of water on ozone reaction. Curve (a), excited molecules+water; curve (b),excited molecules, water and ozoneL . W. BADER AND E. A. OGRYZLO 53amounts are added. However, as the NO2 concentration is increased the normalred emission is masked by a higher energy emission that has a spectral distributionvery similar to that due to the 0 plus NO reaction shown in fig. 5. However, therecan be no atoms present in this system because of the large NO2 concentration.When H20 is added to the stream the NO2 glow decreases in intensity. It disappearswhen sufficient water is added to remove completely 02(1Z3.DISCUSSIONWe believe that the 6340 and 7030A bands described in this paper can be ex-plained only by the presence of dimolecular complexes (dimols) of 02(1Ag), witha bond dissociation energy of about 600cal.The two bands then arise fromsimultaneous electronic transitions in the loosely associated pair. The 6340 Aemission arises when only ground vibrational states of 0 2 are involved, while the7030A band arises when one of the 0 2 molecules ends up in its first excited vibra-tional level. Though such simultaneous electronic transitions do not seem to havebeen reported previously in emission, they have been proposed to account for anumber of absorption bands in high pressure systems.Rank et aZ.10 reported the appearance of some new absorption bands in pureHCl and HC1 mixtures with Ar and Xe that appear at high pressures.The intensityof the bands was found to be proportional to the square of the pressure. Fromthe temperature variation of the absorption they calculated the following bonddissociation energies :D(HC1-HC1) = 2.14 kcal, D(HC1-Ar) = 1.1 kcal, D(HC1-Xe) = 1.6 kcal.Hoijtink et aZ.11 reported a study of the absorption spectrum of mixtures ofnaphthalene and 0 2 in chloroform. They observed a new band at 29,000cm-1with an intensity proportional to the concentration of naphthalene and 0 2 . Theyattribute the band to a simultaneous transition in a naphthalene-02 complex.The effect of a change in temperature was not reported so that it is not possible tosay whether dimols are responsible.More relevant to our work is the absorption spectrum of liquid and gaseousoxygen.Ellis and Kneser 12 were the first to attribute peaks observed in liquidoxygen at 6290 and 5770 to the process :where the 6290A peak arises when all the species are in their ground vibrationalstate, and the 5770A peak arises when one of the 02(1Ag) molecules ends up inits first excited vibrational level. Salow and Steiner 13 studied the pressure depend-ence of these bands in highly compressed gases, and found that they dependedon the square of the oxygen pressure and were independent of the pressure of addedgases, confirming Ellis and Kneser's assignment, More recently Dianov-Klokov 14studied the temperature dependence of the band intensities. He found that theychanged little with temperature, possibly increasing slightly between 77 and 298°K.Dianov-Klokov therefore concluded that the absorption was due to an 0 4 collisioncomplex.This result might appear inconsistent with our emission studies.However,the different temperature dependence may be due to the fact that the absorptionintensities were measured at a constant density of 1.17 g/cm3, which is the densityof the liquid. Under these conditions the average intermolecular distances arefixed at the values that would be assumed in our proposed dimol. Hence, changingthe temperature could well have no appreciable effect on the absorption intensity54 REACTIONS OF 02(1Aq) AND 02(lC;)For any given gaseous system there are two distinct sources for both simultaneousand induced transitions in molecules : (i) collision complexes, or (ii) dimols (stabil-ized dimolecular complexes).With the information available at present it is notpossible to state when the shorter intermolecular distance in the collision complexis more important to the transition probability than the longer lifetime of the dimol.However, it is certain that a number of simultaneous and induced transitions canbe explained only by dimols, and there appears to be no evidence in the literatureinconsistent with the idea that dimols are universally responsible for such transitionsat low temperatures.In a system containing 02(3X;), 02(1 A& and 02(1XB+) we might expect the follow-ing dimols to be present :(1) 04(3x;, 3q); (2) 04(?4g, lAg);(3) 0 4 ( 1 q , l q ) ; (4) 0d3q¶ ‘$1;(5) 04(3z;, l g ) ; (6) 0 4 ( 1 q , lAg).At any given temperature the dimol equilibrium concentration will depend on itsbond dissociation energy and the concentration of the appropriate molecules. Thedissociation energy of dimole (1) can be estimated from PVT data. With someassumptions about the form of the interaction potential, the interaction energy Ecan be calculated from the second virial coefficient.Assuming a Leonard Jones(6 : 12) potential one obtains 15 ~ ( 0 2 - 0 2 ) = 230 cal. Unfortunately, AU andthus the dissociation energy at some temperature T, is not easily calculated fromE. However, Stogryn and Hershfelder 15 have calculated Kp (the equilibriumconstant for dimol formation) from the second virial coefficient.Curve (a) in fig. 7shows a plot of loglo Kp against 1/T from their data. From the slope of the curveat 273”K, - AH = 840 cal, and therefore - AU(273”K) = D(O2-02) = 300 cal.These results, however, still depend on the assumed form for the interaction potential.A more direct estimate of AH can be obtained from “association theory”.16Assuming that the deviations from ideal gas behaviour at moderate pressures isdue to (a) the size of the molecules, and (b) the formation of dimols, trimols, etc.,we can write the equation of state (for one mole of gas) :where b = excluded volume = 4 (molecular volume) ; KD and KT are the equilbriumconstants for the formation of dimols and trimols respectively.If we consider conditions where only the dimol term is significant we obtain uponrearranging :P(V-b)= RT(1-pKD-p2KT-.. .),PV/RT = 1 - P(K, - b/RT).A plot of PV/RT against P for oxygen at several temperatures is shown in fig. 8.The slope is equal to -(Kp-b/RT). Assuming a value of b = 0.0318 the valuesof Kp obtained are plotted in fig. 7, curve (b). From the slope of the curve at273”K, -AH = 940, and - AU = D(O2-02) = 400 cal. It is not possible todecide between this value and that obtained from Stogryn and Hershfelder’s data.Dimol (2) with a dissociation energy somewhat greater than dimol (1) gives riseto the 6340 and 7030A bands. Dimols (3) and (6) could give rise to bands at 3810and 4760A respectively. We have not been able to detect any emission at thesewavelengths.This may be due to the lower concentration of O#C:), or to a smallerdissociation energy for the dimols. It is possible, however, that one of them isresponsible for the N02-glow. From the change in the 7619A emission bandintensity with temperature it appears that emission from dimol (5) does not adL. W. BADER AND E. A . OGRYZLO 55to the emission in this band. This is consistent with the fact that the molar ex-tinction coefficient for the highly compressed gas does not differ significantly fromthat of the low pressure gas.139 14 Because the 12,700A band intensity could not01/Tx lo3 (OK-1)FIG. 7.-A plot of the loglo of the equilibrium constant Kp for dimol formation against l/Tfor oxygen.Curve (a), from the data of Stogryn and Hershfelder; curve (b), from associate theory.P (atm)FIG.8.-Plot of PV/RT against P for oxygen from the data of Birdsall, Jenkens, Dipaslo, Beattieand Apt, J. Chern. Physics, 1955, 23,441.be accurately measured, it was not possible to determine whether dimol (6) con-tributed to this band. From the magnitude of the pressure-induced absorption 14one might expect it to be significant at about 1 atm pressure. However, the rota-tional fine structure observed under these conditions by Herzberg and Henberg16suggests that it is a simple molecular absorption56 REACTIONS OF 02(1&) AND 02(1xi)The research for this paper was supported by the Defence Research Board ofCanada, Grant number 9530-31 and by the Air Force Office of Scientific ResearchGrant number OSR-158-63.1 Noxon and Vallance Jones, Nature, 1962, 196, 157.2Vallance Jones and Gattinger, Planet. Space Sci., 1963, 11, 961.3 Young and Sharpless, J. Geophys. Res., 1962, 67, 3871.4 Noxon, Can. J. Physics, 1961, 39, 11 10.5 Elias, Ogryzlo and Schiff, Can. J. Chem., 1959, 37, 1680.6 Foner and Hudson, J. Chem. Physics, 1956, 25,601.7 Seliger, Anal. Biochem., 1960, 1, 60.8 Bowen and Lloyd, Proc. Chem. Soc., 1963, 305.9 Khan and Kasha, J. Chem. Physics, 1963,39,2105.10 Rank, Sitaram, Glickerman and Wiggins, J, Chem. Physics, 1963, 39, 2673.11 Dijkgraaf, Sitters and Hoijtink, J. MoZ. Spectr., 1963, 6, 643.12 Ellis and Kneser, 2. Physik, 1933, 86, 583.13 Salow and Steiner, 2. Physik, 1936, 99, 137.14 Dianov-Klokov, Opt. i. Spektr., 1959, 7, 377.15 Stogryn and Hershfelder, J. Chem. Physics, 1959, 31, 1531.16 Ogryzlo, J. Chem. Educ., to be published.17 Herzberg and Herzberg, Astrophys. J., 1947, 105, 353

ISSN:0366-9033    

DOI:10.1039/DF9643700046

出版商:RSC    

年代:1964

数据来源: RSC

摘要:

Reactions of Oxygen Atoms in the Photolysis of CarbonDioxide *BY PETER WARNECKGeophysics Corporation of America, Bedford, Massachusetts, U.S.A.Received 10th January, 1964The vacuum ultra-violet photolysis of CO2 has been investigated in the pressure region 150-760 mm Hg. At 1236 A and around 1600 8, the quantum yields for carbon monoxide, oxygen andozone are consistent with the photodissociation mechanism, while an increase in the quantumyields at 1470A indicates that at this wavelength excited C02 molecules are also involved. Fromthe low rates of ozone formation observed in all cases it is concluded that the photodissociationof C02 produces oxygen atoms in the metastable 1D state. This is confirmed by measurements ofthe ratio of the rate constants associated with the reactions,OfO2-l- M +03+M k3O+ O3 +202 k4An average value of k4/k3[M] = 41 has been found.Although the photodissociation of oxygen and ozone in the Earth’s upperatmosphere is known to generate oxygen atoms in the metastable ID state inaddition to ground-state oxygen atoms, little attention could be given to the fateof the excited species owing to a lack of knowledge concerning the appropriatereaction rates.However, ID oxygen atoms can be expected to show a markedcontrast in reactivity on account of the difference in spin and/or internal energy.It is the principal aim of this paper to show that the photodissociation of carbondioxide in the 1200-1 700 A wavelength regionhvCO,~CO+Oproduces oxygen atoms in the excited 1D state, so that the photolysis of C02 mayprovide a convenient framework for a study of the role of 1D oxygen atoms in thereactions2 0 + M-02 + M0 4- 02( + M)+03( + M)(2)(3)0 + 03-+202 (4)which in combination with (1) constitute the main C02 photodecomposition mechan-ism.There is little doubt that oxygen atoms are indeed produced in the primaryprocess of carbon dioxide photodecomposition, since the COa absorption spectrumin the 1200-1700 A wavelength region exhibits two dissociation continua which areshown in fig. 1. Both continua have been associated 1 with a process of type (1).The participation of 1D oxygen atoms in the CO2 photodecomposition mechanismwas first suggested by Mahan 2 who photolyzed C02 in the presence of small amountsof NO but failed to observe the chemiluminescence characteristic for 3P oxygenatoms.In the present study of carbon dioxide photolysis, further evidence wasderived from the rate of ozone formation, which was measured in addition to CO* This work was supported in part by the National Aeronautics and Space Administration.558 co2 PHOTOLYSISand 0 2 quantum yields. Also, the ratio of the rate constants associated withreaction (3) and (4) was determined and compared with the known ratio for 3Poxygen atoms. These investigations were carried out at pressures between 150 and760 mm Hg, utilizing krypton, xenon and hydrogen light sources.1I I I I I I1300 1400 1500 1600 170 0wavelength (A)FIG. 1 .-Vacuum ultra-violet absorption spectrum and continua (dashed) of carbon dioxide,superimposed by the emission lines : Kr 1236 A, Xe 1470 A, and the many lined hydrogen spectrumaround 1600 A.EXPERIMENTALLIGHT SOURCE AND ACTINOMETRYExperiments were performed with a cylindrical flow reactor consisting of two concentricPyrex tubings, of which the outer one served as the gas inlet.The reactor was attachedto a microwave-powered discharge lamp of similar design as a previously described hydrogenlight source.3 Separate discharge tubes, made of Pyrex or quartz, were employed forkrypton, xenon and hydrogen in order to prevent contamination of the desired spectrumwith foreign lines. The reactor and light source were separated by a window of suitablematerial. Lithium fluoride windows were employed in conjunction with krypton dis-charges, while for the other gases barium fluoride plates were inserted. Since BaF2 doesnot appreciably transmit at wavelengths below 1400 A, it provides a suitable cutoff filtereliminating, among other lines, Lyman-alpha at 1216A and the xenon line at 1295A.Accordingly, the vacuum ultra-violet emission spectra associated with the three lightsources consisted mainly of the lines (a) Kr 1236 A, (b) Xe 1470 A, and (c) of the hydrogenmany-lined spectrum centred near 1600 A.The position of these emission lines relativeto the CO2 absorption continua is shown in fig. 1.Integrated light source intensities ranged from 2x 1015 to 2 x 1016 quanta/sec forkrypton and xenon discharges, and up to 1 x 1016 quanta/sec for the hydrogen lamp.They were determined actinometrically from the rate of ozone formation in a flow oP.WARNECK 59oxygen.3.4 A quantum yield of two 49 5 was assumed in accordance with the view thatat atmospheric pressure alrnost all the oxygen atoms resulting from 0 2 dissociation areconverted into ozone. Only the 11 65 A krypton line is probably ineffective in dissociating0 2 , since it falls into a region between bands where the 0 2 absorption strength is low(k ~ 0 . 5 cm-l).' However, since the CO quantum yields obtained in CO2 photolyseswith the krypton lamp were close to unity, it is concluded that the 1165 A line cannotcontribute more than 10 % of the intensity contained in the 1236A krypton line.QUANTUM YIELDSOzone concentrations were determined from the absorption of the 2537 A mercuryline which lies near the maximum of ozone absorption in this spectral region. A measuredflow of C02 was passed through the reactor, where ozone was formed as a photolyticproduct, and subsequently through a 40.5 cm long absorption tube fitted with quartzwindows at both ends.A Pen-Ray mercury lamp and a rubidium telluride (solar blind)photomultiplier in conjunction with a high voltage power supply and a microammeterserved to measure the extent of light absorption. Owing to the particular spectral charac-teristics of the mercury lamp and the photomultiplier combined, no additional filters wererequired for an adequate isolation of the 2537A mercury line. The ozone quantum yieldis calculated from Q(O3) = [O3]u/l, where [ 0 3 ] is the steady-state ozone concentration inthe absorption tube, II is the bulk flow rate in cm3/sec, and I in quantalsec is the effectiveintegrated intensity of the light source.The amounts of carbon monoxide and oxygen produced during the photodecomposi-tion of C02 were investigated in a closed system in which the gas was circulated by meansof a magnetically-driven rotating stirrer.Prior to the introduction of C 0 2 in the desiredquantity, the system was prefilled with approximately 3 mm Hg of argon which served asa reference in the subsequent mass spectrometric analysis. Each sample drawn aftercompletion of a photolysis run was subjected to liquid nitrogen temperature while beingadmitted to the mass spectrometer in order to minimize the contribution to the mass 28peak originating from carbon dioxide.With the exception of carbon dioxide, research-grade gases were employed withoutfurther purification.C 0 2 was found to contain about 0.05 % oxygen as well as 0.25 %nitrogen and/or carbon monoxide. These gases were removed by subjecting a lecturebottle of C02 to liquid nitrogen temperature and pumping off the volatile components.Repeated application of this procedure was required before the desired degree of purifica-tion was achieved (less than 20 p.p.m. contamination as determined mass spectrometrically).RESULTSco QUANTUM YIELDSThe data obtained by mass spectrometric analysis of samples drawn from theclosed system are summarized in fig. 2. CO quantum yields of unity were foundthroughout the investigated pressure region when C02 was irradiated with lightfrom krypton or hydrogen discharges, but the xenon light source resulted in an increaseof the CO quantum yield from unity at lower pressures to an average of Q(C0) = 1-17at 740 mm Hg.Similarly, a comparatively higher oxygen quantum yield was ob-served in the latter case. This indicates a difference in the photolytic mechanismwhich must be active at the wavelengths of the lines emitted from a xenon dischargeon one hand, and a krypton or hydrogen discharge on the other. The results ob-tained with the krypton lamp can be combined with the data reported by Mahan2to demonstrate that at 1236A the CO quantum yield is insensitive to pressure vari-ations in the region 10-760 mm Hg, thus giving additional support to the conclusionthat carbon monoxide is formed directly in the primary process.The observedoxygen quantum yields (Q(O2) = 0.3 for krypton and hydrogen, ( 3 0 2 ) = 0.4 forxenon) were higher than those reported by Mahan, but still below the limit o60 c02 PHOTOLYSISQ(O2) = 0.5 to be expected if all oxygen atoms produced in the primary processrecombined to yield molecular oxygen. However, the missing oxygen has beenshown to exist in the form of ozone.I I I I I 1 I In n nFIG. 2.-Quantum yields of carbon monoxide and oxygen ; 10-min irradiation with intensities ofabout 5x 1015 quanta/sec. Filled circles: at 1236A; open circles: at 1470A; squares: atabout 1600 A.I10 20time (min)intensity 5 x 101s quanta/sec.FIG.3.-Time dependence of carbon monoxide and oxygen formation at 1470 A ; irradiationThe time dependence of carbon monoxide and oxygen formation is plotted in fig.3 for experiments in which a xenon discharge and pressures close to an atmospherewere employed. The observed linearity substantiates the results presented in fig. 2P. WARNECK 61indicating that for the irradiation intervals and intensities applied the quantum yieldsare time independent. Further, since the straight lines obtained meet at the originof the co-ordinates, the absence of perceptible amounts of impurities in the em-ployed C02 is evidenced.OZONE QUANTUM YIELDSMost of the experiments designed to measure the amount of ozone formationwere carried out at atmospheric pressure.As one could expect on account ofthe stoichometry of the reactions involved, the observed ozone quantum yieldsdepended strongly on the concentration of oxygen contained in the carbon dioxide,and also upon the applied flow rates and irradiation intensities. No ozone couldbe detected when a flow of extensively purified CO:! was irradiated with the kryptonor the hydrogen discharge, even if high light intensities and low flow rates wereemployed. As a consequence, in these cases, only an upper limit quantum yield ofQ(03) <0.01 can be given. The ozone quantum yields observed with the xenon lightsource irradiating pure C02 were in the range of Q(O3) = 0.015, but this was still in-sufficient for an accurate determination of the intensity dependence.photon flux x 10-16 (quanta/cm* sec)I I0.5 1.0 1.5i I I~~0 05 1.0 1.5 Llight source intensity X 10-16 (quantalsec)3FIG. 4.-Ozone quantum yields for COz containing 0 057 % oxygen ; average flow rate 1 -9 cm3/sec.Open circles : at 1236 8, ; filled circles : around 1600 A.Larger concentrations of ozone were found when the employed carbon dioxidecontained a trace of oxygen (0.057 %) making it worthwhile to study the corres-ponding ozone quantum yields as a function of radiation intensity under otherwisesimilar conditions.Fig. 4 shows that the data obtained separately with kryptonand hydrogen discharges overlap quite closely, implying that the mechanism ofozone formation is the same in both cases. The increase in the ozone quantumyield with decreasing intensity is in accord with the kinetic mechanism. As theconcentration of oxygen atoms is reduced, their attachment to molecular oxygenby reaction (3) is favoured in comparison to the second-order direct recombinatio62 c02 PHOTOLYSIS(2).Similarly, reaction (4) is more effective at higher intensities owing to the largerabsolute amounts of ozone involved.No systematic study was performed with the xenon discharge under these con-ditions. However, as was the case with purified COz, the rate of ozone formed asa result of the 1470A irradiation was again greater than that observed at the otherwavelengths. For example, with a xenon light intensity of 7.5 x 1015 quantalsecan ozone quantum yield of Q(O3) = 0.225 was determined, while according tofig.4, the other discharges produce an average Q(O3) = 0.125.The pressure dependence of the ozone quantum yield was briefly explored em-ploying carbon dioxide containing 0.057 % oxygen and irradiating it with a con-stant intensity of light produced in the xenon discharge. According to the linearvariation of oxygen concentration with pressure the production of ozone is expectedto decrease as the pressure is reduced, provided the assumed photolytic mechanismis correct. Indeed, fig. 5 shows that the amount of ozone formation when expressedI '7 I/- I I I2 0 0 400 6 0 0 1pressure (mm Hg)FIG. 5.-Dr~"sure dependence of % ozone formation at 1470 A.in % of that observed at 760 mm Hg exhibits an almost linear pressure dependence.Since with decreasing pressure reaction (4) becomes less effective due to the smallerconcentrations of ozone involved, the pressure dependence of the ozone quantumyield seems to indicate that reactions (2) and (3) have similar third-body requirements.Although all of the above reported results concerning krypton and hydrogenradiation are qualitatively consistent with the invoked mechanism, the observedozone quantum yields are considerably smaller than could be expected if the involvedoxygen atoms were in the 3P ground state.With the use of currently accepted valuesfor the rate constants of ground-state oxygen atoms 6 one estimates, for example,ozone quantum yields in the range Q(O3) = 0-1 for a 1 cm3/sec flow of pure carbondioxide, whereas the observed quantum yields are smaller by at least an order ofmagnitude.However, this result would not be unreasonable if the oxygen atomsgenerated in the C02 photolysis were in the metastable 1D state rather than in thP. WARNECK 633P ground state, because in this case the formation of ozone by reaction (3) is spinforbidden so that the corresponding rate constant would be comparatively smaller.Although the formation of 1s oxygen atoms is energetically feasible at 1236& itcan be precluded since the energy available at the onset of the involved continuumis insufficient (see fig. 1). Thus, it appears that the photodissociation of carbondioxide produces 1D oxygen atoms. To provide a more quantitative test, the ratioof the rate constants of reactions (3) and (4) was measured and compared with theknown value associated with 3P oxygen atoms. A carbon dioxide + oxygen mixturecontaining 2.66 % oxygen was prepared and irradiated with light from the kryptondischarge.The ratio of the absorption cross-sections for carbon dioxide and oxygenat 1236A is quite favourable so that only one-tenth of the available radiation in-tensity is absorbed by the oxygen contained in the mixture. However, the highoxygen concentration promotes reactions (3) and (4) relative to reaction (2) renderingthe latter unimportant. Table 1 demonstrates that the resulting ozone quantumyields are in the vicinity of unity, but a considerable variation with the appliedintensities and flow rates is also noticeable.run no.TABLE 1 .-03 QUANTUM YIELDS AND VALUES FOR y = k4/k3[M] IN THE1236A PHOTOLYSIS OF co2 CONTAINING 2.66 % 0 279/8081/8283/8485/86(A)8 5/86(B)87/8 8(A)87/88(B)89/9091/9293/9495/9697/9899/100101/102V I X 10-16cm3lsec quanta Q<o3)StX1.842.042-064.64.64.54.54.74.73.23-153.1 53.13.21.1350.7260.7 11 ~ 6 81.761 -591 -451-140.5920-3730.3020.2041 a071-540.7220.8 10.7820.8 10-7740.7630-8380.8650.9350.93 80.9881.0320.7850.75524.424.230.523.628.533.221.823.019.019.542.329.525.5-4 corrected431.034.941.534139.344.533-238.145.348.237.831.046.234.034.839.146.538.243.850.141.842.250.853.641.235.051.838.2a corrected assuming OZ+h -+20(1~).b corrected assuming O z + h +0(1D)+o(3P).average y1 = 383i-4.8, y2 = 43.435.3.DISCUSSIONThe low ozone quantum yields could not have been obtained if 1D oxygen atomswere rapidly deactivated by collisions with carbon dioxide.The conclusion thatcollisional deactivation is relatively unimportant is corroborated by a study ofKatakis and Taube 7 who found that 1D oxygen atoms produced in the 2537 Aphotolysis of ozone undergo an exchange reaction with C02 rather than deactivation.Fig. 1 shows that at the wavelengths of the xenon and hydrogen radiation theabsorption is predominantly due to the first continuum appearing at longer wave-lengths, while there is negligible contribution of the second continuum towardsshorter wavelengths. Conversely, the line at 1236A is almost solely absorbed bythe second continuum with negligible contribution from the first.Since the resultsobtained separately ,with the krypton and the hydrogen light source are essentiall64 c02 PHOTOLYSISidentical, it can be inferred that the transitions represented by the two absorptioncontinua lead to identical dissociation products, namely, carbon monoxide and1D oxygen atoms. The effect of the xenon 14701$ line can now be considered.In view of the preceding conclusion it is apparent that the increase in the quantumyields observed at this wavelength must be caused by the partial absorption in theoverlying band system, which at this wavelength accounts for approximately 30 %of the total absorption strength. Accordingly, the results are explained in terms ofreactions involving excited C02 moleculesh v co2+co; (5)(6)which must occur in addition to the photodissociation mechanism (1) through (4).These reactions can account for the increase of the quantum yields observed forall the products.At pressures below 150 mm Hg, where the CO quantum yield isunity, the COJ., molecule presumably predissociates or re-emits the photon whichsubsequently is re-absorbed, preferentially in the continuum. The only finding notentirely consistent with this supposition is the lack of pressure dependence con-cerning the 0 2 quantum yields obtained with the xenon discharge.For a quantitative interpretation of the measurements the physical conditionsof the experiments must be described by a suitable theoretical model.Since thereactions induced by COZ photolysis at atmospheric pressures are confined to anarrow region in the vicinity of the window, which is characterized by a certaindegree of turbulence owing to the reversal flow in this region, it appears appropriateto perform calculations on the basis of stirred reactor theory. In the photolysisof C02 containing 2.66 % oxygen the 0 2 concentration is so large that it remainsapproximately constant even if all the oxygen atoms are consumed in the formationof ozone. Also, reaction (2) can be considered negligible. The stirred reactorequations then read :co: + CO2-+2C0 + 0 2 ,(V/R>[OJ = [OI(k3[02ICMI -k4[03I>,(VIR)COI = (w- COI(~,CO,I[MI + k4c031),where the concentrations refer to the outgoing (steady-state) concentrations, v isthe flow rate, R the reaction volume, and Z the integrated irradiation intensity.Ifthe oxygen atoms are largely consumed within the boundary of the reaction volume,(u/R)[O]<I/R, and the equations can be combined to express the ratio of the rateconstants in terms of the previously defined 0 2 quantum yield :Note that the reaction volume R has cancelled.When this equation is employed in conjunction with the data shown in table 1,one obtains the y values entered in column 5. These are about five times greaterthan the highest value available for 3P oxygen atoms, y(3P) = 5, which can be de-rived using the rate constants k4(3P) = 2.5 x 1014 cm3/molecules sec8 and k3(3P) =2 x 10-34 cm6/molecules2 sec6.However, eqn. (A) does not yet take into accountthe simultaneous photodissociation of oxygen in the mixture so that a correctionis required making the discrepancy even greater. Because of the existing un-certainty regarding the dissociation products of oxygen at 1236& two cases wereconsidered: one in which it was assumed that the dissociation products are two1D oxygen atoms, and a second case involving a 1D and a 3P oxygen atom of whicP. WARNECK 65the latter was supposed to undergo immediate ozone formation. The corres-pondingly corrected y values are shown, respectively, in columns 6 and 7 of table I .The averaged values are seen to be approximately the same in both cases, suggestingy x41. Despite a considerable variation in the intensities and flow rates the averagedeviation from the mean is about 12 %, supporting the employed reaction mechan-ism. The high values obtained for y are considered sufficient evidence that theCO2 photodissociation products are carbon monoxide and 1D oxygen atoms.1 Watanabe, Zelikoff and Inn, Absorption Coefficients of Several Atmospheric GaJes, AFCRC2 Mahan, J. Chem. Physics, 1960, 33, 959.3 Warneck, Appl. Optics, 1962, 1, 721.4 Groth, 2. physik. Chem. B, 1937,37, 307.5 Vaughan and Noyes, J. Amer. Chem. SOC., 1930,52, 559.6 Kaufman, Progress of Reaction Kinetics, vol. 1 , ed. Porter (Pergamon Press, 1961), p. 19.7 Katakis and Taube, J. Chem. PhyAics, 1962, 36, 416.8 Phillips and Schiff, J. Chem. Physics, 1962, 36, 1509.Tech. Rpt. no. 52-23, Geophys Res., paper no. 21, 1953

ISSN:0366-9033    

DOI:10.1039/DF9643700057

出版商:RSC    

年代:1964

数据来源: RSC

摘要:

Daytime Atmospheric O ( 9 )BY RICHARD D. CADLENational Centre for Atmospheric Research, Boulder, Colorado, U.S.A.Received 7th October, 1963Atmospheric O(1D) concentrations have been estimated for the altitude range 10-240 km. Theestimates for the 100-240 km region are based on calculations of the 6300 8, line intensity by Brandtwhile those for the region below 100 km are based on calculations of the rates of photolysis byultra-violet radiation of 0 3 . The results suggest that in addition to the strong (50 kR) red day air-glow above 100 km, there may be a weaker glow (2 kR) in the 10-100 kni region. The results alsosuggest that the O(1D) concentrations are sufficiently high that the reaction 0(1D)+N2 -+N20+hmay contribute significantly to the N20 content of the atmosphere and the reaction 0(1D)+H20 -+20H may occur at an important rate in the stratosphere.Electronically excited atomic oxygen in the 1D state in the atmosphere is ofinterest for at least two reasons.One is that it is responsible for the red linesA = 6300 A and A = 6364 A (3P2 - ID2 and 3P1- 1Dz) in the airglow and aurora.The other is the possibility, which is discussed later, that it competes with groundstate (3P) atomic oxygen in certain chemical reactions.line, the more intense of the two, in the night airglowis 50- 100 R.l Dissociative recombination,The intensity of the 6300O,+ + e-, 0 ( 3 ~ ) + O('Dj, (1)O(lS)-+O(lD) + 5577 (2)(3)may be the dominant mechanism,2.3 although other processes may contribute :0 + 0 + 0 - 0 2 + O(W)N + 0 3 -+NO2 + O(lO),0 3 +ihv+O2 + O(lD),or the photolysis of ozone,(4)( 5 )in which the ultra-violet radiation is produced by processes such as0 + 0 - 0 2 + hv.(6)The intensity of the 6300A line in the twilight airglow (referred to the zenith)averages about 1 kR.1 That in the day airglow has been calculated by Brandt4to be about 50 kR referred to the zenith. Brandt's calculations suggest that thered lines in the day airglow result from emissions by O(1O) produced both byultra-violet dissociation of 0 2 in the Schumann-Runge region :and by dissociative recombination (eqn. (1)).The concentrations of O(1D) in the atmosphere are several orders of magnitudesmaller than the concentrations of O(3P). However, because of the difference inmultiplicity, certain reactions of O(3P) violate the spin conservation rule 5 , 6 whereasthe corresponding reactions of O(1D) do not.0 2 + JZV+ O(3P) + O(1D) (7)Examples areand0 + N2+ N20 + i h ~0 + CO-+C02 + hv.6R .D. CADLE 67Unfortunately, almost no information is available concerning rate constants forreactions involving O(1D). However, Laidler 5 has pointed out that the transmis-sion coefficient for a reaction involving a change in multiplicity is probably alwaysless than 10-4. Furthermore, the excitation energy for O(lD), about 45 kcal mole-1,can reduce or eliminate any activation energy. Thus certain atmospheric reactionsinvolving O(1D) may occur at velocities comparable with those involving O(3P)in spite of the difference in concentration.The purposes of this paper are (i) to estimate the concentrations of O(1D) in theatmosphere between 10 and 240 km altitude, (ii) to compare the concentrations ofO(1D) with the concentrations of O(3P), and (iii) to estimate the contributions ofO(1D) produced by photolysis of ozone below 100 km to the day airglow.METHOD OF APPROACHBrandt 4 reported estimated rates of emission of 6300 A radiation resulting fromultra-violet dissociation and dissociative recombination in the altitude range 100-240 km.These rates are readily converted to O(1D) concentrations by dividing bythe transition probability, 0.0069 sec-1.The photolysis of ozone 7 by the strong absorption in the 2200-3140 A regionalmost certainly produces O(lD), probably by0, + hv+O:,followed by :OJ-,02('Ag)+ O('0j (-91 kcal)0; -+.02( 'X,") + O('D) ( - 107-5 kcal)or Of+02(3X,)+ O('D> (-69.8 kcal) (13)(12)and probably largely by reaction (11). It may also be produced by absorptionin the 3140-35OOA region by (13). Thus ozone photolysis almost certainly producesconsiderable O(1D) between 10 and 100 km in the atmosphere. The O(1D) in thisregion is destroyed primarily by the following processes :O(lD)-,O(3P) + hv. (16)Thus the steady-state concentrations of O(1D) can be estimated for any given altitudeby the equation,where f is the absorption rate in photons/molecule sec, k14 and kls are rate con-stants for the corresponding reactions, A4 is the transition probability for (16),and the brackets indicate concentrations.Following Brandt, k15 is taken to be10-12 cm3 sec-1 and [MI] is [02]. The rate constant for reaction (14) is about4.5 x 10-34 cm6 sec-1 (Benson and Axworthy 8 ) and k14 is probably no larger thanthis. A4 is 0-0091 sec-1. Thus the first and the last terms in the denominatorcan be ignored and the equation becomes[O(lD)] = 1012f[03][02]-1.Values off were calculated from the absorption coefficients of ozone, the spectraldistribution of the intensity of solar radiant energy outside the earth's atmosphere,68 DAYTIME ATMOSPHERIC O(1D)and the scale heights for 0 3 . The last were calculated from the theoretical valuesof Dutsch.10 These two equations as written imply that the photochemical yield 4is unity. Volman 11 has reported results which suggest that at atmospheric pressureand very low concentrations of ozone in air, 4 may be 0.05 or slightly less.How-ever, his results can be extrapolated to the lower pressures existing at high altitudesusing the equationwhere kll is the rate constant for 11, kg is the rate constant for collisional deactiv-ation of 0; and [MI is the concentration of 0 2 plus Nz. At 30 km, 4 is estimatedto be 0.8, so in view of other uncertainties in the calculation, 4 is assumed to be4 = kl 1/@11+ k,L-Ml),unity.RESULTSThe resulting values off are shown in fig. 1 along with those obtained by Dutschfor the absorption by 0 3 of radiation up to 11,500A and producing both O(1D)and O(3P). Comparison shows that above 30 km most of the atomic oxygen fromozone photolysis, when first produced, is in the 1D state.I I I0-2 -3 -4log f (sec-1)FIG.1.-Values off for the photolysis of 0 3 leading to O(1D) (this paper), and to O(lD)+O(3P)(Diitsch 10).The estimated concentrations of O(1D) are shown in fig. 2 along with total atomicoxygen concentrations.9 Values for [03] were again the theoretical of Dutsch 10for 40 km and lower, and were those given in the Handbook of Geophysics9 foraltitudes above 40 km. The peak concentration for O(1D) produced by ozonephotolysis occurs at about 50 km rather than at the ozone peak, because of boththe rapid increase in f and decrease in 0 2 with increasing altitude.As expected, the concentrations of O(3P) exceed those of O(1D) by severalorders of magnitude and rate constants for reactions involving O(1D) would havR.D. CADLE 69to be correspondingly greater than those for the corresponding reactions involvingO(3P) in order for the former to be of importance to atmospheric chemistry. Asmentioned above, reactions involving O(1D) might be many orders of magnitudefaster than those involving O(3P). However, simple reactions involving atomsseldom have rate constants exceeding 10-12 cm3 sec-1 for second-order reactionsand 10-32 cm6 sec-1 for third-order reactions. Thus the concentrations of O(1D)are so small that many atmospheric reactions, especially third-order reactions, inwhich it might be involved are so slow that they are not of importance.log [O], CM-3Geophysics.9FIG. 2.-Concentrations of (10) and total 0.The latter are taken from the Handbook ofA possible exception is the reaction with N2 to form N20. The activation energyfor this reaction involving O(3P) is estimated to be 13 kcal mole-1 from the activ-ation energy and endothermicity for the decomposition of N20 (53 kcal mole-1and 40 kcal mole-1). Thus the excitation energy for O(1D) may largely or entirelyeliminate the activation energy. This reaction has been investigated by Grothand Schierholz.12, 13 They irradiated mixtures of nitrogen and oxygen (419 mmN2, 7 mm 02) with the resonance wavelengths of xenon at 1470 and 1295 A. Theresulting photolysis produces O(3P) and O(1D) with kinetic energies of both exceedingthe activation energy of the reaction. The N20 produced was removed from thegas mixture by condensation in a liquid-oxygen trap and the N20 concentrationswere determined with a mass spectrometer.Under these conditions about 10-3 ofthe atomic oxygen formed reacted with N2 to form N2O. However, it was notpossible to distinguish the reactions involving O(3P) from those involving O(1D).The following was suggested as a possible mechanism conttibuting to the N20formation :O+ Q2 + M-+O;+M (1 7)(1 8) 0; + Nz-+NzO + 0 70 DAYTIME ATMOSPHERIC o(l0)However, (18) involves a change in multiplicity unless 0; is a triplet. Further-more, the activation energy for (18) (when ground-state 0 3 is involved) is greaterthan the endothermicity of the decomposition of 0 3 into O(3P) and 0 2 , and if 0;is a triplet with this much excitation energy it would probably decompose rapidly.We can make a rough estimate of the rate constant for the reactionO(1D) + N2 + N2-+N20 + N2 (19)from the work of Groth and Schierholz by assuming (a) that O(1D) reacted at leastas rapidly as O(3P) with N2, (b) that the rate constant for the reaction of O(1G)with 0 2 to forin O3 is no greater than that for the corresponding reaction of O(3P)and is about 4.5 x 10-34 cm6 sec-1, ( c ) that the rate constant for the collisional de-activztions of O(1D) is 10-12 cm3 sec-1, and (d) that reactions (19), (14) and (15)were the main reactions leading to the removal of O(1D).Then,and k19 = 10-36 cm6 sec-1.Similarly, if the reaction is second order,O(~D)+N~+N~O+/ZV, (20)k20 = 10-17 cm3 sec-1.Rates of formation of N20 and estimates 9 of N20 concentrations at variousaltitudes are shown in tab!e 1.They suggest that if the reaction is second orderit may be an important source of atmospheric N20, since N20 is quite inert andmust have long residence times at least below the ionosphere.TABLE END RATES OF FORMATION OF N20 AT SEVERAL ALTITUDESaltitude(km)1020406090120formation rc< tecm-3 sec-1for thirdorderrezction0103102110-410-4for secondorderrenction01041041041010N 10 concentrations(Jcm -3101110121010109107-(a) as taken from Handbook of Geophysics.9It is of interest to compare the integrated rate cf photochemical N20 formation,as estimated from the order-of-magnitude results of table 1, with estimates byGoody and Walshaw 14 of the total rate of photochemical destruction of N20(8 x 1010 cm-2 sec-1) and of the average world wide N20 production by bacterialaction in the soil (1-6 x 1010 cm-2 sec-1). The integrated rate is 1010-1011 CM-2 sec-1if the reaction is second order, and 109 cm-2 sec-1 if the reaction is third order.Another reaction of interest is the oxidation of methane.In the ionospherethe methane undergoes photolysis as the primzry oxidation setp. However, in thestratosphere it can react with atomic oxygen and ozone :O(3P) + CH4+products (21)O(1D) + CH4-+products (22)Q3 + CH4-+products. (23R. D. CADLE 71The rate constants for (21) and (22) are not known. However, that for the reactionof O(3P) with n-butane has been found by Elias and Schiff 15 to bek = 5 x 10-11 exp ( - 4200/RT)or about 2 x 10-15 cm3 sec-1 at 25 km.The rate constant for (21) may not bemuch different, in which case, assuming a methane concentration at 25 km of1012 cm-3,(d[CI&]/d~)(21], 25 kmN lo6 SeC-' Cfn-3.The value of 1012 cm-3 for the methane concentration is based on the assumptionof a constant methane+air mixing ratio up to at least 25 km. However, the ex-change time for the atmospheric layer at 25 km is greater than two years,l6 so inview of the probable high rate of (21), the concentration of methane may be about1010 cm-3 at this altitude. Harteck and Kopsch 17, 18 found an activation energyof 7 kcal mole-1 for (21). Even if the activation energy is this high, the mesosphereis probably partially depleted of CH4 by this reaction.The pre-exponential factor, 5 x 10-11, is so large that spin-conservation rulesmust be followed and the products must be a triplet and a singlet, or two doublets.Perhaps the reaction isIt is very unlikely that the pre-exponential factor for (22) will be greater than5 x 10-11 and it will be much smaller unless the spin-conservation rule is obeyed,in which case the reaction might beThe activation energy for (25) might be smaller than for (24).If it is zero,(d[CH4]jdt)(25), 25 k m - 5 x lo4 cm-3 sec-l.The reaction of O(1D) with CH4 may produce CH2, as in reaction (29, but in theexcited singlet rather than the ground triplet state.The rate constant for reaction (23) has recently been determined by Dillemuth,Skidmore and Schubert 19 to beandThus reaction (23) is not an important mechanism for methane oxidation in theatmosphere.O(3P) + CH4+CH2 + H2O.(24)O(1D) + CH4+CH3 +OH. (25)k = 27 x 10-13 exp (- 15,30O)/RT cm3 sec-1(d[CH4]/dt)(23), 2 5 kmN ~ n i - ~ sec-'.Yet another reaction which may be important is0(1D)+H20-+20H+27 kcal. (26)The corresponding reaction of O(3P) is endothermic by 18 kcal mole-1 and is probablynot important. The concentrations of water vapour (about 1014 cm-3) and O(1D)(about 104 cm-3) at 25 km are such that if the pre-exponential factor in the usualexpression for the rate constant is about 10-12 cm3 sec-1 and the activation energyis low, reaction (26) must occur at an important rate.However, the rate constantfor (26) is not known.The concentrations of O(1D) produced by ozone photolysis below 100 km aresufficiently large to produce a weak day airglow. Integrating the concentrationbetween 20 and 100 km and multiplying by the transition probability gives anestimated intensity of 2 kR for the 6300 A line. This can be compared with theintensities mentioned above of 50 kR for the day airglow originating above 100 km72 DAYTIME ATMOSPHERIC o(l0)and 1 kR for the twilight airglow. It might be possible to detect such an atmosphericglow from a rocket at an altitudsl of about 100 km, shortly before sunset, by spectro-scopically analyzing the light received from the direction away from the sun andslightly above the horizon.The 6300 A line has been observed in the day airglow 2 0 s 21 and while the estim-ated intensities are somewhat conflicting they do not markedly disagree with Brandt’scalculated value.It must be emphasized that most of the above estimates are based on a collisionaldeactivation coefficient for O(1D) of 10-12 cm3 sec-1 and the assumption that oxygenis much more efficient than nitrogen in producing the deactivation. The figure10-12, used by Brandt,4 was originally taken from the work of Seaton22 who con-sidered it to be a minimum effective value for the aurora and airglow.The resultsof the calculations could easily be incorrect by an order of magnitude or more.The calculations must be repeated when more accurate values for the deactivationcoefficient are obtained; in the meantime the above calculations indicate what thesituation may be.1 Chamberlain, Physics of the Aurora and Airglow (Academic Press, New York, 1961), p.571.2 Bates, The Threshold of Space (ed. Zelikoff, Pergamon, New York, 1957), p. 14.3 Chamberlain, Astrophys. J. 1958, 127, 54.4Brandt, Astrophys. J., 1958, 128, 718.5 Laidler, Chemical Kinetics (McGraw-Hill, New York, 1950), p. 382.6 Wigner and Witmer, 2. Physik., 1928, 51, 859.7 Leighton, The Photochemistry of Air Pollution (Academic Press, New York, 1961), p. 50.8 Benson and Axworthy, J. Chem. Physics, 1957,26, 1718.9 Geophysics Research Directorate, U. S. Air Force, Handbook of Geophysics, revised edition10 Dutsch, Chemical Reactions in the Lower and Upper Atmosphere (ed. Stanford Research11 Volman, J. Amer. Chem. SOC., 1951,73, 1018.12 Groth and Schierholz, J. Chem. Physics, 1957,27,973.13 Groth and Schierholz, Ber., 1957,90,987.14 Goody and Walshaw, Quart. J. Roy. Meteorol. Soc., 1953, 79,496.15 Elias and Schiff, Can. J. Chem., 1960, 98, 1657.16 Martell and Drevinsky, Science, 1960, 132, 1523.17 Harteck and Kopsch, 2. Elektrochem., 1930, 36, 714.18 Harteck and Kopsch, 2. physik. Chem. B, 1931,12, 327.19 Dillemuth, Skidmore and Schubert, J. Physic. Chem., 1960, 64, 1496.20 Noxan and Goody, J. Atm. Sciences, 1962, 19, 342.21 Wallace, J. Geophys. Res., 1963, 68, 1559.22 Seaton, Astrophys. J., 1958, 127, 67.(Macmillan, New York, 1960).Institute, Interscience, New York, 1961), p. 167.Martell, 1963, personal communication

ISSN:0366-9033    

DOI:10.1039/DF9643700066

出版商:RSC    

年代:1964

数据来源: RSC

摘要:

Laboratory Studies of the Chemiluminescence from theReaction of Atomic Oxygen with Nitric Oxideunder Upper Atmosphere Conditions *BY G. DOHERTY~ AND NEVILLE JONATHANGeophysics Corporation of America, Bedford, Massachusetts, U.S.A.Received 13th January, 1964Light emission from the reaction of atomic oxygen with nitric oxide has been observed over thepressure range 0-85-400 microns Hg. By calculation of the partial pressures of atomic oxygen andnitric oxide it is concluded that the light emission shows a first-order dependence with respect tothe atomic oxygen and nitric oxide partial pressures but is independent of the total pressure.The results are interpreted as being consistent with a previously postulated three-body mechanism.In addition, they confirm that a previously determined overall rate constant for light emission isapplicable under upper atmosphere conditions.This rate constant is used to show that the chemi-luminescent reaction of atomic oxygen with nitric oxide, which has been postulated as the cause ofthe night airglow continuum, could indeed be responsible provided that the nitric oxide concentra-tion is of the order of 10s molecules/cm3 over the altitude range 90-110 km.In early studies of the continuum in the night airglow Rayleigh demonstratedthat its spectral distribution could not be the result of scattered sunlight.1 Recenttheories have independently suggested that the continuum is the result of the chemi-luminescence associated with the nitric oxide + atomic oxygen reaction?-5 RecentlyShefov 6 and Yarin 7 have shown that the nocturnal continuum distribution is closeto that of a laboratory-produced airglow.Atomic oxygen is formed above 70km by photodissociation of oxygen mole-cules.Wallace 8 gives estimates of the nighttime densities which vary from approxim-ately 1010 atoms/cm3 at 70 km to 1012 atoms/cm3 at 100 km for a 12-h day followedby a 12-h night. The specific amount of nitric oxide present is unknown thoughtheoretical arguments have been put forward for its existence.9 An upper limitof 108 molecules/cm3 has been placed as a result of experiments using a rocket todetermine the ultra-violet absorption spectrum in the altitude range 60-87 km.10Studies of the precise altitude of the continuum have been hampered by the lowlight intensity levels but rocket experiments have suggested the strongest emissionlies between 90 and 112 km.11, 12The essential mechanism for the nitric oxide + atomic oxygen light emission is0 + NO+N02 +hv,followed by the much faster non-light-emitting reaction,Extensive laboratory studies have been made of this reaction in the pressure range1-5 mm Hg.Most experiments have indicated that in the 1 mm pressure rangethe chemiluminescence is the result of a three-body reaction although the lightemission is second order, dependent only on atomic oxygen and nitric oxide con-centrations.13-15 The rate constant for overall light emission has been found by* This work was supported in part by the National Aeronautics and Space Administration.j- present address : Department of Law, Boston College, Boston, Massachusetts.730 +NO2402 + NO74 NO 4- 0 C HEM1 L UMI NES CEN C EFontijn and Schiff to be 1-7 x 10-17 cm3 molecule-1 sec-1.16 However, these workersdoubt the applicability of their value under upper atmosphere pressure conditions.The present study was, therefore, undertaken to extend laboratory measurementsto pressure regions similar to those which exist in the region of the airglow.Theexperimental data may then be used to calculate whether there is sufficient atomicoxygen and nitric oxide present in the atmosphere to produce the observed nightairglow continuuni.EXPERIMENTALThe reaction cell used has been developed along the lines of multipass cells which arecommon in absorption spectroscopy. It is shown diagrammatically in fig.1. It consistedof a 50-l., 3-necked Pyrex flask covered on the outside by aincrease the light-gathering power. The cell was continuallylayer of magnesium oxide toevacuated through the centreMAGNESIUMFIG. 1NITRIC OXIDE I ~ ~ ~ 7TO PUMPSOXYGEN.--Schematic form of reaction cell for observation of chemiluminescence.arm by a 4-in. oil-diffusion pump which had a maximum pumping efficiency in the 1-100micron pressure region when backed by a large mechanical pump. A liquid-nitrogen trapwas maintained between the diffusion pump and the cell at all times. The cell pressurewas measured by two independent McLeod gauges. The reactant gases were either letICOLD fTRAP - I "I - -CISCUAffiEFIG.2.-Apparatus for observation of chemiluminescence.into the cell through the two side arms and mixed near the top centre of the vessel or theywere premixed at the point of entry into the vessel. There was no significant differencein the observed light emission whichever method was used and so the atomic oxygen andnitric oxide were generally mixed at the point of entry. The atomic oxygen concentratioG . DOHERTY AND N . JONATHAN 75was determined in the reaction vessel at the end of each run by titration with nitrogendioxide added through the second side arm. The resulting light emission was plotted asa function of NO2 flowrate.A block diagram of the complete apparatus is shown in fig. 2. Oxygen (MathesonExtra Dry Grade, 99.6 % minimum purity) was passed, via a flowmeter, needle valve andcold trap, through a microwave discharge.The partially dissociated oxygen entered thereaction vessel after .passage through a Wood's light trap. Known flow rates of nitricoxide were added to the atomic oxygen via a stainless steel needle valve and a calibratedsilicon oil capillary flowmeter. 'I he nitric oxide was purified by passage through a columnof Ascarite and a dry-icefacetone trap placed on the low pressure side of the valve. Across check on the purity of the nitric oxide and on the accuracy of the nitrogen dioxidetitration technique was obtained from the fact that the maximum light intensity from thetitration was equal to that attained with a PNO/PO ratio of 1/4. The kinetics of this havebeen studied in some detail 13 but may be explained simply in this case as being due to thefact that the initial reactionis very much faster than bothandO+NO2+02+NO0 + NO+N02 +hvO+NO+M+N02+M.Hence, during the titration, maximum light emission occurs when half the steady-stateatomic oxygen has been destroyed.At this point the atomic oxygen remaining is equalto the nitric oxide which has been formed and the resulting light emission is equal to theproduct of these partial pressures. At pressures of the order of 10 microns it is consideredmore accurate to use the position of maximum light emission rather than the end pointto calcu'ate the 0 atom concentration.17A Perkin-Elmer 1 12G single-beam, double-pass grating spectrometer equipped withan E.M.I.9558 B photomultiplier was used to record the light emission spectrum between3000 and 7000A. The overall light emission was measured by mounting an R.C.A.1P28 photomultiplier, in a light-tight housing, directly on to the aperture of the reactioncell. The resulting d.c. signal was fed into a Victoreen micro-ammeter. Checks weremade to ensure that the photomultiplier and micro-ammeter gave linear readings overthe range of light emission encountered.RESULTSSPECTRUM OF THE LIGHT EMISSIONThe spectrum of the chemiluminescence was obtained between 3500 and 7500Aat total pressures of 100 and 400 microns Hg. Under our conditions of low resolu-tion, the spectrum appcared as the usual airglow " continuum '' with a maximumintensity at approximately 6500A.The light emission at pressures below 100 pwas too low to allow spectroscopic measurements to bt: made. However, crudespectroscopic measurements of the light emission were made by fitting the IP28photomultiplier (S 5 cathode) and E.M.I. 9558 B photomultiplier (S 20 cathode)with various combinations of Corning filters so that approximately lOOOw of theradiation were observed by the photomultiplier at any time. By comparison ofthe readings obtained with and without the filters at 10 p and 100 p it was concludedthat there was no obvious shift in the spectral distribution at the lower pressure.DEPENDENCE OF THE LIGHT EMISSION ON CHANGES I N CONCENTRA-TION OF SPECIES AND VARIATION OF THE PRESSUREThe atomic and molecular oxygen partial pressures were kept constant whilstusing a steady oxygen flowrate of 26 cm3/min before partial dissociation.Smal76 NO 4- 0 CHEMI LUMI NES CEN CEamounts of nitric oxide were added, via a calibrated flowmeter, to the oxygen streamat the point of entry into the reaction vessel. At pressures below 30 microns theresidence time was short enough so that there was virtually no loss of atomic oxygenin the reaction vessel. At pressures above this, the principal losses were due to thethree-body recombination reactionkiO+NO+M-+N02+M,whch has a rate constant of approximately 6-0 x 10-32 cm6 molecule-2 sec-1.133 18-20The partial pressure of nitric oxide remained unchanged since that which was usedup in forming nitrogen dioxide was regenerated immediately by the very fast secondaryreactionkiwhere ki = 2.6+0-6 x 10-12 cm3 molecule-1 sec-1.Hence, according to steady-state reactor theory, the difference between initial and steady-state concentrationsof atomic oxygen at any pressure isO+NO,+NO+02,where [O,] and [O] are the initial and steady-state concentrations of atomic oxygen,[NO] is the initial (and steady-state) concentration of nitric oxide, [MI is the totalconcentration. All these concentrations can be expressed in units of molecules(or atoms)/cm3. The residence time t of the species in the reaction vessel is equalto V/F, where Y is the volume of gas reduced to s.t.p. and P is the total flowratein cm3 s.t.p./sec. Hence, in table 1 it was calculated that at 43.5p, with a nitricoxide partial pressure of 2 p and a residence time of 6-6 sec, there was a 7 % lossin the atomic oxygen concentration.At higher pressures the corrections becamegreater and difficult to apply.TABLE 1.-~N"ENSlc1[y AS A FUNCTION OF THE ATOMIC OXYGEN AND NITRIC OXIDE PARTIALPRESSURES OVER A TOTAL PRESSURE RANGE OF 3 TO 8 0 pintensity I0total pressure atomic oxygen nitric oxidepressure concentration nit$yo$ide (A) ( x 105 A)( P Hg) (p Hg) ( X 106 A)(P Hg) run no.1 3.02 9.53 19.54 30.05 43.543-56 80.080.080.00.130.440.901-351 -93(1 -79)"3-38(3-01)*3.38(2.70)"3-38(2.43)*0.360.902.13.32.02.01.02.03.00.8252-35-147.8210.514.714.3 513.02 . 9 7 ~ 10-71-08 x 10-52 .5 8 ~ 10-52.1 x 10-52-07 x 10-61 . 4 7 ~ 10-5zwx 10-53 . 9 ~ 10-50.630-520.570.580-540*59*0*49*0.53"0.53 ** These values are corrected for loss of atomic oxygen by three-body recombination, as explainedin text.The pressure in the reaction cell was varied from 3 to 80 ,u by partially closingthe valve between the pump and the cell. At the end of each series of additions ofnitric oxide, the atomic oxygen flowrate was measured by titration with nitrogendioxide. Hence, knowing the flowrates of molecular oxygen, atomic oxygen anG. DOHERTY AND N. JONATHAN 77nitric oxide, and the total pressure, the partial pressure of each constituent wascalculated. The light emission was plotted as a function of the nitric oxide partialpressure at each different total pressure.Below 40p, straight line plots were ob-tained but above this point a slight curvature appeared which became severe at80 p and higher. This was due to losses in the atomic oxygen partial pressure as aresult of the three-body non-radiative recombination process. The results of atypical low pressure run are given in fig. 3. It may be seen that the light emission2.6 &0 0.2 0.4 0.6 0.8 1.0 1.2partial pressure nitric oxide (p Hg)FIG. 3.-Light emission as a function of nitric oxide partial pressure at total pressure of 9.5 p.is directly proportional to the nitric oxide partial pressure. If one defines I0 as thelight emission per micron of nitric oxide and atomic oxygen such thatintensity = 10[0][NO],then an essentially constant value is obtained for 10.A typical set of results for aseries of runs over the pressure range 3 to 8 0 p is given in table 1 and fig. 4. Itmay be seen from table 1 that the light emission is directly proportional to theatomic oxygen and nitric oxide partial pressures and independent of the total pressure(and hence third body) over the range 3 to 80 p.The direct relationship between the atomic oxygen partial pressure and the lightemission was further investigated under conditions of constant total pressure and afixed partial pressure of nitric oxide. The atomic oxygen partial pressure wasvaried by changing the power of the microwave discharge and by moving a copperwire into the oxygen stream immediately after the discharge.After each set ofruns the atomic oxygen partial pressure was measured as already explained. Theresults are shown in fig. 5. It can again be seen that the light emission is directlyproportional to the atomic oxygen concentration and I0 has a constant value ingood agreement with the other data78 NOfO CHEMILUMINESCENCE16 0 -14.0 -12.0 -n ,6: 10.0-c. $2XWY0.0 -.I.E 6.0-4.0 -2 . 0 -0 I 2 3partial pressure atomic oxygen ( p Hg)FIG. 4.-Light emission as a function of atomic oxygen partial pressure over total pressure range3-80 p.0 points at which atomic oxygen loss is negligible. @ points corrected for atomic oxygen loss. r 2.2 7 , I , , I , , , , , , , , , I02.00 01 0 . 2 0.3 0.4partial pressure atomic oxygen (p Hg)FIG.5.-Light emission as a function of atomic oxygen partial pressure at total pressure of 9.5 pG. DOHERTY AND N . JONATHAN 79Attempts were also made to study the light emission at pressures below 3 pby cutting down the flowrate of oxygen to 9-10ml/min. In this way the lightemission was studied over the pressure range 0.85 to 7.5 p. These results are givenin table 2. It may be seen that the value of I0 is again constant but is somewhatlower than before. However, 10 is still independent of the amount of third bodypresent otherwise I0 would show a pressure dependence. At these low pressures,a small error in the measurement of either pressure or flowrates may result in this10 difference. Hence, since the constancy of 10 still holds, we attribute this lower10 to an inherent experimental error.TABLE VALUES OF I0 AT OXYGEN FLOWRATES OF 9-10cm3/minintensity 10total pressure pressure atomic pressurebefore NO addition oxygen nitric oxide oc Hl3) (P Hg) (c' Hg) (A) ( X 105 A)0.85 0.04 0.2 owx 10-7 0.421.0 0.065 0.2 0 .5 2 ~ 10-7 0.405.0 0.234 2.0 0 . 1 8 ~ 10-5 0.387.5 0.442 2.0 0.33 x 10-5 0.37EFFECT OF ADDED GASESThe pressure of the reaction vessel was maintained at 3, 30 and 100 p with con-stant flows of atomic and molecular oxygen and nitric oxide. Small amounts ofa foreign gas were added in quantities not exceeding 10 :< of the total flow, Thegases used were dry air, oxygen, nitrogen, and carbon dioxide. The additions didnot cause any noticeable change in the light emission indicating that there was little,if any, quenching by the added gases.Attempts were made to conduct similarexperiments with carbon monoxide but these results showed a slight but definiteincrease in the light emission, probably as a result of the chemiluminescent reaction0 + co+co2 + liv.DISCUSSIONIt is believed that the experimental data described above are in complete agree-ment with the three-body mechanism already postulated to account for the chemi-luminescent reaction between atomic oxygen and nitric 0xide.14 It is not proposedto reproduce the earlier deduced potential energy diagram of the system, but inorder to facilitate comparison between the original derivation of the mechanism andits description here, the original numbers, etc., are maintained.The complete reaction mechanism for light emission can be written 14 asNO + 0 + M+NO;(C) + MNO;(C) + M+NO + o + MNO:( C) -+ N O:(B) (4)NO;( B) + N 0; ( C)NO:+NO2 + hvNO;+M-+NO2+Mwhere NO*,(B) and NO,*(C) are two different excited electronic states.Using thisscheme and neglecting the possibility of appreciable light emission from NOg(C), thisstate having the longer lifetime,zl the steady-state treatment for light emission yields 180 NO4-0 CHEMILUMINESCBNCEAt low pressures it is anticipated that quenching is an efficient process withalmost every collision effective, which means that k7~[M] and k,c[M] are approxim-ately 1 x 106 sec-1 at 300 p. k 6 ~ is given as 4 x 106 sec-1.21Hence, simplifying with k6B% k7~[M],At low pressure k6~%(k3[M]+k7~[M]) and since I is independent of pressureHence,then k4 < (k3[M] +k7c[M]) which means also that k6+ ks.The above equation is consistent with the light emission showing a pressureindependence in spite of the reaction being a three-body process.Such a reactionscheme would also exhrbit the dependence of the light emission on the nature ofthe third body as found by Clyne and Thrush,ls since k7c[M] varies according tothe nature of M. Fontijn and Schiff 16 pointed out that the pressure independencemust be lost at sufficiently low pressures since the condition (k3+k7c)[M]%k4 nolonger holds. It was calculated that this condition should occur at pressures ofseveral microns and hence they cast doubt on the applicability of their rate constantto upper atmosphere calculations.Insertion of the suggested reasonable valuesof k3 M 10-10 cm3 molecule-1 sec-1 and k7cz 10-10 cm3 molecule-1 sec-1 shows thatat a total pressure of 1 p, k4 must be considerably less than 6.6 x 103 sec-1 for thelight emission to remain pressure independent. In view of our results it is there-fore suggested that a more reasonable upper limit for k4 is lO3sec-1 rather thanthe previously suggested value of lO4sec-1. The three-body mechanism is thenconsistent with the experimental data even down to a pressure of 0.85p. Also,our observation that there was virtually no quenching by the added gases can beaccounted for by this mechanism.The experimental data outlined here are also consistent with a direct two-bodyrecombination reaction mechanism in which there is virtually no quenching of theexcited species.Such a scheme has been postulated as a result of experiments similarin some ways to ours but without direct measurement of the reactant concentra-tions.22 Nevertheless, arguments against this have already been given 14 and inaddition it is impossible to explain the different third-body efficiencies 15 in termsof such a scheme.The importance of our data in so far as upper atmosphere reactions are con-cerned is that the previously determined value for the rate constant for light emission1.7 x 10-17 cm3 molecule-1 sec-1 is applicable to overall system pressures at leastas low as 0.85 p. Such a pressure corresponds to an altitude of 93.5 km.23 Therate constant was therefore used to calculate the possible photon flux of the nightairglow continuum using the formulad[NO;]/dt = 1.7 x 10-”[O][NO].Values for the oxygen atom concentrations at various altitudes were taken fromthe calculations by Wallace for a 12-h day and night cycle8 and the nitric oxideconcentration was given the allowed upper limit 10 of 108 molecules/cm3 over theentire 70-1 10 km range.Temperature effects were neglected since the light emissionhas only a slight negative temperature dependence.15 The results are presented intable 3. The total photon flux was calculated to be approximately 3 x 109 photonsG. DOHERTY AND N. JONATHAN 81cm2 sec. The field data obtained by Shefov 6 indicate a total integrated emissionof approximately 3300 rayleighs which corresponds to a flux of 3.3 x 109 photons/cm2 sec.Consequently, the calculated and field data are in agreement but no greataccuracy should be expected when one considers the uncertainty in the nitric oxideconcentrations. The distribution of photons in the range 70-110 km is in apparentagreement with the airglow continuum intensities as measured by rocket experi-ments. This must be the case since the atomic oxygen concentrations follow thisdistribution and a uniform concentration of nitric oxide has been assumed.TABLE 3.-POSSIBLE PHOTON FLUX FROM THE MGHT AIRGLOW CONTINUUM ASSUMING THECAUSE TO BE THE of- NO REACTIONtotal particle atomic oxygen nitric oxideconcentration concentration concent rat ion flux altitude(km) (molecuIes/cm3) (atoms/cm3) (mo~ecu~es/cm~) bhotomlcrn2 set)70 2.1 x 1015 1 x 101080 4.0 x 1014 2 x 101090 5 .9 ~ 1013 1 x 1011100 7 . 8 ~ 1012 1 x 1012110 1.2x 1012 1 x 101226x 1061 x 108IX 1091 . 7 ~ 109Taken together, the experimental and field data indicate that chemiluminescencefrom the nitric oxide+atomic oxygen reaction may be a prime contributor to thenight airglow continuum. This can only be the case if the nitric oxide concentra-tion is of the order of 108 molecules/cm3 in the 90-1 10 km range.We wish to thank Prof. A. Dalgarno for several valuable discussions during thecourse of this work and Dr. Peter West for helpful comments during the preparationof this manuscript.1 Rayleigh, Nature, 1920, 106, 8.2 Nicolet, The Threshold of Space, ed. Zelikoff (Pergamon Press, Oxford, 1957), p. 40.3 Krassovsky, Dokl. Acad. Nauk, U.S.S.R., 1958, 78, 669.4 Bates, The Earth as a Planet, ed. Kuiper (Univ. of Chicago Press, Chicago, 1954), p. 576.5 Broida and Gaydon, Proc. Roy. Soc. A, 1954,222, 181.6 Shefov, Spectral, Electrophotometrical and Radar Researches of Aurorae and Airglo w (NASA7 Yarin, Spectral, Electrophotometrical and Radar Researches of Aurorae and Airglow (NASA8 Wallace, J. Atmos. Sci., 1962, 19, 1.9 Bates, J. Geophysic. Res., 1950, 55, 306 ; Proc. Physic. Soc. B, 1951, 64, 80510 Jursa, Tanaka and LeBlanc, Planet. Space Sci., 1959, 1, 161.11 Koomen, Scolnik and Tousey, J. Geophysic. Res., 1956, 61, 304.12 Heppner and Meredith, J. Geophysic. Res., 1958, 63, 51.13 Kaufman, Proc. Roy. SOC. A , 1958,247, 123.14 Broida, Schiff and Sugden, Trans. Furaday Soc., 1961, 57,259.15 Clyne and Thrush, Proc. Roy. Soc. A , 1962,269, 404.16 Fontijn and Schiff, Chemical Reactions in the Upper and Lower Atmosphere (Interscience17 Jonathan, to be published.18 Harteck, Reeves and Mannella, J. Chem. Physics, 1959, 29, 1333.19 Ogryzlo and Schiff, Can. J . Chem., 1959, 37, 1690.20 Ford and Endow, J . Chem. Physics, 1957,27, 1156.21 Neuberger and Duncan, J. Chein. Physics, 1954,22, 1693.22 Harteck and Chase, to be published.23 Minzner, Champion and Pond, The ARDC Atmosphere (no. 115 Air Force Surveys in Geo-Tech. trans. F-106, 1962, section IV, no. 5, 1961), p. 45.Tech. trans. F-106, 1962, section IVY no. 5, 1961), p. 39.Publications, New York, 1961), p. 239.physics, 1959)

ISSN:0366-9033    

DOI:10.1039/DF9643700073

出版商:RSC    

年代:1964

数据来源: RSC

摘要:

Formation and Reactions of the Excited O2(A3C;) MoleculesBY P. HARTECK AND R. R. REEVES, JR.Dept. of Chemistry, Rensselaer Polytechnic Institute, Troy, New YorkReceived 20fh January, 1964The emission of the Herzberg bands can be readily observed in the laboratory by catalyzingthe recombination of oxygen atoms on nickel. In a clean system the ozone concentration mayincrease due to the reaction with oxygen (O2A3Z;+O2-+O3$O). Addition of a few % hydrogenresults in a strong OH emission (2Z++2n) apparently due to the reaction 03,43Z;+H2+OH+OH2Ci. Addition of NO give a characteristic reddish hue to the emission. The surface catalysisalso was apparent in several glass systems.The night sky radiation includes emission of thc Herzberg oxygen band system(O2A3E; +02X3E-;) which is generally attributed to the production of the OzA3E;molecule by oxygen atom recombination processes.1 This emission has been ob-served in the laboratory,z and recently by use of a surface catalyst to obtain arelatively intense emission.3 In this paper we describe some observation of theHerzberg emission when enhanced by a surface catalyst.In addition, the effectsof NO, H2 and Nz on the emission are discussed.EXPERIMENTALTwo similar experimental arrangements were used to study the surface-catalyzedexcitation of the Herzberg emission of the oxygen molecules using nickel as the catalyst.One of these consisted of a glow discharge, producing oxygen atom which were pumpedby a 1397 Welch pump into a 45 mm quartz reaction tube containing the nickel catalyst.FIG.1 .-Schematic of apparatus.Spectra of the Herzberg emission from pure oxygen and with the addition of nitric oxideor hydrogen were recorded using a 0.5 m Jarrell-Ash scanning monochromator. Theother was specifically used to detect the formation of any ozone in the system by absorption8P. HARTECK AND R . R . REEVES JR. 83of the mercury 2537A line. The 2537A line was monitored by a photomultiplier tubewhich was in a bridge circuit.A third experimental arrangement shown in fig. 1 was also used which consisted ofa 200 1. reaction vessel which could be readily operated at pressures from 5 to 20 microns.This vessel was evacuated by a 6-in. glass pipe system connected to a CVC MHG-900mercury diffusion pump and a Welch vacuum pump (1398).The vessel could be pumpedat a rate of about 200 l./sec. The pressure was monitored by an NRC Alphatron 530pressure gauge and the light emission was observed with an EM1 6256B photomultipliertube using various filters to obtain the desired wavelength region for observation. D.c.amplifiers were used to produce a signal level which could be plotted on an oscilloscopeequipped with an attached Polaroid camera for direct recording of results.The following technique was used to add a gas stream with a well-defined 0-atom con-centration. The oxygen atoms were produced in a glow discharge and pumped by a 1397Welch pump through a long 45 mm Pyrex tube, where the pressure was generally in theregion from 500 microns to 1 mm. The light intensity in the 45 mm tube at the pointat which the sidestream was removed was monitored by a 931 A photomultiplier tube.The change in intensity with addition of various reactants including NO, H2 and N2, wasalso studied.A sidestream was taken from this tube to feed the 200 1. vessel with the samerelative oxygen-atom concentration.RESULTSUsing a stream of oxygen with an atom concentration of approximately 20 %, theHerzberg bands were observed by eye as a blue luminosity being emitted from the nickelcatalyst surface to a distance of several mm from the surface at pressures from 100 to 300microns. Addition of molecular hydrogen to the stream resulted in emission of the (0,O)and (1,O) OH bands (A2P-tX2II) in the ultra-violet. Addition of appreciable amountsof hydrogen resulted in a marked decrease in the Herzberg emission with correspondingincreased intensity in the OH emission.Further addition of molecular hydrogen reducedthe Herzberg emission to an unobservable level and finally OH bands also disappearedapparently because of interference with the surface catalysis process (spectra shown in fig. 2).When small amounts of nitric oxide were also added to the oxygen-atom stream, inaddition to the oxygen afterglow, a red coloration was observed in the same region wherethe Herzberg bands are emitted. This emission was similar to the emission of the oxygenafterglow from NO and 0-atoms, but shifted to the red. Large amounts of NO quenchedthe Herzberg emission.The addition of molecular nitrogen had no specific effect on the Herzberg emission.These studies were made using a nickel catalyst.It is possible to obtain this type of cata-lysis under normal conditions of fairly clean nickel or cobalt and a reasonably clean oxygenatom stream. To obtain a blue emission readily visible to the eye requires very clean oxygenand a conditioned catalyst. It was not always easy to reproduce the best conditions.Experimentally it was observed that a partial pressure of about 1 p ozone was obtainedwhen excited oxygen molecules were produced by surface catalysis in the 100-300 p pressureregion. Various experimental conditions were used : changing pumping speed, geometricconfiguration of the nickel catalyst, partial pressure of the oxygen atoms, and the totalpressure.Despite this large number of variations the ozone concentration remained inthe region of 1 p and substantially higher concentrations could not be obtained. Withoutthe catalyst, which could be removed by a magnet, the partial pressure of the ozone droppedto less than 0-1 p. Thus, about 10 % of the oxygen atoms must have been converted toozone via the excited OzA3X+, Herzberg level.A weak surface catalysis could also be observed in the large 200 1. glass reaction vesselafter it had been operating for some weeks. This was attributed to undefined surfaceconditioning. A typical surface catalyzed emission is shown in fig. 3. This photographwas made with NO present and has the red coloration mentioned above. This catalysiscovers a surface area of approximately 300 cm2.The Herzberg emission in the blue wasnot intense enough to be visible to the eye, but was readily observed by the photomultiplierFIG. 3.-Reaction vessel with surface catalysis evidcnt at upper left.[To j k e poge 84FIG. 4.-Photograph of Herzberg band emission84 EX c I TE D 02(~3z MOLE c u L E sFig. 4 is a photograph taken in the ultra-violet region only, where the emission of theHerzberg bands can be seen from various areas of a tube through which O-atoms areflowing. All these experiments were performed in a considerably lower pressure region,i.e. 5-20 p. Herzberg emission from the 200-1. vessel was measured as a function of oxygenatoms and nitric oxide concentrations. Generally the Herzberg emission increased pro-portional to the oxygen atoms and was reduced by the addition of NO.However, con-ditions were not stable enough to give reliable quantitative results. The addition ofN o emission observed H 2 z 1 0 '__- - - _ _ _FIG. 2.-Emission of Herzberg bands.hydrogen under these conditions yielded entirely different results compared to the experi-ments in the 100-3OOp region using the nickel catalyst. Apparently hydrogen enhancesthe surface catalysis effect at low pressure and the intensities of the Herzberg emission andthe reddish emission in the visible (assumed to be due to NO2 formation) were increasedin most cases by a factor of 10-100. In a freshly cleaned system no emission of the Herzbergsystem could be observed from the 200-1. flask at those very low pressures.DISCUSSIONBy the use of a catalyst such as nickel it is possible to produce the Herzbergbands with a strong emission intensity.This technique permits some experimentalobservations which are otherwise virtually impossible to obtain in the laboratory.Since the Herzberg emission is observed in the night-sky radiation, it is pertinent tothe chemistry of the upper atmosphere. It is significant that the Herzberg emissionproduced by surface catalysis extends only a few mm from the catalyst indicatinga lifetime of approximately lo-ssec. Since the radiation lifetime is much longer(- 1 sec), the lifetime must be limited by deactivation processes or chemical reactionsP. HARTECK AND R. R. REEVES JR. 85One possibility is the interaction of the excited species with a ground-state oxygenmolecule to produce ozone.The experimental results indicate that ozone is formed,but the maximum partial pressure is limited to about 1 p. This may be partly dueto ozone also quenching the excited oxygen molecules and becoming destroyed inthe process. Alternately, it also may be that the collision of two excited oxygenmolecules results in a deactivation of both by formation of two 0-atoms and theoxygen ground-state molecule. These reactions are given as reactions (l), (2) and(3) in table 1. Those believed to occur with NO and H2 are also listed in table 1.The ozone formation by reaction (1) may be considered analogous to the reactionof excited CO with CO : 49 5co"+co-,co2+c. (6)Studies of the absorption of the iodine line at 2062A by NO indicate that a similarreaction also occurs :NO* +NO+N02 + N (74+N20 + 0.(7b)Excited nitrogen molecules formed by surface catalysts, however, have been foundto be much less reactive than expected.5 The reaction with N2 is impossible sincethere is no indication of a molecule N3 with a reasonable binding energy.It is of basic interest to know what fraction of the 0-atoms result in excited02A3Z$ molecules using the nickel catalyst. An estimate can be made from theintensity of the emission of the Herzberg bands and the fraction of excited mole-cules that will emit radiation compared to the number formed. The diffusion ofthe excited species from the surface corresponds to a lifetime in the order of 10-5sec, limited by deactivation processes.The radiative lifetime is about 1 sec andtherefore the fraction emitting would be 10-5 of those formed. Since the intensityis in the order of 1012 light quanta/sec, then 1017 excited molecules would be prim-arily formed. Since about 3 x 1018 0-atoms/sec stream over the catalyst, roughlyone-tenth must form the 02A3Zfi molecule. This agrees favourably with themeasured ozone concentration corresponding to a 10 % conversion of atoms toozone, but the estimate is only an approximation.The rates of reaction for ozone formation (0 + 0 2 + M-+03 + M) and the ozonedestruction by 0-atoms (0 + 03+202) are of major importance for the upper atmo-sphere. The values given in the literature for these rates are not consistent,7and the steady-state of ozone in a mixture of oxygen and 0-atoms varies also.Afterthe observation of the Meinel bands in the night sky and the laboratory experiment86 EXCITED o~(A~c;) MOLECULESsimulating this emission,s it became evident that the H-atom-ozone reaction wasvery fast and that minor amounts of H-atoms could give misleading results in thestudy of the rate of the reaction between ozone and 0-atoms. Therefore, it seemedthat a system entirely free of hydrogen atoms should give reliable results. However,due to surface catalysis additional ozone can be formed making the ozone concen-tration higher than that corresponding to kinetic equilibrium. Therefore the errorsmay be in either direction and reliable results can be obtained only when both effectsare under control.The authors thank Mr. T. Rolfes and Mr. W. Chace for assistance in obtainingthe experimental results, and Dr. B. A. Thompson for her helpful suggestions. Thiswork was carried out under research grant no. AF-AFOSR-174-63 from the AirForce Office of Scientific Research.1 Barth, J. Geophys. Res., 1962, 67, 1628.2 Barth and Patapoff, Astrophys. J., 1962, 136, 1962.3 Mannella and Harteck, J. Chem. Physics, 1961, 34,2177.4 Groth, Pessara and Rommel, 2. physik. Chem., 1962, 32, 192.5 Harteck, Reeves and Thompson, Z. Natirrforsch., in press.6 Harteck and Safrany, unpublished results.7 Kaufman and Kelso, Chemical Reactions in the Lower and Upper Atmosphere (Interscience,8 Gamin and McKinley, J. Chem. Physics, 1956, 24, 1256.N.Y., 1961), p. 255

ISSN:0366-9033    

DOI:10.1039/DF9643700082

出版商:RSC    

年代:1964

数据来源: RSC