8 Reactions of Atoms and Small Molecules Studied by Ultraviolet Vacuum-ultraviolet and Visible Spectroscopy* By R. J. DONOVAN Department of Chemistry University of Edinburgh West Mains Road Edinburgh and 0. HUSAIN Department of Physical Chemistry University of Cambridge 1 ensfield Road, Cambridge 1 Introduction This Report is confined to a consideration of the reactions of atoms and small molecules in the gas phase studied by absorption or emission spectroscopy in the ultraviolet or visible regions. There is inevitably some overlap with the more recent Annual Reports; however we have mainly confined our attention to work carried out during the period 1966-1971 and only consider earlier work where recent studies are absent or to place work in context. Several recent reviews of a more general nature but containing material relevant to the present Report should be mentioned.Three volumes of the series ‘Annual Survey of Photochemistry’ have now appeared lo covering the period 1967-1969. The main bulk of these surveys deals with the photochemistry of large organic molecules ; however there are several chapters concerned with the reactions of atoms and small molecules. Volume two of the Chemical Society’s Specialist Periodical Reports on ‘Photochemistry’ covers the more recent litera-ture July 1969-June 1970,Ib and contains a very useful section on developments in techniques which include nanosecond and picosecond flash photolysis, molecular modulation and photofragmentation spectroscopy. Volume 3, published in May of this year covers the literature published during the period July 197GJune 1971.A number of other useful reports covering wider aspects * ’. . . can this cockpit hold The vasty fields of France? . .’ W. Shakespeare King Henry the Fifth Chorus. ’ ( a ) ‘Annual Review of Photochemistry’ ed. N. J. Turro et al. Wiley-Interscience, New York 1969 vols. 1 2 and 3; (b) ‘Photochemistry’ ed. D. Bryce-Smith (Specialist Periodical Report) The Chemical Society London 1970 vol. 1 ; 1971 vol. 2 ; 1972, vol. 3; (c) A. F. Trotman-Dickenson and G. S. Milne ‘Tables of Bimolecular Gas Phase Reactions’ NSRDS-NBS-9 U.S. Government Printing Office Washington 124 R. J . Donovan and D. Husain of atomic and radical reactions have appeared recently.“-’’ Two reviews of particular relevance are that of chemiluminescent reactions in the gas phase by Thrush,Ik and a review of energy-transfer processes by Callear and Lambert.” Atoms Studied.-The following atoms will be considered in varying degrees of detail : H C N O Na Si P S C1 K Fe Cu Zn Ge As Se Br Rb Cd Sn Sb Te I c s Hg T1 Pb Bi Noble-gas atoms which have been reviewed recently,’” will not be included in this Report.Unfortunately the highly important technique of e.s.r. lies beyond the scope of this Report and further space does not permit detailed description of a large body of fluorescence data. Consideration of rate data for atomic states differing in spin and orbital symmetry will generally be made within the context of correlation diagrams many of which have been given in a recent review by the authors.2 The chemistry of the higher spin orbit state 2Pt of the halogen atoms in the np5 ground-state electronic configuration has also been reviewed recently by the author^,^ where the details of specific reactions may be found.A number of reviews on atoms are referred to in the relevant sections in the text. A general account of methods for studying atomic reactions has recently been given by Wagner and W~lfrum.~’ It is our hope that this part of the Report particularly will indicate the state of the subjects as well as drawing attention to recent developments. The order of presentation of the atoms essentially on a Group by Group basis does not imply any priority felt by the authors. D.C. 1967; ( d ) E. Ratajczak and A. F. Trotman-Dickenson ‘Supplementary Tables of Bimolecular Gas Phase Reactions’ Publications Department U.W.I.S.T. Cardiff, Wales 1971 ; ( e ) K. Schofield ‘An Evaluation of Kinetic Rate Data for Reactions of Neutrals of Atmosphere Interest’ Planer-and-Space Sci. 1967 15,643 1336; (f) D. L. Baulch D. D. Drysdale D. G. Horne and A. C. Lloyd ‘High Temperature Reaction Rate Data Series’ High Temperature Reaction Rate Data Centre Leeds University, Leeds England 1970 vols. 1-5; ( g ) H. S. Johnston ‘Gas Phase Reaction Kinetics of Neutral Oxygen Species’ NSRDS-NBS-20 U.S. Government Printing Office, Washington D.C. 1968; ( h ) J. T. Herron ‘An Evaluation of Rate Data for the Reactions of Atomic Oxygen O(3P) with Methane and Ethane’ Internat. J . Chem. Kinetics 1969 1 527; ( i ) A. C. Lloyd ‘A Critical Review of the Kinetics of the Dis-sociation-Recombination Reactions of Fluorine and Chlorine’ Internat.J . Chem. Kinerics 1971 3 39; ( . j ) F. Kaufman Ann. Rev. Phys. Chem. 1969 20 45; ( k ) B. A. Thrush Ann. Rev. Phys. Chern. 1968 19 371; (4 A. B. Callear and J. D. Lambert, ‘The Transfer of Energy Between Chemical Species’ in vol. 3 of ‘Comprehensive Chemical Kinetics’ ed. C . H. Bamford and C. F. H. Tipper Elsevier Amsterdam, 1969 p. 182; ( m ) D. W. Setzer and D. H. Stedman ‘Progress in Reaction Kinetics’, ed. K. R. Jennings and R. B. Cundall Pergamon Press Oxford 1971 V O ~ . 6 . R. J. Donovan and D. Husain Chern. Rev. 1970 70,489. ( a ) D. Husain and R. J. Donovan Adv. Photochern. 1971 8 I ; ( 6 ) H. Gg. Wagner and J. Wolfrum Angrw. Chem. Internat.Edn. 1971 10 604 Reactions of Atoms and Small Molecules 125 2 Monatomic Species Hydrogen.-Hydrogen Atoms Investigated by Absorption of Lyman M Radiation. (a) H atoms inflow systems. The study of resonance absorption of light by hydrogen atoms in the vacuum U.V. is a highly sensitive method for monitoring this transient species and is the basis of a number of direct kinetic measurements on this atom. Michael and Weston4 have monitored the concentration of hydrogen atoms in a discharge flow system using atomic absorption spectroscopy by attenuation of Lyman CI radiation (H 2’P3,+ -+ l2SS; ; A = 121.6 nm) detected by a nitric oxide ionization chamber. These authors report rate data for the reactions of H atoms with ethylene and acetylene (Table 1) and Barker and Michael5 report those for Table 1 Rate constants for the reactions of hydrogen atoms with olefns obtained ,from time-resolved resonance Juorescence ( F ) and time-resolved atomic absorption spectroscopy (A) employing the Lyman M transition React ion OleJn k/cm3 molecule- s - Conditions C2H4 (1.0 _+ 0.15) x lo-’’ (1.36 0.19) x (3.22 0.17) x 7.11 x lo+ 3.87 x 10-13 (9.1 0.8) 10-13 c2 D4 (1.0 0.15) x lo-” Isobutene (3-8 & 0.57) x lo-’’ trans-But-2-ene (1.0 0.15) x lo-’* C3H6 (1-61 & 0.04) x room temp.298 K 300 K room temp. room temp. 298 K room temp. ro@m temp. room temp. 298 K ( 121 1 ; 1 cal) (10.18 0.26) x lO-”exp - -Ref. Method 11 F 12 F 4 A 5 A 6 A 9,lO A 11 F 11 F 11 F 14 F 14 F Molecule C2H5 (6.0 2.0) x lo-” 50Torr He 12 F 298 K 300 K 4 A H + C2H4 obtained by this method.Michael et aL6 have further studied this latter reaction by pulsed mercury-photosensitized decomposition of H2 coupled with Lyman a attenuation measurements and report a further value for the bi-molecular rate constant for H + C2H4 extrapolated to high pressure (Table 1). Lyman CI photometry of hydrogen atoms generated by steady-state mercury photosensitization has yielded a value for the absolute cross-section7 of 10-8 A2 for the reaction of Hg(63P1) with H,. J. V. Michael and R. V. Weston J . Chem. Phys. 1966 45 3632. ’ J. R. Barker and J. V. Michael J . Chem. Phys. 1969 51 850. ‘ J. R. Barker D. G. Keil J. V. Michael and D. T. Osborne J . Chem. Phys. 1970 52, ’ J. V. Michael and C.Yeh J. Chem. Phys. 1970,53 59. 2079 126 R. J . Donovan and D. Husain (6) H atoms generated by pulse radiolysis. Dorfman and Bishop' have employed Lyman a attenuation to monitor the concentration of hydrogen atoms generated by pulse radiolysis. By this method Dorfman and his c o - w ~ r k e r s ~ ~ ' ~ have characterized the bimolecular rate constant for the addition of H to C2H at high pressure under conditions in which the energized C2H5 initially formed is rapidly quenched by collisions. The resulting value is in good agreement with that given by Braun et aI."3'2 from resonance fluorescence measurements (see later) and is to be preferred. Bishop and Dorfman' have further reported rate data for H atom-molecule recombination reactions following pulse radiolysis (Table 2).Myerson and Watt13 have also employed absorption of Lyman a radiation to follow the course of H atoms generated in shock-heated hydrogen in order to study the rate of molecular dissociation. Table 2 Rate constants for hydrogen atom-molecule recombination studied by pulse radiolysis with Lyman a attenuationa Recombination Reaction k/cm6 molecule-2 s- ; at 298 K H + 0 + H -+ H0 + H (4.7 f 1.1) x 10-32 H + 0 + Ar + HO + Ar (1.6 0.2) x 10-32 H + C O + H -+ HCO+H (1.1 f 0.2) x 10-34 H + C O + A r - + HCO+Ar (7.2 _+ 1-1) 10-35 H + N O + H - + HNO+H (3.9 _+ 0-6) x 10-32 Hydrogen Atoms Studied by Resonance Fluorescence. The technique of time-resolved resonance fluorescence for transient atoms was developed by Braun and Lenzi" initially for hydrogen atoms.Hydrogen atoms generated by flash photolysis in the gas phase were optically excited by a Lyman a resonance lamp and the resulting fluorescence was monitored photoelectrically as a function of time. By this method Braun and Lenzi" report rate constants for H atom addi-tion reactions with various olefins (Table 1). Braun and his c o - ~ o r k e r s ~ ' * ~ ~ have extended their measurements on H atom addition to olefins in fine detail to other olefins (Table 1). These workersI5 have also studied the reaction of H atoms with H2S by this method and report a rate constant of k = 1.29 & 0.15 x lo-" exp ( - 1709 60 cal/RT) cm3 molecule- s - for the hydrogen atom abstrac-tion reaction. H(22P) and H(2'S). Reactions of the excited state of the hydrogen atom H(22P) and H(2'S) have also been the object of spectroscopic measurements.The former state emits strongly in reverting to the H(1'S) ground state (Lyman a z = 1.6 x W. P. Bishop and L. Dorfman J. Chem. Phys. 1970 52 3210. J. A. Eyre T. Hikida and L. Dorfman J . Chem. Phys. 1970 53 1281. l o T. Hikida J. A. Eyre and L. Dorfman J . Chem. Phys. 1971 54 3422. W. Braun and M. Lenzi Discuss. Furuday Sac. 1967 No. 44 p. 252. I Z M. J. Kurylo N. C . Peterson and W. Braun J. Chem. Phys. 1970 53 2776. A. L. Myerson and W. S. Watt J. Chem. Phys. 1968 49 425. l 4 M. J. Kurylo N. C . Peterson and W. Braun J . Chem. Phys. 1971 54 4662. l 5 M. J. Kurylo N. C . Peterson and W. Braun J. Chem. Phys. 1971 54 943 Reactions of Atoms and Small Molecules 127 10W9 s)16 whereas the latter is optically metastable (z = 0.14 s ) .' ~ A convenient method for studying H(22P) is that of exciting ground-state hydrogen atoms .in a flow discharge system by Lyman CI radiation and of monitoring the fluorescent Lyman a radiation in the presence of quenching gases. Tanaka et a1.'87'9 have demonstrated efficient energy transfer from both H(22P) and D(22P) to molecular nitrogen measuring Lyman a radiation by means of a but-l-ene ionization gauge. Wauchop and Phillips20,2 ' have reported molecular emissions of excited molecules derived from the reactions of H(22P) generated by Lyman a excitation in the presence of reactant molecules including the (0,O) transition of OH(A2C-X 2 n ) from the reaction of H(22P) with 02. Phillips et ~ 1 . ~ ~ also report quenching cross-sections of H(22P) using the Lyman a excitation method (Table 3).Where a Table 3 gases Collisional cross-sections (02) for the quenching of H(2*P) by various Quenching gas He Ar H2 D2 N2 d / A ' < 3 1.1 f 0.5 84 f 8 3-0 f 1.5 84 k 8 62 k 6 3.3 f 1.0 4-5 1- 1.5 4.9 _+ 2.5 5.2 f 2.0 12.0 f 4.5 8.6 f 3.0 17.0 _+ 5.0 Ref. 23 22 23 22 23 23 22 22 22 22 22 22 22 comparison can be made the cross-sections are lower than those given by Braun et a1.,23 who carried out a later study using a system specifically optically thin to Lyman c1 radiation and whose values therefore are to be preferred. Lyman a emission has been employed to study indirectly the metastable state H(22S). Fite et ~ 1 .~ ~ have generated H(22S) by electronic excitation of H(12S) and have measured the yield by Lyman a emission following the collisional quenching of H(22S) in the overall process : H(22S) + Q -+ H(12S) + Lymancr + Q l 6 J. E. Mental1 and E. P. Gentien J. Chem. Phys. 1970 52 5641. J. Shapiro and G. Breit Phys. Rec. 1959 113 179. I. Tanaka and J. R. McNesby J . Chem. Phys. 1962 36 3170. T. S . Wauchop and L. F . Phillips J. Chem. Phys. 1967 47 4281. T. S . Wauchop and L. F. Phillips .I. Chem. Phys. 1969 51 1167. l 9 I. Koyano and 1. Tanaka J. Chem. Phys. 1964 40 895. 2 2 T. S . Wauchop M. J. McEwan and L. F . Phillips J . Chem. Phys. 1969 51 4227. 2 3 W. Braun C. Carloe T. Carrington G . V. Volkenburgh and R. A. Young J . Chem. 2 4 W. L. Fite R. T. Brackman D.G. Hummer and R. F. Stebbings Phys. Rev. 1959, Phys. 1970 53 4244. 116 363; 1961 124 2051 128 R. J. Donovan and D. Husain They report the following overall cross-sections :24 Q a2/A2 H2 70 N2 100 0 2 60 H20 1000 Mental1 and GentienI6 have generated H(2,S) and H(2,P) by photodissociation of H and have shown that the observed Lyman M radiation is obtained principally from the H(2,P) derived from the collisional quenching of H(2,S). Comes and Wenning25 have carried out a similar study on H(2,S) generated by photo-dissociation of H in the vacuum U.V. and report the following cross-sections (02) : Process a2/A2 H(2,S) + H -+ Lyman M + H + H( 1,s) 50-100 H(2,S) + H -+ H(12S) + H (energy transfer without radiation) It is particularly important in this type of investigation to exclude even weak fields as these give rise to mixing of the 2,s and 2,P states resulting in the relaxa-tion of the former.Sodium Potassium Rubidium Caesik and Thallium.-Extensive kinetic investigations of electronically excited alkali-metal atoms in the 'Pk,+ state have been carried out by means of resonance fluorescence methods. This has been reviewed by Krause,26n who has considered alkali-alkali atom and alkali-noble-gas atom collisions. Whilst a number of further detailed studies including quenching by various gases have been reported since Krause's review,26a this work will not be reviewed here. A recent investigation by Hrycyshyn and Krause26b describes resonance fluorescence of Rb(5,P,,+) in the presence of a number of different gases.Data for the collisional mixing of the 5 2 P + w 5'Pt states and deactivation to the 52S state are reported. For example with hydro-gen the cross-section for the process Rb(5,P3-+ 5,P+) (15 A2) is larger than that for Rb(S2P+ -+ 5,S,) (3 81,); on the other hand for N, the former process is characterized by a cross-section of 23A2 and the latter by one of 4381,. Hrycyshyn and Krause's paper26b may be referred to for other recent work on resonance fluorescence from alkali-metal atoms and itself constitutes a brief summary of recent work on alkali-noble-gas atom collisions. Zare et have recently reported cross-sections for the four processes : ca. 50 K(4p 'P+,+) + Rb(5s 2S+) -+ K(4s 'S+) + Rb(5p 'P+.+) Agreement with some of the cross-sections reported by Krause et is observed 2 5 F.J. Comes and U. Wenning Z . Nuturforsch. 1969 24a 587. 2 6 (a) L. Krause Applied Optics 1966 5 1375; (6) E. S. Hrycyshyn and L. Krause, Canad. J . Phys. 1970 48 2761; ( c ) V. Stacey and R. N. Zare Phys. Rev. 1970 A l , 1125; M. H. Ornstein and R. N. Zare ibid. 1969 181 214; (6) E. S. Hrycyshyn and L. Krause Canad. J . Phys. 1970 47 215; ( e ) L. E. Brus J . Chem. Phys. 1970 52 1716 Reactions of Atoms and Small Molecules 129 but others are a factor of ca. 6 lower than those given by Krause and no satis-factory explanation has yet been put forward for this discrepancy. Brus26e has observed the time dependence of emission from Na(3,PJ) and T1(72S,) generated by the pulsed photodissociation of metal halide vapours in the far-u.v. The quenching of the emission by halogens is reported and discussed in terms of a 'harpoon' mechanism encountered in molecular-beam studies.Time-resolved atomic absorption measurements have been reported leading to kinetic data for the Group IA elements and for thallium with the emphasis on the latter. Thallium. Collisional rate data for the optically metastable state (6,P+) of the thallium atom 0.97 eV above its 6,P+ ground state have been described. Dudkin et ~ 1 . ~ ~ " have measured the change in lineshape of the transition 7,S j 6'P+ (1 = 535.0 nm) attenuated by T1(6,P3) generated from the continuous photolysis of thallium iodide vapour. These authors give the following estimates of the collisional deactivation cross-sections for this atomic state :27a Quenching gus O2 NH H2 Cross-section/A2 1 1 10- 2-10- 3 Pickett and Anderson27b have studied T1(6,Pt) by time-resolved absorption at 1 = 535.0nm on the afterglow of a pulsed discharge and report a quenching cross-section of 5.4 A' of T1(ti2P,) by T1(6,P,).The most detailed investigation on this atom has been that of Bellisio and Davidovits28 who have studied T1(6,P,) following the pulsed irradiation of TIC1 TIBr and TI1 in the U.V. in the temperature range 2 8 5 4 0 0 "C. These authors report a sizeable body of quench-ing collision cross-sections for this atomic state (Table 4). Davidovits et also report a cross-section of 159 A2 for the chemical reaction of TI(6,PJ with molecular iodine. Sodium Potassium Rubidium and Caesium. Davidovits and c o - ~ o r k e r s ~ ' * ~ ' have employed atomic absorption of resonance radiation to monitor the rates of the chemical reactions of a ground-state alkali-metal atoms with molecular iodine.The alkali-metal atoms are generated by the flash photolysis of alkali-metal salts at elevated temperatures (55-20 "C) and the following cross-sections for reaction with I are reported :30,31 Atom C T / A 2 Na 97 K 127 Rb 167 c s 195 '' (a) V. A. Dudkin V. I. Malyesh and V. N. Sorokin Optics and Spectroscopy 1968, 20 3 13 ; (6) R. C. Pickett and R. Anderson J. Quant. Spectroscopy Radiative Transfer, 1969 9 697. '* J. A. Bellisio and P. Davidovits J. Chem. Phys. 1970 53 3474. 2 9 A. Gedeon S. A. Edelstein and P. Davidovits J. Chem. Phys. 1971 55 5171. 3 0 D. C. Brodhead P. Davidovits and S.A. Edelstein J. Chem. Phys. 1969 51 3601. 3 1 S. A. Edelstein and P. Davidovits J . Chem. Phys. 1971 55 5164 130 R. J . Donovan and D. Husain Table 4 Collisional cross-sections (0’) for the deactivation 0fn(6~P+) by various gases Quenching gas o2/A2 <2 x 10-3 < 2 x 10-3 t 2 x 10-3 < 2 x 10-3 <2 x 10-3 4.4 39 0.037 0.010 5.2 28 0-026 0.39 0.13 0.8 3 0.1 1 159” Ref. 29 Mercury Cadmium Zinc Iron and Copper.-Mercury . Earlier studies of the collisional quenching of the metastable species Hg(63P0) generated following flash excitation of Hg(6’So) and which may be monitored by kinetic absorption spectroscopy have been r e v i e ~ e d . ~ ~ - ~ ~ Callear and his co-workers have re-investigated this work in greater detail and have demonstrated that the earlier data must be considerably modified.Callear and M c G ~ r k ~ ~ have shown that pumping into the 3P0 state occurs by the process : N2(A3C,+) + Hg(6’SO) -+ Hg(63P0) + N2(X1Zg+), Table 5 Cross-sections (0’) for the collisional quenching of Hg(63P0) Results of Results of Callear and M c G ~ r k ~ ~ Callear and Williarn~,~ Gas 02/A2 d J A ’ NO 16.2 0.34 co2 0.033 0.00 14 Hg 0.95 0.01 8 N2 being generally present to cause collisional quenching of the initially formed Hg(63P,) to Hg(63P0) in the earlier experiments. Callear and Wood37938 report 3 2 A. B. Callear Applied Optics 1965 Suppl. 2 ‘Chemical Lasers’ p. 145. 3 3 J. G. Calvert and J. N. Pitts ‘Photochemistry’ Wiley New York 1966. 3 4 A. B. Callear and R. G. W. Norrish Proc.Roy. Sac. 1962 A266 299. 3 5 A. B. Callear and G. J. Williams Trans. Faraday Soc. 1964 60 2158. 3 6 A. B. Callear and J. C . McGurk Chem. Phys. Letters 1970 6 417. 37 A. B. Callear and P. M. Wood Chem. Phys. Letters 1970 5 128. 3 8 A. B. Callear and P. M. Wood Trans. Faraday Sac. 1971 67 272 Reactions of Atoms and Small Molecules 131 Table 6 gases3 Rate constants for the collisional deactivation of Hg(6,P0) by various k/cm3 molecule-' s - ; at 295 K (5.37 _+ 0.35) x lo-" (2-51 f 0-16) x lo-'' (1.035 _+ 0.062) Y lo-" (1-81 f 0.07) x lo-'' (4-39 f 0.23) x (1.12 f 0.04) x lo-'' (5-90 & 0.21) x (8.88 0.43) x (4.20 f 0.06) x lo-'' (4.41 0.18) x (3.80 0.20) x 10-13 this pumping process to be characterized by a rate constant of k = 2.9f0.15 x 10- l o cm3 molecule-' s- ' which is clearly rapid.Examples of the extent to which the earlier quenching data are modified by this process are given by Callear and M c G ~ r k ~ (see Table 5) who also report a new set of deactivation rate data for quenching of this metastable atom (Table 6). These authors36 also report data on termolecular quenching of Hg(6,PO) in the presence of NH, as shown in Table 7. Table 7 Quenching rate constants36 at 295 K for the process : Hg(63P0) + NH + M 5 Hg + NH3* + M M k/cm6 molecule-' s -Ar 0.92 f 0-06 x N2 1.88 2 0.02 x 10-31 He %0.9 1 0 - 3 1 The second-order quenching constant for NH (Table 6) is in good agreement with the results of the sensitized fluorescence experiments of Phillips et ~ 1 . ~ Callear and have further shown that CO(v" = l) which results on the quenching of Hg(6,P1) arises from the spin-orbit relaxation process : Hg(63P,) + CO(u" = 0) + Hg(63P0) + CO(v" = l), and that this process occurs with approximately unit collisional efficiency.Quenching of Hg(6,Pl) into Hg(63P0) by ground-state mercury atoms has been shown by Waddell and Hurst40b to proceed with low efficiency ( k = 3@-10.0 x 10- '' cm3 molecule- ' s- ') by applying photon-transport theory to experi-mental data on the quenching of Hg(6,P1). J 9 C . J. Freeman M. J. McEwan R. F. C. Claridge and L. F. Phillips Chern. Phys. Letters 1970 5 5 5 5 . 4 0 ( a ) A. B. Callear and P. M. Wood Trans. Faraday SOC. 1971 67 2862; (b) B. V. Waddell and G. S. Hurst J . Chem. Phys. 1970 53 3892; (c) B. deB.Darwent and F. G. Hurtubise ibid. 1952 20 1684; (d) M. D. Scheer and J. Fine ibid. 1962 36, 1264; ( e ) A. C. Vikis and H. C . Moser ibid. 1970 53 1491 2333 132 R. J. Donovan and D. Husain Measurement of the number of electrons emitted from a silver surface following collision with Hg(63Po) atoms a technique described initially using nickel4" and later with has been employed recently by Vikis and Moser40e for a study of the collisional quenching of this atomic state. Where a comparison may be made the reported quenching cross-sections relative to that for ethylene are in better agreement with the more recent work of Callear and M c G ~ r k ~ ~ (Table 6) than the ratios these authors give following chemical analysis which ratios are somewhat higher and apparently less reliable.New methods of studying Hg(63Po) on a time-resolved basis include microwave pulsing coupled with kinetic absorption spectro~copy.~'*~~ Strausz et al. report kinetic measurements by attenuation of radiation during continuous photoly~is.~~ The authors feel that the work of Azada et ~ 1 . ~ ~ should be re-emphasized at this period since their work performed in 1928 constituted what was presumably the first resonance fluorescence experiment on a transient species. Thus in their experiment H S ( ~ ~ P ~ ) was generated following irradiation of a Hg-N mixture with light of wavelength 253.7 nm [Hg(63P,) -+ Hg(6'S0)]. H S ( ~ ~ P ~ ) was then monitored in fluorescence at 546.1 nm (73S -+ 63P2) following excitation by a secondary source raising the Hg(63P0) atoms to the 73s state (73S + 63P0; R = 404-7 nm).Phillips and his c o - w o r k e r ~ ~ ~ ~ ~ ~ - ~ ~ have carried out a number ofinvestigations on Hg(63Po) by sensitized luminescence combined with phase-sensitive detection. A modified approach has been carried out recently by H u n ~ i c k e r . ~ ~ Phillips' work has led to a body of quantitative data on the lifetimes of the complexes that mercury in the excited state forms with molecules including alcohols and amines and to quenching data particularly for Hg(63Po). These data compare well with those obtained from Callear's experiments (see above) using kinetic absorption spectroscopy. Of recent work on Hg(63P,) by resonance fluorescence, a particularly detailed investigation has been described by Krause et a!.'' using low mercury vapour densities to avoid large corrections for pressure broadening and radiation tra~ping,~' leading to accurate values for the total collisional cross-sections.4 1 A. B. Callear J. A. Green and G. J. Williams Truns. Furuduy SOC. 1965 61 1831. 4 2 A. B. Callear and R. E. M. Hedges Truns. Furuduy Sac. 1970 66 605 615. 4 3 J. M. Campbell S. Penzes H. S. Sanhu and 0. P. Strausz Internut. .I. Chem. Kinetics, 44 T. Azada R. Ladenburg and W. Tietze Phys. Zeit. 1928 29 549. 45 C. G. Freeman M. J. McEwan R. F. C. Claridge and L. F. Phillips Trans. Furaduy Soc. 1971 67 67. 4 6 C. G. Freeman M. J. McEwan R. F. C. Claridge and L. F. Phillips Truns. Furuduy SOC. 1971 67 2004 2567 3247. 4 7 R. H. Newman C. G. Freeman M. J. McEwan R. F. C. Claridge and L.F. Phillips, Truns. Faraduy SOC. 1970 66 2827. 4 8 R. H. Newman C. G. Freeman M. J. McEwan R. F. C . Claridge and L. F. Phillips, Chetn. Phys. Letters 1971 8 226. 4 9 H. E. Huntziker Chem. Phys. Letters 1969 3 504. s o J. S. Deck J. Pitre and L. Krause Canad. J . Phys. 1971 49 1976. 5 1 J. S. Deck and W. E. Bayliss Canad. J . Phys. 1971 49 90. 1971 3 175 Reactions of Atoms and Small Molecules 133 Zinc and Cadmium. Callear and Bre~kenridge’’.’~ have reported a method of cadmium resonance flash excitation coupled with kinetic spectroscopy in which Cd(53P,,l,o) and also Cd(SISo) were monitored in absorption. This work includes quantitative measurements on the relative yields of CdH(D) from the processes : Cd(53P,) + H2(D2 HD) -+ CdH(D) + H. Strausz et report kinetic absorption spectroscopic observations of Cd(53P,,,,o) and Zn(43P2 l,o) following the flash photolysis of alkyl compounds.Detailed measurements on the fluorescence decay following pulsed excitation of the triplet resonance line in cadmium have been reported by Dodd et a2.” Whilst the observed lifetime in the excited state was lower than the expected value in the apparent absence of a quenching gas the method employed may clearly be extended to the determination of accurate collisional cross-sections. Iron. Callear and old ma^^'^ have described detailed rate data on electronically excited iron atoms (Fea’D) generated by flash photolysis and monitored in absorption by kinetic spectroscopy. The resulting data are presented in Table 8. Table 8 gases56 Rate constants for the collisional deactivation of Fe(a5D3+,) by various Quenching gas Ar N2 He co H2 D2 Fe k/cm3 molecule- ’ s - ; at 293 K (2.1 f 0-3) x (6-2 f 0.7) x (2.9 f 0.5) x (7.4 & 0.7) x (6.1 f 0-7) x (1.9 0.3) x (1.1 * 0.2) x 1 0 - ’ O Copper.Coilisional quenching cross-sections for Cu(4p ’P+,+) have been reported by Bleekrode and van Benthem,’ ’ following phase-sensitive amplification of the fluorescence at 327.4 and 324.8 nm (4p ’P+,* -P 4s ’S,) from copper vapour. This investigation has yielded the cross-sections shown in Table 9. Table 9 Quenching cross-sections (o’ f 15 %) for Cu(4p ’P+,+) in the presence of gases Gas 02/A2 for ’P+ 02!A2fot ’P4 H2 22 23 N* 14 19 co2 50 36 Ar 0.2 0.8 5 2 A. B. Callear and W.H. Breckenridge Chem. Phys. Letters 1970 5 17. 5 3 A. B. Callear and W. H. Breckenridge Truns. Furuduy Soc. 1970 67 2009. 5 4 P J. Young G . Greig and 0. P. Strausz J . Amer. Chem. SOC. 1970 92 413. s 5 J. N. Dodd W. J. Sardle and D. Zisserman Proc. Phys. SOC. London 1967 92 497. 5 6 A. B. Callear and R. J. Oldman Trans. Furuduy SOC. 1967 63 2888. 5 7 R. Bleekrode and W. van Benthem J. Chem. Phys. 1969 51 2757 134 R. J. Donovan and D. Husain Carbon Silicon Germanium Tin and Lead.-Carbon. The principal develop-ment in the kinetic study of carbon atoms has entailed their direct investigation by atomic absorption spectroscopy. The foundation of this development is the work of Braun et a1.,58 who generated C(23P,) C(2lD2) and C(2'S0) by the flash photolysis of carbon suboxide and who monitored these atomic species by plate photometry in the vacuum U.V.Rate data reported by these authors for reactions with various gases are given in Table 10. Further detailed kinetic investigations have been described by Husain and Kirs~h,'~-~' who have employed atomic absorption spectroscopy by attenuation of resonance radiation following the production of C(23P,) and C(2lDZ) by the pulsed irradiation of C302 in a static system (Table 10). Meaburn and Perner6' have reported some approximate half-life data for C (2'S,) monitored in absorption following pulsed radiolysis. Wolf et ~ 1 . ~ ~ have detected the optically metastable atoms C(21D2) and C(2'S0) Table 10 Rate constants k/cm3 molecule- s- ' at 300 K for the reactions of carbon atoms C(ZI3P, 2'D, 2lSO) with various molecules Gas c(23pJ) Ref.C(292) Ne - ._ (1.1 f 0.4) x lo-'' Ar - - 510-15 Kr - - (9.4 f 1.6) x 10-13 - < 3 x 10-l6 - He Xe - - (1.1 0.3) x lo-'' NO (7.3 f 2.2) x lo-" 5 9 ~ 6 0 ~ (4.7 & 1.3) x lo-" 1.1 x lo-'' 58 9.2 x lo-" CO '(6.3 f 2.7) x 60a (1.6 f 0.6) x lo-" (M = He) CH4 <2.5 x lo-'' 6 0 ~ (2.1 f 0.5) x lo-'' < 5 x lo-'' 58 3.2 x lo-'' N,O (2.5 & 1.6) x lo-" 6 0 ~ (1.4 0.5) x lo-'' COZ < 60a (3.7 k 1.7) x lo-" -. - -3.7 x 10-l' c2 H4 0 2 (3.3 f 1-5) x lo-" 6 0 ~ -2.6 x lo-" 3.3 x lo-" 58 < 5 x 10-l2 H20 63.6 x 60a -1.7 x lo-" 4-15 x lo-" - C3H6 - -H '(7.1 k 2.5) x 60a (2.6 0.3) x lo-'' N2 "(3-1 1.5) x 60a (4.2 f 1.2) x lo-'' (M = He) (M = Ar) 2.2.5 x Ref. C(2'S0) Ref.60b 60b 60b 60b 60b 61 58 - -- -- -- -- -- -- -61 b<3*5 x 62 61 b-3.0 x 62 58 61 61 61 b-5.0 x 62 58 61 59b < 5 x 58 58 b-2 x 62 596 58 - -61 b < l * O x 62 - -- -- b-5 x lo-'' 62 - -- -- -- -Third order k/cm6 molecule-* s ~ Data for C(21S,) from ref. 62 presented here by assuming first-order kinetics combined with half-life data. s 8 W. Braun A. M. Bass D. D. Davis and J. D. Simmons Proc. Roy. Soc. 1969 A312, 417. 5 9 ( a ) D. cusain and L. J . Kirsch Chem. Phys. Lerrers 1971 ,8 543; (b) ibid. 1971,9,412. 6 o ( a ) D. Husain and L. J. Kirsch Truns. Furuduy SOC. 1971 67 2025; ( 6 ) ibid. p. 2886. 6 1 D. Husain and L. J. Kirsch Trans. Furaduy Soc. 1971 67 3166. 6 2 G. M. Meaburn and D.Perner Nature 1966 212 1042. 63 E. Y. Y. Lam P. Gaspar and A. P. Wolf J. Phys. Chetn. 1971 75 445 Reactions of Atoms and Small Molecules 135 in absorption following the plasmolysis of a number of organic moIecules but kinetic data have not been reported. All the rate data determined by direct investigations on C(23PJ 21D2 and 2 l S O ) are given in Table 10. Rate data for the quenching of C(21D2) in the presence of the noble gases were considered by Husain and Kirsch6" in terms of the Landau-Zener for-mula,6L6s using an empirical equation of the form : P = 2 exp ( - AE,2,) [ 1 - exp ( - AE$o)l where P is the probability of collisional relaxation E, is the spin-orbit interaction energy and A is an empirical constant chosen for the best fit with the experimental data.Husain and Kirsch6'' also consider previous data for the species 0(2'D2), S(3'D2) and 'CH in terms of this equation. Deactivation probabilities of these species give a better fit to the empirical equation compared to the data for C(2'D2), where the variation in deactivation probability across the noble gases is much larger. The data for C(23PJ) C(2'D2) and C(2'S0) with molecules (Table 10) are shown by Husain and Kirsch61 to satisfy the predictions of the correlation diagrams given previously by the present authors.2 These considerations include the approximate data on C(21S0).62 The absence of detailed kinetic information on this state constitutes at present the main gap in our understanding of the rate data for the carbon atom in specific quantum states.The yield of C(21S0) from photolysis of C 3 0 2 is very and whilst this species can be photoelectrically detected by attenuation of atomic radiation in single-shot e~perirnents,~~ it would appear that detailed kinetic data on this state must best come from the application of signal-averaging methods. Silicon Germanium Tin and Lead. Detailed direct kinetic studies on defined quantum states of Si Ge Sn and Pb remain to be published although some kinetic spectroscopic observations have been made hitherto. No detailed data have yet been reported for silicon to our best knowledge though the surfaces appropriate to the reactions of this atom have been discussed by the authors.2 Ge(4'D2 43PJ) has been detected photographically in absorption following flash p h o t o l y ~ i s .~ ~ Sn7 and Pb72,73 atoms have been monitored photographically in absorption at the high variable temperatures which exist in experiments on the effects of additives on the 'knock' and 'anti-knock' behaviour of fuels in the internal combustion engine but detailed kinetic data on these atoms have not been reported. 6 4 L. Landau Phys. Z . Sowjerunion 1932 2 46. 6 5 C . Zener Proc. Roy. Soc. 1932 A137 696. " C. Zener Proc. Roy. SOC. 1933 A140 660. 6 7 D. R. Bates Proc. Roy. Soc. 1960 A257 22. 6 8 C. A. Coulson and K. Zalewski Proc. Roy. Soc. 1962 A286 437. 6 9 D. Husain and J. G. F. Littler unpublished results. '' A. B. Callear and R. J. Oldman Spectroscopy Letters 1968 1 149. A. B. Callear and R. G. W. Norrish Proc. Roy. SOC. 1961 A259 304. 7 2 K.H. L. Erhard and R. G . W. Norrish Proc. Roy. Soc. 1957 A234 178. l 3 K. H. L. Erhard and R. G . W. Norrish Proc. Roy. SOC. 1961 A259 297 136 R. J. Donovan and D. Husain Nitrogen Phosphorus Arsenic Antimony and Bismuth.-Nitrogen. The main development in kinetic studies of nitrogen atoms by electronic spectroscopy has principally concerned the optically metastable states N(22D_t,t) and N(22P+,t), 2.38 and 3-58 eV respectively above the N(24S,) electronic g r ~ u n d - s t a t e . ~ ~ There have hitherto been many studies of N(24S,) generally by chemical-titration techniques on flow discharge systems and these have previously been re-~ i e w e d . ~ ~ - ~ ~ It is only in the past year that detailed kinetic studies on the excited states have been carried out directly although Tanaka et dsO had detected the 4S 2D and ' P states by atomic absorption spectroscopy in the afterglow of a discharge.Two time-resolved methods have been used both employing attenua-tion of atomic resonance radiation in the vacuum u.v. the basis of which had earlier been laid for flow systems by Kaufman et ~ l . ~ ' ' ~ Lin and Ka~fman'~ have generated N(22D,) in a flow discharge system and monitored the atoms by absorption of radiation from a microwave-powered flow lamp. Husain Kirsch, and Wiesenfelds4 have monitored both N(22D,) and N(22PJ) by time-resolved absorption following pulsed photolytic dissociation of N 2 0 in the Schumann region. The resulting kinetic data for the two types of investigation namely in flows3 and static ~ysterns,'~ are presented in Table 11.The data for N(22D,) reported by Lin and KaufmanB3 are to be preferred. The later investigations4 involved the production of very low yields of N(2*DJ) and the measurements could not take account of population of N(22D,) from the collisional deactivation of N(22PJ). The agreement with the data of Lin and Kaufmans3 would indicate that this effect is not significantly large. Furthermore Lin and Kaufmans3 were able to carry out product analyses particularly for the transient atoms involved, and the results of these analyses together with the data of N(22PJ) and that quoted here for N(24S,) permit discussion within the context of correlation diagrams which have been presented earlier in detail for these systems by the authors2 and which account for the data.The data for N(22D,) in Table 1 183-88 are in general accord with those reported hitherto by Young et ~ l . ' ~ who 7 4 C . E. Moore National Bureau of Standards Circular 467 'Atomic Energy Levels', '' 1. M. Campbell and B. A. Thrush Ann. Reports 1965 62 17. 7 6 A. Nelson Wright and C. A. Winkler 'Active Nitrogen' Academic Press New York " 7 8 G. G. Mannella Chem. Rev. 1963 63 1. 7 9 K. R. Jennings and J. W. Linnett Quart. Reo. 1958 12 116. 8 o Y . Tanaka A. Jursa and F. LeBlanc in 'The Threshold of Space' ed. M. Zelikoff, 8 1 8 2 C.-L. Lin D. D. Parkes and F. Kaufman J . Chem. Phys. 1970 53 3896. K 3 C.-L. Lin and F. Kaufman J. Chem. Phys. 1971 55 3760. 8 4 D. Husain L. J. Kirsch and J. R. Wiesenfeld Discuss. Furuduy Soc. 1972 in the press. F.Kaufman 'Atmospheric Reactions involving Neutral Species - An Evaluation', Amer. Geophys. Union Meeting San Francisco California 1968. '' J. F. Noxon J . Chem. Phys. 1962,36,926. n 7 M. A. A. Clyne and B. A. Thrush Proc. Roy. Soc. 1961 A261 259. " G. B. Kistiakowsky and G. G. Volpi J . Chem. Phys. 1957 27 1141. M 9 G. Black T. G. Slanger G. A. St. John and R. A. Young J . Chem. Phys. 1969 51, vols. 1-111 U.S. Government Office Washington D.C. 1958. and London 1968. B. Brocklehurst and K. R. Jennings Progr. Reuction Kinetics 1967 4 1. Pergamon Press London 1957 p. 89. F. A. Morse and F. Kaufman J . Chem. Phys. 1965 42 1785. 116 Reactions of Atoms and Small Molecules Table 11 atoms N(24S, 22DJ 22PJ) with various molecules at 300 K 137 Rate constants k/cm3 molecule-' s-' ,for the reactions of nitrogen Gas W2DJ) Ref.N( 2 PJ) Ref. N(24S,) (1.7 0.5) x (2.3 f 1.1) x 10-14 (9.3 f 2.2) x 10-l2 ( 5 f 1) x 10-l2 (1.6 f 0.7) x -(6 & 2) x (6-1 & 3.7) x lo-" (7 5 2.5) x lo-" (4-8 _+ 0.9) x (3.5 f 1-2) x 10-l2 ( 5 2) x 10-13 (1 0-6) x < 1.6 x 84 (3.0 f 1.1) x 84 84 < 3 x 84 -s 6 x 83' - 83 - < 3 x lo-" 86 -84 (4.6 & 2-5) x 84 7.8 x l o - ' ' 83 85 84 (3.4 f 1.1) x l o - ' ' 84 3.6 x l o - " 83 83 83 83 83 a - - -- - -- - -84 (3.4 f 1.5) x 84 <4*2 x - - -- - -- - -- - -(I 0.87 eV endothermic reaction presumed third-order recombination with M = He (see ref. 75); At ca. 400 K . employed an indirect method in which the NO(B 213 -+ X 211) emission following the photolysis of N20 is ascribed to the production of NO(B 211) in the reaction : N(22DJ) + N 2 0 -+ NO(B211) + NO(X2H) The data reported for N(22PJ) by Husain et aLS4 are not complicated by popula-tion from other states in contrast with those for N(22DJ).Electronically excited nitrogen atoms are important in the chemistry of the upper atmosphere and the reader is referred to the papers of Kaufman et ds3 and Husain et aLS4 for a discussion of the data in Table 1 1 within this context. In particular the data for N(22DJ) + O2 (Table 11) are in accord with the reaction between these species accounting for their being the source of NO in the sub-100 km regiong0 in terms of model calculations of NO density profile^^^^^^ and the data from rocket m e a s ~ r e m e n t s .~ ~ ~ ~ Becker Groth and Jud9' have re-ported detailed observations of resonance fluorescence for N(24S,) but have not reported detailed kinetic data. Phosphorus. Kinetic spectroscopic studies of the optically metastable states P(32DJ) and P(32PJ) respectively 1-40 and 2.32eV above the P(34S,) ground state have now been reported by Acuna Husain and Wie~enfeld.~~ These excited states were monitored by attenuation of resonance radiation in absorp-tion in the U.V. following production of the atoms by pulsed irradiation of PCl . 90 9 1 9 2 D. F. Strobel D. M. Hunten and M. B. McElroy J. Geophys. Res. 1970 75 4307. 9 3 C. A. Barth Planet. Space Sci. 1966 14 623. 94 L. G. Meira J. Geophys. Res. 1971 76 202. 9 5 K. H. Becker W. Groth and W.Jud Z. Naturforsch. 1969 24a 1953. 9 6 A. U. Acuna D. Husain and J . R. Wiesenfeld J. Chem. Phys. 1972 in the press. M. B. McElroy Canad. J. Chern. 1969,47 1916. R . B. Norton and C. A. Barth J. Geophys. Res. 1970 75 3903 138 R. J. Donovan and D. Husain The correlation diagram2 is again a suitable vehicle for considering the resulting rate data (Table 12) and comparing these with those for the appropriate states of atomic nitrogen (see earlier). Indeed the spectroscopic observations of Balfour and Douglas9' on PH(a'A) and the calculated value98 for the energy of PH(b 'C+) lead to an identical correlation diagram for P+ H and N+ H . Basco and Yee99 have detected P(32PJ 32D,) photographically but no rate data have been reported by these authors. Table 12 Rate constants k/cm3 molecule-' s-' at 300 K for the collisional quenching of P(~,D,) and P(32~J) by some gasesg6 Gas P( 3 2D,) P( 3 PJ) PCI (9.7 0.9) x lo-" (1.1 * 0.1) x lo-'" H2 (4.0 f 0.7) x (3.1 k 0.8) x He v.small v. small 0 2 (1.4 _+ 0.2) x lo-" (2.6 & 0.2) x lo-" Arsenic Antimony and Bismuth. A number of time-resolved spectroscopic measurements on As Sb and Bi atoms have led to quantitative rate data. Basco and Yee99 have detected As(~~S, 42D,,,) photographically in absorption follow-ing flash photolysis and Callear and Oldman'OO report kinetic rate data on the collisional quenching of As(~~D+) (Table 13) in which any population into this state from As(~~D,) is neglected. Strausz et a/."' report collisional quenching data for electronically excited antimony and bismuth atoms following flash photolysis.The data are derived principally from emission measurements and Table 13 Rate constants for the quenching of As(~~D,) + As(~~S,) by various gases"' Gas Ar Ar" Kr Xe SF6 co N2 TemperaturelK 296 296 296 403 296 296 296 403 296 296 296 296 -a Quenching of As(4'Dt) not As(4*D,). k/cm3 molecule- s -(1.1 0.2) x 10-15 <4.6 x 10-15 < 10-15 < 10-15 (1.7 f 0.3) x 5.4 x (4.7 & 0.6) x lo-" (4.0 0.6) x (1-3 0.2) x lo-" (2.8 f 0.3) x lo-" (1.2 * 0.2) x 10-11 (7.8 f 1.2) x 10-13 (1.9 0.4) x 9 7 W. J. Balfour and A. E. Douglas Cunctd. J . Phys. 1968 46 2277. 9 8 P. C . Jordan J. Chem. Phys. 1964 41 1442. 9 9 N. Basco and K. K. Yee Nature 1967 216 998.l o o A. B. Callear and R. J. Oldman Trans. Furuduy SOC. 1968 64 840. l o ' J. Connor P. J. Young and 0. P. Strausz J . Amer. Chetn. SOC. 1971 93 822 Reactions of Atoms and Small Molecules 139 Table 14 Half pressures for quenching of the emission from Sb(6s 'P2) lo' Quenching gas Xe co2 N2 H2 CH4 CzH4 CO, C2H6 Quenching half-pressurelTorr" 75 9 12 4 1 0.3 0.5 3 1 Torr = 133.3 N m-'. are not detailed on a time-resolved mode. Rather Stern-Volmer plots are con-structed leading to collisional quenching data for Sb(6s 2PJ) and Bi(7s 4PJ , 7s 2PJ) some of which are given in Tables 14 and 15. Table 15 Quenching cross-sections (02) for the collisional deactivation of Bi(7s "P+) Quenching gas 2 J A 2 H2 0 2 N2 co2 CH4 co Xe C2H6 C2H4 <0.15 110 < 3.5 46 620 << 6 < 0.5 < 0-6 330 Oxygen Sulphur Selenium and Tellurium.-Oxygen.Oxygen atom chemistry in the gas phase in general is continually under review particularly by symposia with strong interests in the chemistry of the upper atmosphere (see for example, ref. 102). Methods used for monitoring oxygen atoms spectroscopically include (a) time-resolved atomic absorption spectroscopy for ground-state atoms O(23PJ) (b) resonance fluorescence for O(23PJ) (c) the 'forbidden' emission from O(2l0,) to O(z3PJ) and ( d ) forbidden emission from 0(2lS,) to 0(2'D2). The main emphasis of this section is on the extent to which these methods bear upon the quantitative investigation of collisional rate processes.Whilst space does not permit discussion in detail of the nature and use of the relevant atomic transitions special mention may be made of the work of Kaufman et al.82 on the oscillator strengths of resonance transitions of ground-state N and 0 atoms, Symposium on Laboratory Measurements of Aeronomic Interest ed. H. I. Schiff, Canad. J . Chem. 1969 47 1703 140 R. J. Donovan and D. Husain studied in flow systems. Further the theoretical papers of Kaufman and Parke~,"~ and of Braun and Carringt~n"~ are particularly helpful in considering the procedure of experiments which involve measurement of the attenuation of resonance radiation where the finite thickness of the emission sources self absorption and resonance light scattering must be taken into account.We may only mention observations of the photoelectron spectra of H N and 0 atoms by Jonathan et a1."' 0 ( 2 3 PJ) Investigated by Time-resolved Atomic Spectroscopy in the Vacuum U.V. (a) Kinetic investigation of O(Z3PJ) in absorption by time-resolved attenuation of atomic resonance radiation. Donovan Husain and Kirsch'06" have employed time-resolved atomic absorption in the vacuum U.V. in their study of O(Z3PJ), generated by pulsed irradiation in a static system and monitored by attenuation of resonance radiation [O(33S,)+ O(Z3PJ); ;I = 130nml. Detailed rate data for the recombination of oxygen atoms with O2 and CO are reported by these authors. These data which are presented in Table 16,'06*'07 are for the most Table 16 Rate constants for the recombination of ground-state oxygen atoms with molecules studied by time-resolved atomic absorption ( A ) and time-resolved resonanceJIuorescence ( F ) at 300 K M k/cm6 molecule - s - Ref Method Reaction 0 + O2 + M Ar (5.0 f 1.2) x 10-34 106a A (4.4 0.6) 10-34 107a F He (4.9 k 2.6) 10- 3 4 106a A Kr (4.9 k 2.1) 10-34 106a A N2 (7.0 k 1.0) 10-34 107a F He (1.4 -t 0.7) x 10-35 106b A (6 & 1.5) x 107b F Ar (1.4 & 0.7) 10-35 106b A (7 4 3.5) x 10-36 107b F N2 (1.4 k 0.4) 10-35 1076 F Ar 13.5 x 10-32 107b F Reaction 0 + CO + M Reaction 0 + NO + M part in good agreement with those obtained by resonance fluorescence tech-niques (see later).In the case of the slow recombination of O+CO+M this system is so sensitive to the effect of impurity that the lower of the published values for the rate with CO are in general to be preferred.l o 3 F. Kaufman and D. A. Parkes Trans. Faraduy Soc. 1970 66 1579. I o 4 W. Braun and T . Carrington J . Quant. Spectroscopy Radiative Transfer 1969,9 1133. I o 5 N. Jonathan A. Morris D. J . Smith and K. J. Ross Chem. Phys. Letters 1970,7,497. l o 6 (a) R. J. Donovan D. Husain and L. J . Kirsch Trans. Furaduy Soc. 1970 66 2551; (b) ibid. 1971,67 375. l o ' (a) T. G. Slanger and G. Black J . Chern. Phys. 1970 53 3717; ( 6 ) ibid. p. 3722 Reactions of Atoms and Small Molecules 141 (b) O(23PJ) investigation by resOnancepuOrescence. A highly sensitive method for the study of O(23PJ) is that of time-resolved resonance fluorescence applied by Black and Slanger'07 to the study of these ground-state atoms.O(Z3PJ) was generated by the pulsed irradiation of 0 in the vacuum U.V. and the atomic fluorescence at 130 nm was monitored following excitation from an atomic emission source. Black and Slanger report third-order recombination rate data for oxygen atoms with 02 CO and NO (Table 16) and compare their results with those obtained from a number of methods.'07 In particular the value for the recombination rate of 0 + NO + Ar was greater than generally observed hitherto. ' 0 7 a Becker Groth and Jud lo8 have also reported resonance fluores-cence for O(z3PJ) but have not given quantitative kinetic data. Smith'09hasemployed theCSradicalasaspectroscopicmarker(A 'n +- X 'C') to monitor oxygen atoms generated from the photodissociation of NO, and has shown that both CS and SO radicals are formed in vibrationally excited states in the reaction : 0 ( 2 3 ~ ) + CS,(PC;) -+ so(x3z- u" Q 4) + cs(xlc+ ti' G 3) With this method Smith' l o reports absolute rate data for the addition of oxygen atoms to olefins which are in good agreement with those from earlier measure-ments.0 ( 2 ' D 2 ) . The chemistry of the metastable oxygen atom 0(2'0,) has recently been reviewed by the authors.2 The majority of the rate constants reported for this atom are derived from indirect methods. The flash photolysis of ozone mixtures combined with kinetic absorption spectroscopy in the U.V. of the species O,(u" = n) O3 itself and OH continues to be employed to obtain kinetic information on the oxygen atom. By monitoring the Schumann-Runge system of O,(B +- X 'Xi) Bair et al." ' have recently observed Oz(u" < 30), attributed to the reaction of O(2'0,) with O, the vibrationally excited molecules apparently being formed to the dissociation limit.McGrath et a/.' confirm rapid quenching of O(2'0,) by 0 to yield electronically excited oxygen mole-cules,' and they have employed isotopic labelling experiments' ' to demonstrate that vibrationally excited ground-state molecules are formed from this collisional process. McGrath and his co-workers' l4 further conclude that O(23P,) gener-ated from the long-wavelength pulsed photolysis of 03 yields 0 2 ( u " < 16) following reaction with the parent molecule. By monitoring OH(A 'C+ +- X ,n), Bair et ~ 1 . " ~ report rapid reaction for 0(2'0,)+H20 ( k = 3.1 x lo-" cm3 molecule-' s-I) yielding OH(u" < 1) which quickly relaxes to OH (0'' = 0).l o ' K. H . Becker W. Groth and W. Jud Z . Naturforsch. 1969 24a 1953. j o q I. W. M. Smith Discuss. Faraday Soc. 1967 44 194; Trans. Faraday Soc. 1968 64, ' l o I . W. M. Smith Trans. Faraday Soc. 1968 64 378. ' ' I V. D. Baiamonte L. G . Hartshorn and E. J. Bair J . Chem. Phys. 1971 55 3617. ' I 3 W. D. McGrath and D. W. McCullough Chern. Phys. Letters 1971 8 353. 3183. K. F. Langley and W. D. McGrath Planet. Space Sci. 1971 19 416. D. M. Ellis J. J. McGarvey and W. D. McGrath Nature 1971,229 153. D. Biedenkapp L. G . Hartshorn and E. J. Bair Chem. Phys. Letters 1970 5 379 142 R. J. Donovan and D. Husuin Similar peasurements have led McGrath and Langley116 to conclude that OH(v" < 3) will not carry a chain reaction on the photolysis of 03-H20 mixtures.Donovan et have employed OH(A 2C+ t X ,II) as a spectroscopic marker following hydrogen-atom-abstraction reactions by O(2'0,) and report relative quenching rate data for this excited atom which are in accord with earlier measurements.2 Although the Einstein coefficient for emission from O(2'0,) to the ground state is very low"* [0(2'02)+ O(23PJ)+hv (630nm); A = 6 . 9 ~ 10-3s-1] Kvite and Vegard"' succeeded in observing this emission in a pumped discharge. Noxon'20 has observed the emission in a static system following the photolysis of CO and reports quenching rate data (Table 17). These data are in sensible Table 17 Rate constants for the collisional deactivation ofO(2' D2)l 2o Quenching gas k/cm3 molecule- s -N2 (9 -t 4) x lo-" 0 2 (6 & 3 ) x lo-" co < 5 x lo-" CO (0.3 & 0.1) x lo-" agreement with those obtained by various other measurements,2 apart from the case of C 0 2 which is clearly too slow.'2o In the opinion of the authors the most fruitful direction for the extension of kinetic spectroscopic measurements on O(2 ' D ) would be towards time-resolved atomic absorption or resonance fluorescence in the vacuum U.V.involving allowed transitions coupled with signal averaging since pulse-counting methods for emission from O(2l0,) involve extremely low pulse counts.'2' 0(2'S0). Kinetic studies that have been reported for 0(2'S0) have all employed the forbidden transition 0(2'S0) -+ O(2'0,) (A = 577.7 nm) in emission, characterized by an Einstein A coefficient"8 (A,) of 1-28s-l.This aspect of the oxygen atom has been reviewed recently.2 Table 18122-'30 lists the main body of rate data for 0(2'S0) the majority of these investigations having been 1 1 6 K. F. Langley and W. D. McGrath Planet. Space Sci. 1971 19,413. 11' R. J. Donovan D. Husain and L. J. Kirsch Chem. Phys. Letters 1970 6 488. 'I8 R. H. Garstang Monthly Notices Roy. Astron. SOC. 1951 111 115. G. Kvite and L. Vegard GeoJvs. Publ. 1947 17 3. J. Noxon Canad. J . Chem. 1969 47 1873; personal communication mentioned in J . Chern. Phys. 1970 52 1852. R. Gilpin H. I. Schiff and K. H. Welge J . Chern. Phys. 1971 55 1087. S. V. Filseth F. Stuhl and K. H . Welge J . Chern. Phys. 1970 52 239. R. A. Young and G. Black J .Chem. Phys. 1966,44 3741. 1 2 5 R. A. Young G. Black and T. G. Slanger J . Chern. Phys. 1969 50 309. 1 2 ' F. Stuhl and K. H. Welge Canad. J . Chern. 1969 47 1870. R. A. Young G. Black and T. G.'Slanger J . Chern. Phys. 1968 48 2067; 49 4769. 1 2 8 G. Black T. G. Slanger G. A. St. John and R. A. Young Canad. J . Chern. 1969,47, 1872. 1 2 9 S. V. Filseth and K. H. Welge J . Chern. Phys. 1969 51 839. 13' E. C. Zipf Canad. J . Chern. 1969 47 1863. 12' E. C . Zipf Bull. Amer. Phys. SOC. 1967 12 225 Reactions of Atoms and Small Molecules 143 Table 18 Rate constants at 300 K for the collisional deactivation of 0(2lS,) by various gases Quenching gas k/cm3 molecule- s - Ref: 0 N2 co H2 NH3 NO C2H6 C2H4 (2.1 i 0.4) 10-13 1 10-13 3.2 x 10-13 5 x 10-13 3.7 x 10-13 1.8 10-13 < 10-17 < 5.9 10-17 < 5 10-17 < 3 x 10-15 3.6 x 10-13 3.18 x 10-13 2.5 10-14 2.6 x 10 - 1 4 4.6 x 10-13 3 x 10-13 4.9 x 10-15 6.1 x 10-15 3.6 x 1.4 1-7 x < 2 x 10-'6 < 10-16 9.4 x 10-14 4.9 x 10-14 -4 x 10-10 7 x lo-" 2.8 x 3.0 x 10-15 2.5,3.4 10-15 1.1 x 10-15 I x 10-15 5 x 10-l0 8.0 x lo-" 3.5 x lo-" 4 x 10-'O 5.5 x 10-'O 4.7 x 10-14 3.5 x 1.0 x 10-12 9.6 x -8.9 x lo-" 1.1 x 10-11 1.5 x lo-" 1.6 x lo-" 5.9 x 10-13 122 123 124 124 125 126 127,128 129 124 123 124 124,125 126 128 127 123 130 124 125 126 128 123 124,125 126 123 126 123 123 124 124,125 126 127 128 123 123 124,125 128 127 123 124 125 123 123 124 125 123 124 126 127,12 144 R.J . Donovan and D. Husain Table 18 (continued) Quenching gas k/cm3 molecule-' s - R eJ Ne Ar Kr Xe 5.0 x lo-'' -5.9 x lo-" 1.2 x lo-" 2 x 10-16 <5.9 x 10-17 < 3 x 10-15 < 10-17 < 10-17 (5.9 x 10-17 < 3 x 10-15 3.9 x 10-16 3.9 x 5.0 x 6.7 x 123 124,125 124,125 123 124,125 128 123 124 123 124 124,125 128 123 123 carried out by Welge and c o - w ~ r k e r s ' ~ ~ ~ ' ~ ~ ~ ' ~ ~ and by Black et u1.1249125,127,128 Very recently Welge et al.13' have studied the temperature dependence of the collisional quenching of 0(2'S0) generated in the pulsed photolytic mode, and report the rate constant for deactivation by C02 k = 5 & 2 x lo-" exp [(-2700+400cal)/RT] cm3 molecule-' s-'.Hamson and Okabe'32 have studied the collisional stimulation of the atomic emission from 0(2'S0) and report (Table 19) relative efficiencies of the gases M in the process: M + 0(2'S0) -P 0(2'D2) + M + hv(577-7nm) Table 19 Relative eflciencies ( E ) for collisional stimulation' 32 of the emission 0(2'S0) -P O(2'0,) + hv Gas E Xe 40 Kr 2.9 Ar 1.7 N2 1 H2 0.39 He 0-05 Sulphur. Donovan et a!.' 3 3 have employed atomic absorption spectroscopy in the vacuum U.V. for a quantitative kinetic investigation of the reactions of ground-state sulphur atoms S(33PJ);'34 generated flash photolytically. Absolute rate ' ' I K. H. Welge A. Zia E. Vietzke and S. V. Filseth Chein. Phys. Letters 1971 10 13. 1 3 ' R.E. Harnson jun. and H. Okabe J . Chein. Phys. 1970,52 1930. 1 3 3 R. J. Donovan D. Husain R. W. Fair 0. P. Strausz and H. E. Gunning Trans. Furaday Soc. 1970 66 1635. 1 3 4 A. B. Callear Proc. Roy. Soc. 1963 A276 401 Reactions of Atoms and Small Molecules 145 constants for the processes: s + ~ 2 ~ 4 -+ c2n4s S + C2H4S -+ C2H4 + S2 of k = (1.2f0.15)~ and (3-0+0.7)x lo-" cm3 molecule-'^-^ respec-tively at 300K are reported. Connor et have extended these measure-ments for sulphur-atom addition to other olefins and have obtained the values for the second-order rate constants (k/cm3 molecule- ' s-' ; at 298 K) shown in Table 20. Values relative to that for C2H4 are in accord with previous results from conventional photochemical measurements. Table 20 298 K Second-order rate constants for addition of sulphur atoms to olejins at Olefin k x 10' '/cm3 molecule- ' s- ' Ethylene 0.1 5 f 0.22 Propylene 1.0+ 0.2 But -1 -ene 1.5 * 0.2 trans-But-2-ene 2.0 & 0.3 Isobutene 6.0 0.8 Tetramethylethylene 10.0 * 1.3 The electronically excited metastable state S(3 'D,) has not yet been observed directly.Indirect methods include monitoring transient S2(a 'Ag) following the reaction of S(3'D2) with OCS'36 and monitoring NS resulting from the reaction of this atom with N20.'37 The more energized and indeed more metastable atom S(3'S0) may be readily followed by atomic absorption spectroscopy in the vacuum U.V. following pulsed irradiation of OCS in the Schumann region and rate data for collisional quenching are reported'38 (Table 21).An interesting Table 21 Rate constants for the collisional quenching at 300 K of S(3lSO) 136b,138 Quenching gas k/cm3 molecule- ' s -ocs (1.0 f 0.2) x 10- ' H2 (4.0 1.0) x 1 0 - 1 5 Xe < 10-13 Ar < 5 x 10-lS He < 1.3 10-15 general discussion employing these data is that on the quenching of the 'D and ' S states of Group VI atoms in terms of curve crossing using the semi-empirical potential-energy diagram given for Xe0.136b Relaxation of the O(2'0,) state J. A. Connor A. Van Roodselaar R. W. Fair and 0. P. Strausz J . Anier. Chetn. Soc., 1971 93 560. ' 3 6 ( a ) R. J. Donovan D. Husain and L. J . Kirsch Nature 1969 222 1164; ( h ) Trans. Faraday Suc. f970 66 774. 1 3 ' W. H. Breckenridge and R. J. Donovan Chetn. Phys. Letters 1971 11 520.1 3 ' R. J . Donovan Trans. Faraday Soc. 1969 65 1419 Table 22 Rate constants for the spin-orbit relaxation of Se(43P,) on collision Quenching gas Ar N2O co2 H2 co N2 0 2 Proposed quenching process Se(43Po) + Ar -+ Se(43P,) + Ar Se(43Po) + N,O ( v 2 = 0) -+ Se(43P,) + N 2 0 (v, Se(43Po) + C 0 2 ( v 2 = 0) -+ ~ e ( 4 ~ ~ ) + CO,(V, Se('l3P0) + H,(J) -+ Se(43P,) + H,(J') se(43P0) + CO (u" = 0) -+ Se('I3P2) + CO (u" = se('l3PO) + N (u" = 0) -+ Se(43P,) + N (u" = se(43P0) + O,(u" = 0) -+ Se(43P,) + 0 (u" Reactions of Atoms and Small Molecules 147 with Xe is shown to be facilitated by the crossing of potential curves for XeO which possess the same symmetry in the Hund's coupling case (c). Very recently, Davis Klemm and Pilling' 39 have described the application' of time-resolved resonance fluorescence to the ground-state sulphur atom S(33PJ) and have reported detailed kinetic rate data for this atom in the presence of molecular oxygen.In general the data for the reactions of S(33PJ 31D2 3lSO) reported from various techniques2 satisfy the predictions of the correlation diagrams.2 Selenium. Callear and Tyerman 140a,' report the results of detailed kinetic absorption studies on selenium atoms following the flash photolysis of CSe,. A non-Boltzmann distribution in the J levels of the 43P2,,, ground state is initially p r o d ~ c e d ' ~ ~ ~ ~ ' (J = 1 1990cmP1; J = 0 2534cm-') and spin-orbit relaxation on collision with various partners has been investigated by these authors the results of which studies are shown in Table 22.140c Further sufficient N may be added to ensure the maintenance of a Boltzmann distribution between the J states for a kinetic study of the addition of Se(43PJ) to 0lefins.'~~~7~ The resulting rate data are presented in Arrhenius form [k = A exp ( - E/RT)] in Table 23.The resulting activation energies may be correlated with the ionization potentials of the olefin indicating polarization in the transition Table 23 Arrhenius rate parameters for the addition of Se(43PJ) to 01eJins'~~" Olefin Ethylene Propylene But- 1-ene cis-But-2-ene trans-Bu t-2-ene Isobutene Buta- 1,3-diene Pent- 1-ene Vinyl chloride Acrylonitrile 10" &m3 molecule-'^-^ 1.8 f 0.6 2.15 f 0.7 5-15 f 1.7 3.3 f 1-4 2.9 & 1.3 3.95 1-7 8-8 _+ 3.8 -4 1.3 & 0.7 'v 0.2 Elcal mol-2810 & 200 2350 f 230 2250 f 230 1210 f 270 590 f 270 1010 f 250 880 f 250 2210 f 440 2440 280 720 f 800 Tellurium.There are limited quantitative kinetic data on tellurium atoms, obtained by spectroscopic methods. OsborneI4 observed a non-Boltzmann distribution in Te(53P2. l,o) following the flash photolysis of hydrogen telluride, together with Te(S'D,) by atomic absorption at A = 277-1 nm (6s 3 S , +- 5p4 IDz). Unfortunately collisional relaxation of the 'D state was not investigated. Connor Greig and S t r a ~ s z ' ~ ~ have also observed Te(53PJ) by atomic absorption in the U.V. following pulsed irradiation of TeMe and H,Te, 1 3 9 D. D. Davis R. M. Klernrn and M. J. Pilling 1972 in the press.I4O ( a ) A. B. Callear and W. J. R. Tyerrnan Nature 1964 202 1326; ( b ) Trans. Faraday SOC. 1965 61 2395; (c) ibid. 1966 62 2313; ( d ) ibid. p. 371 ; ( e ) ibid. p. 2760. I 4 l M. J . Osborne Thesis University of Cambridge 1962. 1 4 2 J. Connor G. Greig and 0. P. Strausz J. Amer. Chem. Soc. 1969 91 5695 148 R. J. Donovan and D. Husain and they report rate constants at 300 K for the processes : Te + (CH,),Te -* Te + 2CH or C2H6 Te + C2H4 -+ C2H4Te of 2.8 x 10- l o and 4 x have extended these measurements to propylene and tetramethylethylene (TME) and report the rate constants at 298 K shown in Table 24. These authors also cm3 molecule-' s- ' respectively. Connor et Table 24 Rate constants for addition of Te(5 'PJ) to olefins OleJin k/cm3 molecule- ' s - ' Ethylene (2-2 & 0.5) x Propylene TME (6.5 1.3) x (2.0 f 0.5) x lo-', report rate constants for these processes at 353 K the addition to TME indicating a small negative activation energy ( - 1.6 kcal mol-').Connor et further report negative activation energies for oxygen- and sulphur-atom addition to TME. This is a novel type of observation and will clearly need to be studied in further detail before its importance can be fully assessed. Fluorine Chlorine Bromine and Iodine.-Extensive advances have been made in recent years in the direct kinetic study of halogen atoms by spectroscopic methods, particularly those in the higher-energy spin orbit state of the ground-state electronic configuration X(np5 'P+). This area has been reviewed in detail by the author^.^.^' A number of methods have been developed for the kinetic study of the optically metastable 'P+ atoms of which time-resolved atomic absorption spectroscopy has yielded the main body of data.These methods comprise principally : (a) Time-resolved absorption spectroscopy in the U.V. and the vacuum-u.v. following flash-photolytic initiation. (b) Photoelectric measurement of time-resolved atomic absorption spectros-copy by attenuation of atomic resonance radiation following pulsed photolytic initiation. (c) Atomic absorption spectroscopy on flow systems. (d) Time-resolved atomic emission in the i.r. following flash-photolytic (e) Atomic emission in the i.r. from a flow system. ( f ) Time-resolved resonance fluorescence. Tables 25-29' 43-' 6 2 and 30 31 list the kinetic data for the halogen atoms, obtained primarily by spectroscopic methods.Those derived by method ( d ) lie initiation. I J 3 R. J. Donovan and D. Husain Trans. Furuduy Soc. 1966 62 1050. ' 4 4 ( u ) D. Husain and J . R. Wiesenfeld Narure 1967 213 1227; (6) Trans. Faruduy Soc., 1967 63 1349. 1 4 5 ( a ) R. J. Donovan and D. Husain Tram. Furadcry Soc. 1966 62 1 I ; ( h ) ibid. p. 2023 Reactions of Atoms and Small Molecules 149 Table 25 iodine atoms I(5Pt) at 300 K Rate constants for the collisional deactivation of electronically excited Quenching species He Ar Xe 1(5'P+) N2 co N2O CF31 CH31 C2HJ n-C3H,I i-C3H71 n-C4H,I HI t-C,H,I DI HCl D2 k/cm3 molecule - s - ' < 5 x < 2 x lo-'* < 1.6 x lo-'" 2.1 x lo-', < 1.6 x lo-'' 1-5 x 6.5 x 10-1' 1.2 x 10-15 1.2 x 10-lS 1.7 x 1.3 x 3.1 x 10-15 2.4 10-17 4.5 x 4.6 x 1.3 x lo-'' 3.5 x 10-16 1.7 x lo-'' -3.7 x -1.9 10-13 2.0 x 10-13 2.0 x 1 0 - 1 3 3.8 x 10-i3 1.3 x 10-i3 1.2 x 10-l3 1.4 x 10-l4 1.1 x 10-13 2.2 x 10-is 1.0 x 10-15 3.2 x 2.9 x 1.5 x Ref: 143 144,145a 145b 3a 145a 145a 144b 146 145b 144b 145b 144b 145b 144b 145b 144b 147 144b 143 148,149 149 150 149 149 149 149 143 151 143 146 143 144b 146 1 4 6 J.J. Deakin and D. Husain J.C.S. Faraduy I f 1972 68 41. 14' 14' R. J . Donovan F. G. M . Hathorn and D. Husain J . Chem. Phys. 1968 49 953. I J 9 R. J. Donovan F. G . M. Hathorn and D.Husain Trans. Furaduy Soc. 1968,64 3192. Is' R. J . Donovan and D. Husain Nature 1966 209 609. I S P. Cadman J . C. Polanyi and I . W. M. Smith J . Chirn. phys. 1967 64 11 1 . I s ' P. Cadman and J. C. Polanyi J . Phys. Chern. 1968 72 3715. R. J. Donovan D. Husain and C. D. Stevenson Trans. Faraduy Soc. 1969 65 2941. I S 4 J . J . Deakin D. Husain and J . R. Wiesenfeld Chem. Phys. Lerters 1971 10 146. R. J. Donovan F. G . M . Hathorn and D. Husain Trans. Furaday Soc. 1967,64 1228. 1 5 6 R. J. Donovan and D. Husain Nature 1965 206 171. M . I. Christie R. S. Roy and B. A. Thrush Trans. Faraday Soc. 1959 55 1149. l S x R. J . Donovan and D. Husain Truns. Furaduy Soc. 1966 62 2987. R. J . Donovan and D. Husain Truns. Faraduy Soc. 1966,62,2643. 160 R. J .Donovan and D. Husain Trans. Furaday Soc. 1968 64 2325. 16' R. J. Donovan D. Husain A. M. Bass W. Braun and D. D. Davis J . Chern. Phys., 1969 50 41 15. 16' D. D. Davis W. Braun and A. M. Bass Internur. J . Chern. Kinetics 1970 2 101. F. G. M . Hathorn and D. Husain Trans. Faraduy SOL. 1969 65 2678 150 R . J. Donovan and D . Husain Table 25 (continued) Quenching species H2 CH4 n-C4H 10 (CHd3CH CH,=CH -CH,Br CH2=CH, CH,=CH -CH,CI CH,=CH-CH,I CF,=CFH CF,=CF2 CH3-CH=CH2 But-1 -ene trans-But-2-ene cis-But-2-ene Isobutene Tetramethylethylene CZH2 D2O H2O ICN 0, NO k/cm3 molecule- ' s -8.8 x 2.15 x 10-13 1.3 x 10-13 5.9 x 10-14 LO x 10-13 1.1 x 10-13 4.6 x 10-14 1.0 x 10-13 5.7 x 10-l4 1.7 x 10-13 1.9 x 10-13 4.2 x 10-13 1.3 x 10-13 2.4 x 10-13 2.1 x 10-13 5.3 10-14 3.7 x 10-15 1.6 x 10-13 3.4 x 10-13 1.6 x 10-13 3.6 x 2.1 x 10-13 2.2 x 10-13 3.0 x 10-i3 1.1 x 10-l2 6.3 x 10-13 3.1 x 10-14 9.4 10-13 7.2 x 10-13 6.0 x 10-14 2.7 x 4.2 x 7.0 x 6.2 x 9.3 x lo-', 2.6 x lo-" 1.1 x lo-" 1.6 x lo-'' 1 Ref: 143 152 146 1456 144b 146 1456 146 145a 1446 146 146 146 147 147 147 153 146 153 153 153 146 153 146 153 153 153 146 153 146 1456 145b 146 147 145b 154 1456 146 outside our immediate context but are included for comparison because they involve optical spectroscopy.Experiments on atomic recombination employing the molecular spectra of the halogens particularly bromine in the flow discharge system the shock tube and the flash photolysis arrangement continue unabated.A recent example of this is the detailed work of Clyne et on emission from Br,(311,f,) derived from two Br(42Pt) atoms in a flow discharge system demon-strating recombination into the u' = 5 level higher vibrational levels being 1 6 3 (a) M . A. A. Clyne J. A. Coxon and A. R. Woon-Fat Trans. Furuduy Soc. 1971, 67 3155; ( 6 ) A. G. Clarke and G . Burns J . Chern. Phys. 1971 55 417; (c) K. L. Kornpa J. H. Parker and G. C. Pirnentel J . Chem. Phys. 1968,49,4257; J . H. Parker and G. C. Pimentel ibid. 1968 48 5273 Reactions of Atoms and Small Molecules 151 Table 26 iodine atoms I(52P+) at 300 K Rate constants for the chemical reactions of electronicalIy excited Reactant species k/cm3 molecule- s -Br2 1.5 x ICl 3.4 x 10-12 I Br 4.3 x 10-12 I 2 5.0 x c12 2.1 10-13 [Compare the values for the reactions : I(52P+) + C12 6.1 x I(5'Pi) + Br (2.7 x I(52P+) + IBr - 10-17 CH31 < 1.7 x 10-15 NOCl 6-2 x NOBr 9.6 x Ref: 155 155 155 155 156 157 155 1551 147 147 149 thermally populated.The paper by Clarke and on trajectory calcula-tions is a useful brief review of recent work on bromine atom recombination by shock-tube and flash photolysis experiments. Stimulated emission in the i.r. from vibrationally excited HF has been employed by Pimentel and his co-w o r k e r ~ ' ~ ~ ' to determine relative rates of formation of given vibrational levels from the reactions of fluorine atoms generated by flash photolysing UF6 with hydrogen-containing molecules.The stimulated emission effectively 'freezes out' the initial energy distribution without the loss associated with the longer lifetime for spontaneous emission. Davis Braun and Bass'62 have applied the method of time-resolved resonance fluorescence" to the detailed kinetic study of C1(32P,:) (Table 29). The excellent agreement between Braun's data for the reaction of C1(32P,) with molecular Table 27 Rate constants for the collisional removal of electronically excited bromine atoms Br(42P,) at 300 K Quenching species k/cm3 molecule- s - Ref: Ar N2 co CF4 CF,Br HBr D2 H2 CH4 D2O H 2 0 Br, 0 2 NO IBr < 2 x 10-16 2.5 x 10-15 7.3 x 10-15 2.1 x 10-13 5.0 x 1.1 x 10-12 5.7 x 10-12 4.7 x 1 0 - 1 2 4.2 x 9.6 x 3.2 x lo-" 1.9 x lo-" -3.4 x lo-" 4.7 x lo-" 3 x 10-12 158 158 158 158 159 159 158 158 158 158 158 159 159 159 16 152 R.J. Donovan and D. Husain Table 28 chlorine atoms C1(32P,) at 300 K Rate constants for the collisional removal of electronically excited Quenching species Ar H CF3 C1 CC14 HCl H2 ICl k/cm3 molecule- s-< 2 x 10-16 - 7 x 10-'O 2.5 x 5 x lo-" 6 x 7 x 10-l2 3 x lo-" Ref: 161 161 161 161 161 161 160 hydrogen and those of previous investigations may be considered to establish this method because the rate of this particular chemical process effectively constitutes a kinetic standard. The rate constants for the hydrocarbons (Table 29) indicate that previous absolute data for the reaction of C1 atoms with these Table 29 Rate constants for the reaction' of C1(32Pt) with various molecules at 298 K from time-resolved resonance fluorescence experiments (A = 133.58 nm)162 Reactant gas k/cm3 molecule- s -H2 (1.4 _+ 0.1) 10-14 CH4 (1.5 f 0.1) x 10-13 CH2C12 (5.5 0.5) x 10-13 Cyclo-C,jHl (2.0 f 0.2) x 10-'O C2CL (5.3 0.5) x C2H6 (6.7 & 0.7) x lo-" ' I All are atomic abstraction reactions with the exception of C1 + C,CI,.molecules should be revised by a factor of two. Brewer and Tellingh~isen'~~~ have reported the observation of I(52Pt) by resonance fluorescence and have employed the technique to examine diffusion and recombination of iodine atoms, particularly on the vessel walls.Donovan et al. 164b have demonstrated electronic-vibrational energy transfer for the process : Br(42P,) + HBr(v" = 0) -+ Br(42P,) + HBr(v" = 1) by atomic and molecular kinetic spectroscopy. Clyne and his c o - ~ o r k e r s ~ ~ ~ * ~ have carried out detailed work on atomic resonance absorption in the vacuum U.V. on the atoms C1 Br and I calibrating the degree of absorption against the stoicheiometry of rapid atomic reactions. These investigations clearly form the basis for future rate determinations of reactions of ground-state halogen atoms. Of very recent work on I(52P+) an example which may be mentioned is the detailed kinetic measurements employing attenuation of atomic resonance radiation in the u.v. by Deakin Husain and Wie~enfeld.'~~.' 54 Barile and 64 ( a ) L.Brewer and J. B. Tellinghuisen J . Chrrn. Phys. 197 I 54,5 I33 ; ( b ) R. J. Donovan, 16' l b 7 C. A. Barile and R. B. Solo A.C.S. Abstract Div. Phys. Chem. No. 44 1969. D. Husain and C. D. Stevenson Trans. Faruday SOC. 1970,66 2148. M . A. A. Clyne and H. W. Cruse Trans. Furaduy Soc. 1971 67 2869. M. A. A. Clyne H. W. Cruse and R. T. Watson J . C.S. Faruduy I I 1972 68 153 Table 30 iodine atoms 1(52P+) in noble gases Experimentally determined digusion coejicients D &rn2 s- at 1 H e Ref N e Ref Ar R ef: 0.95 0.09 168 0.40 & 0.03 168 0.36 & 0.06 168 0.11 1.1 3a 143 0.212 (calc.) 169 0.25 k 0.02 154 0.064 0.548 (calc.) 169 0-41 k 0.05 144b 0.27 3a 145a 0.60 151,152 0.108 (calc.) 16 154 R. J. Donovan and D. Husain Table 31 Experimentally determined mean radiative lifetime of electronically excited iodine atoms I(52P,) %/S 0.028 f 0.016 0.02 & 0.005 0-024*05 0-045 > 0.017 0.17 0.04 0.13 (calc.) 0.11 (calc.) Ref: 168 154 145a 144b 167 172 170 171 report briefly the application of this technique to the study of the excited iodine atom but have not given detailed rate data.Abrahamson et a1.'68 describe further work on time-resolved i.r. emission from I(52P& reporting values for the diffusion coefficients of I(52P+) in noble gases and the mean radiative lifetime. There remains the difference between the mean radiative lifetime of J(52P+) measured by kinetic methods (Table 31),'67-'72 including the highly sensitive method of attenuation of resonance radiation,'46>' 5 4 and that measured with what is essentially an 'equilibrium' method of Derwent and T h r ~ s h ." ~ The kinetic methods which are in sensible agreement (Tables 30 31) may yield values of z which are too high on account of the contribution by non-cavity-assisted stimulation emission because these systems generally involve a popula-tion inversion between 1(52P,) and I(S2P3). On this basis however the agree-ment is surprising as the concentration of I(52P+) employed in time-resolved emission measurements'68 is some lo3 times greater than that employed in the resonance absorption experiments. 146,1 5 4 The calculated values for z should be relatively reliable as they do not depend strongly on the radial part of the wavefun~tion.~~~,' ' A general theory permitting direct comparison with the now large body of data on the spin-orbit relaxation of the atoms in Group VII is still needed.Andreev and Nikitin' 73 have presented a theory for energy transfer from I(52P+) to N and CO involving weak dipole and quadrupole interactions and employing a L a n d a ~ - Z e n e r ~ ~ - ~ ~ formalism. The theory is numerical in its development, and sensible agreement between the theory which involves vibrational excitation, and experiment is observed. Fluorine atoms have not as yet been studied by electronic absorption spectroscopy as the topical transitions lie at wavelengths less than the low-wavelength limit of transmission of lithium fluoride the material generally used to contain the reaction system during vacuum-u.v.investigations. 16' E. W. Abrahamson L. J . Andrew D. Husain and J . R. Wiesenfeld 3 . C.S. Faruday If, 1972 68 48. 169 J . 0. Hirschfelder C. F. Curtis and R. B. Bird 'Molecular Theory of Gases and Liquids' Wiley New York 1954. " O R. H. Garstang J . Res. Nut. Bur. Stand. Sect. A. 1964 68 61. ' ' I D. E. O'Brien and J . R. Bowen J . Appl. Phys. 1969 40 4767. ' 7 2 R. G . Derwent and B. A. Thrush Chem. Phys. Letters 1970 6 115; ibid. 1971,9,591. E. A. Andreev and E. E. Nikitin Theor. Chim. Acta 1970 17 171 Reactions of Atoms and Small Molecules 155 3 Diatomic Molecules Table 32175-253 lists those diatomic molecules which have recently been observed by emission or absorption spectroscopy in the visible or U.V. regions under conditions which are suitable for kinetic studies.We have not included species observed in electric discharges or those observed under ill-defined conditions. In a few cases only the spectra have been reported ; however in principle kinetic studies are possible and the spectra of these transient diatomic species are included to give as complete a coverage as possible. l T J J. I . Steinfeld Accounts Chern. Res. 1970 3 313. K. D. Beyer and K. H. Welge Z . Naturforsch 1967 22a 1161. D. L. Akins E. H. Fink and C. B. Moore J . Chern. Phys. 1970 52 1604. (a) Ch. Ottinger and W. Poppe Chern. Phys. Letters? 1971 8 513; (b) Ch. Ottinger, R. Velasco and R. N. Zare J . Chem. Phys. 1970 52 1636. (a) J. W. C. Johns F. A. Grimm and R. F. Porter J . Mof. Spectroscopy 1967,22,435; ( h ) G.Herzberg and J. W. C. Johns Proc. Roy. Soc. 1967 A298 142. l i 7 A. B. Callear and R. E. M. Hedges Trans. Faraday Soc. 1970 66 2921. I n " W. Braun J. R. McNesby and A. M. Bass J . Chern. Phys. 1967 46 2071. 18' (a) G . E. Bullock and R. Cooper Trans. Faraday Soc. 1971 67 3258; (b) B. L. Lutz, l n 2 J. C. Boden and B. A. Thrush Proc. Roy. SOC. 1968 A305 107. 1 * 3 18' 18' T. G. Slanger and G. Black J . Chern. Phys. 1971 55 2164. I n -1 8 9 J. P. Simons and A. J. Yarwood Trans. Faraday SOC. 1963,59 90. 1 9 " W. J. R. Tyerman Trans. Faraday Soc. 1969 65 2948. R. N. Dixon and H. W. Kroto Trans. Faraday Soc. 1963 59 1484. lY2 L. J . Stief V. J. Ce Carlo and R. J. Mataboni J . Chetn. Phys. 1967 46 592. I y 3 H. Okabe J . Chein. Phys. 1970 53 3507. I')' N. Basco and K.K. Yee Chern. Coinin. 1968 15. H. Okabe and M. Lenzi J . Chetn. Phys. 1967 47 5241 ; P. Ballmark 1. Kopp and R. Rydh J . Mol. Spectroscop.v 1970 34 487. J. A. Meyer D. H. Klasterbaer and D. W. Setser J . Chem. Phys. 1971 54 2084. 1 4 ' J. Billingsley and A. B. Callear Trans. Faraday Soc. 1971 67 257. I " ' L. A. Melton and W. Klemperer J . Chern. Phys. 1971 55 1468. I Y y A. B. Callear and M. J. Pilling Trans. Faraday SOC. 1970 66 1618 1886. lo" .4. G. Briggs and R. G. W. Norrish Proc. Roy. Soc. 1964 A278 27. ' " I 2 0 2 2 0 3 '04 D. G. Horne and R. G. W. Norrish Nature 1967 215 1373. ' 0 5 2"6 D. Kley and K. H. Welge J . Chem. Phys. 1968 49 2870. '07 D. W. McCullough and W. D. McGrath Chem. Phys. Letters 1971 8 353. R. J. Donovan L. J. Kirsch and D.Husain Chem. Phys. Letters 1970 7 453. 2 0 9 M. Ogawa J . Chem. Phys. 1970 53 3754. ' l o D. Kearns Chem. Rev. 1971 71 395 and references therein. *' R. P. Wayne Adv. Photochem. 1969 7 31 1. ' I 2 R. G. Derwent and B. A. Thrush Trans. Faraday SOC. 1971 67 2036. 2 1 3 W. Demtroder M. McClintock and R. N. Zare J . Chem. Phys. 1969,51 5495. 'I4 G. A. Oldershaw and K. Robinson J . Mol. Spectroscopy 1971,38 306. 'I5 G. A. Oldershaw and K. Robinson Trans. Faraday SOC. 1968,64 2256. ' I 6 N. Basco and K. K. Yee Spectroscopy Letters 1968 1 17; R. N. Dixon and H. M. "' Canad. J . Phys. 1970 48 1192. R. J. Donovan and D. Husain Trans. Faraday Soc. 1967,63,2879. R. A. Young and G. V. Volkenburgh J. Chem. Phys. 1971,55,2990. G. M. Lawrence Chern. Phys. Letters 1971 9 575.C. Morley and I. W. M. Smith Trans. Faraday Soc. 1971 67 2575. R. J. Donovan D. Husain and C. D. Stevenson Trans. Faraday Soc. 1970 66 1. . 1 U b N. R. Greiner J . Chem. Phys. 1970 53 1070. N. R. Greiner J . Chem. Phys. 1970 53 1285. N. R. Greiner. J . Chenz. Phys. 1970 53 1284 and references therein. M. Kaneko Y. Mori and I. Tanaka J . Chern. Phys. 1968,48,4468. Lamberton J . Mol. Spectroscopy 1968 25 12. N. Basco and K. K. Yee Chem. Comm. 1967 1255 156 R. J. Donovan and D. Husain Table 32 Diatomic molecules which have recently been observed Molecule HZ HD Hez(a3Z ) Liz BH CH CN co (u" = 1) System observed B ' Z -+ x1c: B T I -+ X ' C ; B'C+ + x'z+ e3n +- a3Z: A'n +- X ' C + CZC+ +- xzn BZZ- -+ XzII A2rI -+ X Z C + B2C+ - X 2 C + E2C+ +- XZC+ A'n + X'Z+ a3n -+ X ~ Z + Re$ 175 176 177 178 179 180 l m lm 58,181,182 40a 183 184-1 86 2 ' 8 N.Basco and K . K. Yee Nature 1967 216 998. ' 1 9 N. Basco and K . K . Yee Chern. Cornin. 1967 1146. 2 2 0 0. P. Strausz R. J. Donovan and M. de Sorgo Ber. Bunsengesellschaft phys. Chern., 2 2 1 2 2 2 R. J. Donovan D. Husain and P. T . Jackson Trans. Faraday Soc. 1969 65 2930. 2 2 3 R. J. Donovan and D. J. Little Spectroscopy Letters 1971 4 213. 2 2 4 R. Colin Canud. J. Phys. 1969 47 979. 2 2 5 R. W. Fair and B. A. Thrush Trans. Furuduy Soc. 1969 65 1557; M. Elbanowski, 2 2 6 R. J. Donovan D. Husain and P. T. Jackson Trans. Furaday Soc. 1968 64 1798. '" M. A. A. Clyne and H. W. Cruse Trans. Furaday Soc. 1970 66 2214.2 2 8 E. D. Morris J. Van den Bogaerde and H. S. Johnston J. Amer. Chem. Soc. 1969, 2 2 9 N. Basco and S. K. Dogra Proc. Roy. Soc. 1971 A323 29 401. 2 3 0 A. G. Briggs and R. G. W. Norrish Proc. Roy. Soc. 1963 A276 51. 2 3 1 W. J. Tango and R. N. Zare J. Chem. Phys. 1970 53 3094. 232 R. K. Gosavi G . Greig P. J. Young and 0. P. Strausz J . Chem. Phys. 1971,54 983. 2 3 3 G. A. Oldershaw and K. Robinson Trans. Furuduy Soc. 1971 67 2499. 2 3 4 G. A. Oldershaw and K . Robinson Trans. Furaday Soc. 1970 66 532. 2 3 5 R. J. Donovan and P. M . Strachan Trans. Furuduy Soc. 1971 67 3407. 2 3 6 B. Lindgren J . Mol. Spectroscopy 1968 28 536. 2 3 7 G. A. Oldershaw and K. Robinson Trans. Furaday Soc. 1971 67 907. 2 3 8 M. A. A. Clyne and H. W. Cruse Trans. Faraday Soc.1970 66 2227. 2 3 9 N. Basco and S. K. Dogra Proc. Roy. SOC. 1971 A323 417. 240 G. A. Oldershaw and K. Robinson J. Mol. Spectroscopy 1968 32 469. 2 4 1 2 4 2 P. Ballmark and B. Lindgren Chem. Phys. Letters 1967 I 480; (6) N. Basco and 243 S. Caich and P. J. Thistlethwaite J. Chem. Phys. 1970 53 3381. 2 4 4 N. Basco and K. K. Yee Spectroscopy Letters 1968 1 19. 2 4 5 N. Danon A. Chatalie and G. Pannetier Compt. rend. 1971 272 C 1411. 2 4 6 G . A. Oldershaw and K. Robinson J. Mol. Spectroscopy 1971 37 314. 2 4 7 D. Husain E. W. Abrahamson and J. R. Wiesenfeld Trans. Faraday SOC. 1968 64, 2 4 8 R. B. Kurzel and J. I . Steinfeld J. Chem. Phys. 1970 53 3293; ibid. 1971 55 3304. 249 R. B. Kurzel J. I. Steinfeld D. A. Hatzenbuhler and G. E. Leroi J. Chem. Phys., 197 1 55,4822.2 5 0 K. Sakurai S. E. Johnson and H. P. Broida J . Chem. Phys. 1970 52 1625. 2 5 1 D. G. Horne R. Gosavi and 0. P. Strausz J. Chem. Phys. 1968 48 4758. 2 5 2 0. P. Strausz G. Greig and H. E. Gunning J. Chem. Phys. 1970 52 3684. 253 0. P. Strausz G. Greig and H. E. Gunning J. Chem. Phys. 1970 52 4569. 1968 72 253. B. Morrow Canud. J . Phys. 1966,44 2447. Roczniki Chem. 1969 43 1883. 91 7712. G . A. Oldershaw and K. Robinson Trans. Faraday Soc. 1968 64 616. K. K. Yee Spectroscopy Letters 1968 1 13. 833 Reactions of Atoms and Small Molecules 157 Table 32 (continued) Molecule cs CSe CF cc1 CBr NH N2 NO (u'' = 1) NO NS so s2 SCl c10 c12(3&+u) K2 FeO ZnBr ZnI GeO GeCl GeBr Gel System observed A'rI + xlc+ B,C,e,c,E,F,G +- X '2' a3rI -+ x'Z+ A'rI + X'C+ A2Z + x2rI 'A +- X 2 n A3rI 4-b x3c-c'n -P a'A A3C -+ X'C; B,C -+ A A,C +- x2n A,C,D -+ X2rI C2C +- x2n A2C+ f-) x2n 3rI +- X3c-(?) a'A -+ X3fg 2(a'A,) -+ 2(X3C;) biz,+ -+ X3C; B'rI,+ X'Z:f B,C,D +- X2fi B2C +- X 2 n l C'c; +- x'c,+ A+-X A2C f-) x2rI C,D,E,F,G,H +- X2rI B3C- + x3c-C,D +- x3c-4-b t t t t x3c, x3c, a'Ag a'A, X 2 n A2rI +- x2n 0; t "n,:, B'rI -+ X'C; -B21 +- X 2 n D,C +- X2rI B21 +- X 2 n , B2C t X 2 n , G,F,E,D,C +- X2rI } Ref: 109 187 188 1406 189 190 191 192-195 196 lm 197 198 199 137 200 117,201-206 11 1,114,207 208,209 208,209 208,209 210,211 2 10-2 12 210,211 213 214 215 2 16-2 18 2 16-2 18 219 220 22 1 109,222,224 222,223 188,225 188 136a 226 136a 226 230 23 1 56 232 232 233,70 234 227-229 233 214,21 158 R.J . Donovan and D. Husain Table 32 (continued) Molecule ASH (AsD) As 0 AsCl AsBr H Se Se2 SeBr BrO Br2 CdH (CdD) As2 HBr (0'' = 1) CdBr Cd I SnCl SnBr SnI SbH (SbD) SbN SbCl SbBr SbI Sb2 HTe Te2 TeO TeCl TeBr TeI I 0 Ba 0 I 2 HgH HgCl HgBr HgI System observed A 3 n +- x3c-B2Z +- X 2 n A,B& E,G IJ M N +- X 'C P -A(?) +- x2n --'state 6' +- X'C+ A2rI +- x2n C + - X D + - X 0; +- "n,:, -E,DC +- X 2 n } B2Z +- x2n 3n +- x3c-'n +- x'c A,B,C +- X3C A,B,C +- x3c A,B,C +- X 3 C F,E,G,I,M,U +- X'X; Rydberg +- X 2 n --B +- x2n,,+ B +- x2n,,* B,C +- x2ni A2rI +- x2n B3n& -+ X'C,+ A'Z -+ X'C C,B +- X 2 C + A2n,,+ +- X 2 C + 2nt +- xzc C D ( 2 n +- X 2 C C,D(2rI;:;) + X 2 C Ref: 216 100 218,235,100 215 100 236 140b 237 164b 238,239 230 52,53 54 232 232 240 241,233 24 1 242 243 244 245 245 218,235 141 142 141 141 246 246 237 147,238 250 42 42,251 252 253 247-249 There has been considerable interest in energy transfer from diatomic molecules and some of the most detailed of recent energy transfer studies have employed resonance fluorescence techniques.A brief but useful review discussing work on NO I, N, HD OH CO NO, SOz and CH30H studied by this technique has been given by Steinfeld.'74 The more important or detailed studies relating to reactions and energy transfer involving diatomic molecules are discussed individually below Reactions of Atoms and Small Molecules 159 HD.-Resonance fluorescence from the B('X+) state (u' = 3 J' = 2 ; u' = 5, J' = 2 ; and u' = 6 J' = 5) following excitation with an argon resonance lamp has been studied in the presence of the collision partners 3He 4He Ne D,, and HD.'76 The most interesting conclusions were that transfer for AJ = + 2 predominated for spherically symmetric partners (3He 4He and Ne) while transfer for AJ = lfrl occurred for the less symmetric diatomic molecules D and HD.Data are reported for 297+2 K 0n1y.l~~ Li Na and K .-Resonance fluorescence from the alkali-metal dimers Li, Na, and K has been reported by Zare et u1.,178b,2139231,254 following excitation with a continuous-wave argon ion laser.For Na, nineteen different fluorescence progressions (B'n -+ X'ZC,') were observed.,' The lifetime of the u' = 10 J' = 12 level of the B'H state of Na was determined using a level-crossing technique [z = (6.41 _+ 0.38) x lop9 s] and represents one of the fastest transitions yet observed in the visible region (it is faster by a factor of two than that of the atom). The laser fluorescence technique has also allowed a precise determination of the bond strength of Li,(X'C,') (D = 1.026 f 0.006 eV).254 Energy transfer from Li,(B'rIT,) has shown that changes in AJ are dependent on the A doublet c o n ~ e r n e d . ' ~ ~ ' . ~ ~ ~ 'Propensity rules' are presented and a simple classical model is proposed to account for the observa-t i o n ~ .' ~ ~ ' ~ ~ ~ ~ However this model clearly requires revision in the light of later work which demonstrates that the sign of the AJ = k l asymmetry changes in a systematic manner with the collision partner (He Ar Kr Xe)."'" Table 33 Rate constants for the reactions of CN radicals at 300 K1* l a Molecule k*/cm3 molecule- s -Ethylene Propylene Buta-1,3-diene Benzene (1.9 f 0.3) x lo-'' [(2.2 + 0-4) x lO-''] (2.7 f 0.3) x lo-'' (4-3 f 0.5) x lo-'' (2.8 + 0.3) x lo-'' Methane (7.3 f 0.2) x [(8-3 f 0.3) x Ethane (2.4 f 0.2) x lo-" [(2*4 & 0.4) x lO-"] * Rate constant for the zeroth vibrational level of the ground electronic state; rate constants for the fourth vibrational level are given in square brackets where known, CN.-Observation of CN radicals in the vibrational levels u" = 0 and 0'' = 4 has been reported following pulse radiolysis of cyanogen-argon mixtures.l 8 la The rate of decay was monitored photoelectrically via the B + X system, and rate data are reported for reaction with several alkenes and alkanes (Table 33). The rates for reaction with alkenes approach the collision frequency, and indicate a very low or zero activation energy. The results are in general 2 5 4 R. Velasco Ch. Ottinger and R. N. Zare J . Chem. Phys. 1969 51 5522 160 R. J. Donovan and D. Husain agreement with data from other techniques and parallel with those for the analogous chlorine atom reactions. C0.-The decay of the CO(a311) state has been monitored directly via the Cameron bands CO(a3n -P X’Cf) following excitation with a tesla dis-charge 184 and by indirect photoexcitation.’’ Values for the self-quenching rate constants of CO(a3n; u’ = 1 and 0) have been reported as 2.8 x 10-lo and 1.2 x 10- lo and for quenching by NO as 7.0 x 10- l o and 3-1 x 10- l o cm3 molecule- ’ s- ’ respectively. Quenching by NO yields NO(A2C) and NO(B211) the yield for the A state being 15-23% depending on the vibra-tional state of CO(a3n) and 10 % for the B state.’” Indirect photoexcitation of CO was achieved via the ‘resonance’ transitions CO(d3A t X’C’) and CO(U’~C+ X’C’) which are relatively strong due to mixing with the close-lying A’II state.’85 The (a3n) state is populated via radiative transitions from the d and a’ states.Using a modulated source a lifetime for the a3n state was determined as 4.4 1.1 ms. Quenching by NO CO N, O, H,, and C02 was studied using this system.’85 Lawrencels6 has also reported a somewhat greater value for the radiative lifetime of CO(a3n) uiz. 7.5 1 ms ; however his data for quenching of this state by C 0 2 are in good agreement with the work of ref. 185. CS.-The vibrational relaxation of CS (u” = l) formed in the reaction 0 + CS -P SO + CS has been studied by monitoring the (2,l) band of the CS(A’n X’C’) system phot~electrically.’~~ Rate data for relaxation by orthohydrogen parahydrogen HD ,He D, 4He N,O CO, H,O D,O, H,S and D2S are given.’87 The data strongly indicate that the probability for vibration-vibration exchange is enhanced if the transitions involved are both i.r.-active.Thus N 2 0 is found to be two orders of magnitude more efficient than CO, although both molecules possess near resonant transitions for transfer from CS(u” = 1). The data also support the proposal made by ma ha^^,,^ and implicit in Sharma’s theory,256 that vibration-rotation transfer may occur with collision partners which possess small moments of inertia. N .-The ‘forbidden’ Vegard-Kaplan system (A3Cu+ .+ X’C,’) has been reported following electronic energy transfer from metastable argon atoms in a flow The radiative lifetime of the A state was determined as < 3.5 s in agreement with other recent data. The reaction of N,(A3C,f) with acetylene and cis-but-2-ene is reported to be similar to that for other triplet sensitizers and occurs at gas-kinetic collision frequencies.Quenching of the A state by 02 CO SO, N,O NH C2H2 propene benzene buta-lP-diene, and cyclopropane has also been examined.’96 Callear and Wood2” have described a simple technique for monitoring the decay of N,(A3Z:,f) following energy transfer from NO(C2n) in a flash photolysis experiment. The decay of N,(A) is observed by monitoring the NO y-emission [NO(A2C+ -P X’n)] 2 5 5 B. H. Mahan J . Chem. Phys. 1967,46,98. 2 5 6 R. D. Sharma and C. W. Kerr J . Chem. Phys. 1971 55 1171. 2 5 7 A. B. Callear and P. M. Wood Trans. Furaduy SOC. 1971 67 598 Reactions of Atoms and Small Molecules 161 which results from the process : N,(AZC,+) + NO(XZnj -+ N2(X1C,+) + NO(A2C). Rate data for quenching of N,(A) by water are presented and the rate is shown to be lower than previously supposed.In a further study using monochromatic flash photolysis quenching of the A state by C,H, C2H4 C2H6 C,H,, C4Hlo N20 NH, O, CO NO H, and Hg is reported.258 An elegant and sophisticated technique259 has been applied to the measurement of the lifetime of N2(C311,). The technique involves excitation of the state by a monoenergetic electron beam and only those emissions which are coincident with an in-elastically scattered electron with the correct energy are counted. Thus effects from cascading are avoided. Lifetimes for the first three vibrational levels have been determined [also for the first two vibrational levels of H,(U~Z:)].~~” The technique should be of considerable importance for deter-mining lifetimes of states which have strong radiative transitions in a con-venient wavelength region.NO.-Further work on energy transfer from the A C and D states of NO using resonance fluorescence techniques has been reported by Pilling and Callear,Ig9 extending the earlier work of Smith and Callear. A particularly elegant piece of work has been reported by Melton and Klemperer,’”’ who employed an atomic lamp (electrodeless) operating in a homogeneous magnetic field to tune one of the Zeeman sublevels into exact coincidence with an NO y-band absorp-tion line. By selectively exciting I4NO in the presence of I5NO it was demon-strated that the high cross-section for relaxation of NO(A2C+) (u” = 1) is due to fast electronic energy exchange uiz : ~ ~ N o ( A ~ c + ) ( ~ ’ = 1) + 1 5 ~ 0 ( ~ 2 n ) ( ~ ” = oj -+ 15NO(A2Cf)(u’ = 0) + ‘‘NO(X211)(u” = I?) It has been proposed that the exchanges take place via dipole-dipole inter-action similar to that proposed previously in exciton theory and Forster’s theory.260 The recombination of N + 0 produces NO(C2n) by inverse predissociation and at low pressures the NO(A2C +) state is formed exclusively from NO(C211).By monitoring the C -+ X ( 6 ) and A -+ X ( y ) emission the f value for the transition C + A was obtained asf = 0.61.261 The vibrational relaxation of nitric oxide in the ground electronic state has been investigated at temperatures between 100 and 433 K and the rate has been shown to have a negative temperature ~oefficient.’~’ The formation of nitric oxide dimers and the importance of ternary collisions have been demonstrated.OH.-Greiner has reported detailed quantitative rate data for the reaction of OH with H202 CO H2 D, CH4 and higher alkanes (Table 34) obtained by monitoring the A2Cf +- X211 system at 309 nm in a b s o r p t i ~ n . ~ ~ ~ - ~ ’” A. B. Callear and P. M. Wood Trans. Furuduy Soc. 1971 67 272. 2 5 9 R. E. Imhof and F. H. Read J . Phys. (B) 1971 4 1063. ’’’ R. G . Gordon and Y.-N. Chen J. Chem. Phys. 1971 55 1469. ’” W. Groth D. Kley and U. Schurath J. Quant. Spectroscopy Radiative Transfer 1971, 11. 1475 Table 34 Rate data ,for reactions involving OH radicals Reactant H202 co H2 D2 CH4 n - C A 10 i-C,H neo-C,H 2,3-Dimethyl butane 2,2,3-Trimethylbutane 2,2,3,3-Tetramethylbutane 2,2,4-Trimethylpentane C2H6 C3H8 CyClO-C6H 1 2 n-C8H18 c2 H4 k/cm3 molecule-' s-' ; at 300 K 8.5 x (1.47 k 0.06) x (7.05 k 0.58) x (2.1 * 0.3) x (8.8 f 0.3) x (2.94 f 0.38) x (2.14 * 0.12) x (8.75 _+ 0.24) x (7.95 _+ 0-43) x lo-'' (5.03 f 0.12) x lo-'' (1.37 & 0.2) x (2.39 f 0.05) x (7.45 * 0.22) x 10-l2 (8.42 -t 1.2) x (1.16 k 0.1) x (3.90 f 0.15) x (5.70 0.33) x log,,(A/cm3 1.3 (2.09 f 1.99) (6.77 k 2.09) (5.49 f 1.99) (1.86 & 1.99) (1.41 f 0.21) (8.72 f 1.99) (1.42 f 0.20) (2.34 f 0.20) (4.78 f 2.06) (7.95 & 2.09) (1.42 0.19) (1.62 f 0.19) (1.55 0.19) (1.26 .+ 0.20) (1.20 f 0.18 Reactions of Atoms and Small Molecules 163 The source of OH radicals employed was the far-u.v.photolysis of H20, rather than H202 which was found to undergo rapid reaction with OH to yield HO radicals. It was suggested that similar reactions in the pyrolysis of H,O might account for the discrepancy between the pyrolysis and flash photolysis results. Horne and Norrish204 have also reported a study of OH radical reactions using similar techniques to those of Greiner. The reaction of OH with H2 has been studied over the temperature range 300-500 K and shown to be first-order for both reactants at the extremes of this range. These data do not however extrapolate well with results from combustion studies, and a curvature in the Arrhenius plot has been proposed to account for this The results for reactions with alkanes have been discussed in terms of the principle of additivity of bond properties with kinetic data.The reported negative activation energy for abstraction of some tertiary hydrogen atoms is rather s ~ r p r i s i n g . ~ ~ ~ - ~ ~ ~ W has discussed the discrepancies in activation energy for a number of reactions. Reaction of OD with CH and C2H has also been reported the rates being equal to those for OH within the experimental err~r.~O~-'O~ Fluorescence from OH(A2C+ -P X 2 n ) following photodissociation of H 2 0 by krypton resonance radiation has been employed to study rotation-vibration energy transfer in the A OH(AZC+ ; u' = 0 K' = 20) + Ar N --+ fact. 2 0 1-20 3 uiz: OH(AZC+ ; c' = 1 K' = 15) + Ar N + AE. The process is shown to be efficient the rate constant in both directions F,:ing - 10- l 1 cm3 molecule- s- '.Radiative and predissociation probabilities for OH(A2Z+) have been measured using phase shift techniques.264 The radiative lifetime for u' = 0 was reported as 850 f 130 ns yielding an ,f value for the (0,O) band of (7.7 The higher vibrational levels show shorter lifetimes due to p r e d i s s ~ c i a t i o n . ~ ~ ~ Predissociation of OH(A2Cf) has also been discussed by Durmay and M ~ r r e l l ~ ~ who propose that continuum states of the ground electronic state may be responsible. SH.-Although the self disproportionation of OH radicals has not been studied using U.V. absorption or emission techniques the only quantitative study of SH [using the (1,O) band of the A2C + X211 system at 305 nm] yields a value of k = 2.3 x lo-'' cm3 molecule-'^-^ for this reaction.220 A significant channel in the reaction was found to be the production of S2(a1A,); however, the contribution of this channel to the total reaction was not established quantitatively.02(a1Ag) and 02(b1Cg+).-Two reviews of the physical and chemical reactions of these two excited states of the oxygen molecule have appeared.210*211 The 1-1) x 2 6 2 W. E. Wilson J . Chern. Phys. 1970 53 1300. '" K. H. Welge S. V. Filseth and J. Davenport J . Chem. Phys. 1970 53 502. 2 6 4 W. H . Smith J . Chem. Phys. 1970 53 792. 2 6 5 S. Durmaz and J . N. Murrell Trans. Furuduy Soc. 1971 67 3395 164 R. J. Donovan and D. Husain more recent of these by Kearns,2'0 deals extensively with the reactions with organic molecules and with the theoretical treatment of such reactions.Only very recent results will thus be discussed here. The formation of 02(b'C,+) following quenching of the triplet state of I-fluoronaphthalene has been reported by Andrews and Abrahamson,266 who observed the forbidden emission (b'Cl + X3Z:) at 762 nm. An analogous study by Kearns et has shown that 02(b'X:) is also formed directly in the photosensitized reaction between gaseous quinoxaline and oxygen and that the quantum yield for formation of the 'C state is at least equal to that for the 'A state and probably greater in accord with the predictions made by Kearns2" on the basis of theoretical treatments. Contrary to previous suggestions the most recent experimental data support the predominant formation of O,(b'C:) following quenching of 0 ( 2 ' D 2 ) by 0 2 rather than formation of 02(a'A,).268-270J13 This is in agreement with the prediction made by the authors2 on the basis of correlation diagrams.It may be noted however that the direct formation of 0 2 ( X 3 Z ) is in fact a spin-allowed channel in the quenching of 0(2'D2) by 0 2 , and the results of McGrath et using isotopically labelled oxygen show that vibrational levels of the ground electronic state up to t"' = 14 are populated in this quenching process. Welge et ~ 1 . ~ ~ ' have reported data for the quenching of O2(b1Z:) by a large range of molecules (Table 35). The results of less direct methods271 are generally in good agreement with the results of Welge et a1.268 However the data for NH and H2 show a wide discrepancy. The results of Derwent and Thrush2' support the lower value for quenching by H .These authors have also reported absolute intensity measurements for the 'dimol' emission by two 02(a'A,) species and the rate constant for 'energy pooling' to yield 02(b'X,f ; u' = 0 and 1). Removal of O,(b'C,+) by quenching on the walls and in the gas phase by H20 have been reported.212 A further comparison with less com-plete quenching data from other sources is given in refs. 210 and 271. The formation of 02(a'A,) in the photolysis of 0 has been shown to arise from the primary photochemical step for all wavelengths in the u.v.270,272 The observation of the vacuum-u.v. spectrum of 02(a'Ag) following the photolysis of O, has been reported,208 and a further series of bands between 83 and 90 nm have been attributed to this state.273 Extinction coefficients for several transi-tions of OJa' A,) in the vacuum-u.v.have been reported ;209 however further work to determine the contribution from continuum absorption by Oz(a'A,) [the concentration of 02(X3Cg-) was determined from a measurement of the continuous absorption and that of 02(a'A,) obtained by mass balance] and a more precise measurement of the atomic oxygen concentration will be necessary "' L. J. Andrews and E. W. Abrahamson Chem. Phys. Letters 1971 10 113. 2 b 7 D. R. Kearns and C . K. Duncan J . Chem. Phys. 1971 55 5822. 2 b 8 S. V. Filseth A. Zia and K. H. Welge J . Chem. Phys. 1970 52 5502. 2 h 9 F. Stuhl and H. Niki Chem. Phys. Letters 1970 7 473. 2 7 0 M. Gauthier and D. R. Snelling J.Chem. Phys. 1971 54 4317. 2" R. J. O'Brian and G. H. Myers J . Chem. Phys. 1970 53 3832. 2 7 2 I. T. N. Jones and R. P. Wayne Proc. Roy. Soc. 1970 A319 273. 2 7 3 R. E. Huffman J. C . Larrabee and V. C . Baisley J . Chem. Phys. 1969 50 4594 Reactions of Atoms and Small Molecules 165 Table 35 298 K Rate constants k/cm3 molecule-' s-' for quenching of O,(b'C,f) at Quenching species co co2 SF6 0 2 H2O D2O CH,I CH,OH C2H50H CH3COCH3 NH, CH, C2H6 n-GH1, C2D, CZD2 C2H2 CCI, N2O NO2 SO2 H2 D2 HD N2 He Ne Ar Kr Xe C2H4 C6H6 NO Resirlts in ref. 268 4.4 x lo-', 5.7 x 10-16 4.5 x 10-l6 3.3 x 10-l2 4.3 10-15 __ 8.6 x lo-', 1.1 x 10-13 3.1 x 10-13 3.6 x -1.4 x (ref. 269) 8.3 x 10- l4 (ref.269) 4.5 x 10- l 3 (ref. 269) -7.0 x lo-', 4.1 x 10-14 1-1 x 10-12 1.8 x 1 0 - 1 5 3-1 x lo-', ----1 x 10-'6 -1 x 5.8 x -1 x 10-'6 - 1 x 10- l 6 Results in ref. 271 (1.5 _+ 0.3) x --1 1 0 - 1 5 (4.0 & 0.6) x (4-0 & 0.5) x (3.0 f 0.6) x lo-', (4.0 f 0.8) x lo-'' (3-1 _C 0-6) x lo-" (3.0 & 0.5) x (7.0 & 1.5) x lo-', --(2.0 f 0.4) x -(6.5 & 1.5) x < 3 x 10-15 (4 1) x 1 0 - 1 ~ (3 1) x 10-l5 -(2.5 f 0.5) x lo-', (4-0 & 0.6) x (2.0 f 0.5) x 10-l4 (1.8 f 0.4) x lo-', < 1 x < 1 x 10-l6 ( 3 1) 10-15 -before the extinction coefficients can be used for quantitative work. Quenching of 02(a'A,) by iodine,'72 bromine,274 and sulphur-containing and the reaction with O3 molecule^^^^^^^^ and N atoms278 have been reported.The reaction with nitrogen atoms is particularly interesting as the rate constant exhibits a low pre-exponential factor. It has been suggested that this is due to the low probability for a non-adiabatic transition between the quartet potential surface (correlating with reactants) and the doublet surface correlating with the products. The photoelectron spectrum of 02(a'A,) has been o b s e r ~ e d . ~ ~ ~ 2 - 4 M. A. A. Clyne J. A. Coxon and H. W. Cruse Chem. Phys. Letters 1970 6 57. 2 ' 5 J. N. Pitts R. A. Ackerman and R. 1. Rosenthal J . Chem. Phys. 1971 54 4960. "' R. A. Ackerman J. N. Pitts and R. P. Steer J. Chem. Phys. 1970 52 1603. "' I. D. Clark I. T. N. Jones and R. P. Wayne Proc. Roy. Soc. 1970 A317 407.2 7 8 I . D. Clark and R. P. Wayne Proc. Roy. Soc. 1970 316 539. 2 7 9 N. Jonathan A. Morriss K. J. Ross and D. J. Smith J. Chem. Phys. 1970 53 3758. N. Jonathan A. Morriss K. J. Ross and D. J. Smith J . Chem. Phys. 1971 54 4954 166 R. J. Donovan and D. Husain S2(a1Ag) and SO(b"C+).-The formation of S2(a1Ag) in the photolysis of OCS has been shown to result from the reaction of S(31D2) with OCS.'36*220 The decay of S2(a1Ag) which has been monitored via both the f+- a and g+ a transitions was found to be very considerably faster than that of the analogous state of oxygen under all conditions used. The forbidden emission SO(blC+ -P X 3 C - ) has been observed from H2S-02 flames.281 C10 BrO and 10.-Reactions involving the halogen oxides have received considerable attention.Johnston et a1.228*28 have employed the 'molecular modulation' technique to examine the photochemical reaction between chlorine and oxygen. The ClOO radical was observed for the first time in the gas phase and the decay of CIO was shown to be dependent on the total pressure in the range 50-760 Torr supporting the 'high pressure' mechanism proposed by Clyne and C O X O ~ ~ ~ ~ viz. C10 + C10 + M -+ Cl202 + M Cl202 -+ c1 + 0, However a recent and very detailed study by Basco and Dogra using flash photolysis and employing a number of sources of C10 appears to show that the decay of C10 is independent of the total pressure in the range 75-200 Torr, thus directly conflicting with the results obtained with the molecular modulation technique. At low pressure the mechanism for decay of C10 is :284 c10 + c10 -+ c1 + c100, and Clyne et al.have now extended their flow-tube studies for pressures up to 8 T ~ r r . ~ ' ~ ~ The decay was found to remain second-order with Ar and SF6 as third bodies over the entire range 0.5-8 Torr. Agreement with Johnston et al.228 on the pressure region for which a change from second- to third-order kinetics is expected was achieved if the dissociation energy of ClOO was revised viz. D;,,(Cl-OOj = (29 5 3) kJ mol- '. The rate constant for bi-molecular removal of C1O at low pressures was given as 10-'o.71'o.04 exp[( - 1 150 50)K/T] cm3 molecule- s- ' in the temperature range 273-7 10 K.284a Following matrix-isolation studies the vibrational wavenumber of the C10 ground electronic state has been revised284b to 995 cm-' and re-analysis of the gas-phase spectrum confirms this.284c The bimolecular dispro-portionation reaction of BrO is considerably more rapid than that for C10, having a higher pre-exponential factor and lower activation energy exp[(-450 f 300jK/T] cm3 molecule-' s-I).Thus the bi-molecular removal mechanism is likely to dominate up to pressures of one ( k = 10-9.3kO.l "' A. hl. Bouchoux J . Marchand and J . Janin Specrrochim. ACIU 1971 27A 1909. 2 8 2 H. S. Johnston and E. D. Morris J . Amer. Chern. Soc. 1968 90 1918. 2 8 3 M. A. A. Clyne and J. A. Coxon Truns. Faraduy Soc. 1966 62 1175; Proc. Roy. Soc. 1968 A303 207. 284 ( a ) M. A. A. Clyne and I. F. White Truns. Furuduy Soc. 1971 67 2068; (b) L. J. Andrews and J. I. Raymond J .Chem. Phys. 1971 55 3087; (c) P. A. G . O'Hare and A. C. Wahl J . Chem. Phys. 1971 54 3770 Reactions of Atoms and Small Molecules 167 atmosphere or more. The cross-disproportionation reaction between C10 and BrO has been reported by Basco et and the rate was found to be almost two orders of magnitude greater than for two C10 radicals at 298 K. A less detailed study of I 0 has shown that the bimolecular disproportionation reac-tion is as fast as that for BrO within the experimental error.238 The reactions of C10 and BrO with ground-state singlet molecules (H CH, C,H, C,H,, and N,O) have been shown to be slow and that chain reactions previously proposed for C10 are more reasonably accounted for by the C1 atoms produced in such reactions. Further data for the reaction of BrO with NO and 0 are given.238 BrO has been detected following the flash photolysis of N,O + Br, and NOz + Br mixtures and has been attributed to the reaction of oxygen atoms with Br .2 8 5 Under the conditions used it is possible that the reactions observed actually involved 0 ( 2 ' D 2 ) as no steps were taken to quench the excited state which is produced in the photolysis of N,O and NO with cu. 200 nm radiation. I .-Further detailed work on vibrational and rotational energy transfer in the B31"I& state of iodine has been presented by Steinfeld et uf.248,249 The 514.536 nm line of a continuous-wave argon ion laser (1 W) was used to excite the J' = 11 and 15 rotational states of the u' = 43 vibrational level of 12(B31"I,f,) and the resonance fluorescence was observed.Changes in vibra-tional level from Av = + 5-Au = - 8 were reported. However the processes with highest cross-section lay within an energy range equivalent to kT of u' = 43 (i.e. Av = &3 predominates). Thus while more quanta of vibrational energy are transferred for collisions involving u' = 43 relative to collisions involving u' = 25 or u' = 15 it is that approximately the same amount of energy is transferred from each of these vibrational levels of colli-sions. The relative magnitudes of the collision cross-sections are well predicted from the oscillator matrix elements of the repulsive term in the intermolecular potential which couples the initial and final vibrational states (provided these are weighted by a factor to allow for detailed balancing).In the treatment of data a procedure for removing the effects of multiple collisions was used. An interesting dependence on the mass of the most efficient collision partner was observed for the levels u' = 15 25 and 43. The maximum efficiency was observed when the collision duration was approximately equal to the period of the vibration undergoing relaxation. Thus as 7,ib increases on going from u' = 15 to u' = 43 so the probability for relaxation when plotted against mass passes through a maximum at higher masses. However the correlation appears to be only qualitative. Excitation to the 0' = 50 and u' = 53 vibra-tional levels of I,(B3n&) using a cadmium lamp has shown that 'electronic quenching' from these levels occurs at every collision (a modified collision diameter was used to take account of long-range forces; this reduced the collision cross-sections from values apparently greater than gas-kinetic to values close to the gas-kinetic cross-section).As the level L?' = 50 lies within 285 R. E. Tomalsky and J. E. Sturm J . Chrrn. P h j s . 1970 52 472 I68 R. J. Donovan and D. Husain 2kT of the dissociation limit it has been proposed that 'collisional release'286 may account for some of the 'quenching'. The quenching appears to be inde-pendent of mass. In a later the transfer of rotational energy was studied in more detail. The maximum rates of transfer are observed for values of AJ between + 4 and + 8. Transfer rates for higher values fall rapidly indicating negligible rates outside the limits AJ = + 14 and AJ = + 18 (for Au = 0 1, and 2).However for stronger collisions involving Au = 3 there is a greater spread in AJ' and transitions up to AJ = +28 are found. A number of interesting points arising from the data are further discussed including the asymmetry of the microscopic rate constants for rotational energy transfer. 4 Triatomic Molecules The triatomic molecules and radicals which have been observed using U.V. and visible spectroscopic techniques are listed in Table 36.287-3 The most detailed kinetic studies have been made for the species CH, CF NO,, and SO, which are discussed individually. CH .-The rate of formation of triplet methylene following relaxation from the singlet state formed in the vacuum-u.v. photolysis of keten and diazomethane, has been studied by means of the strong vacuum-u.v.transition of wavelength 2 8 6 A. B. Callear and T. Broadbent Truns. Furuduy Soc. 1971 67 3030. 2 R i G. Duxbury J. Mol. Spectroscopy 1968 25 1. 2 8 8 (u) M. J. Pilling W. Braun and A. M. Bass J. Chrtrr. Phys. 1970 52 5131 ; ( h ) M. J. Pilling A. M. Bass and W. Braun Chew. Phys. Letters 1971. 9 147; ( c ) G. Herzberg and J. W. C. Johns J . Chem. Phj.s. 1971 54 2276. 2 8 9 I. Dubois G . Herzberg and R. D. Verma J. Chem. Phys. 1967 47 4262. 2 9 0 R. N. Dixon and G. Duxbury Chetn. Phys. Letters 1967 I 330. "' R. N. Dixon G. Duxbury and D. A. Ramsay Proc. Roy. Soc. 1967 A296 137. 2 9 2 R. N. Dixon G. Duxbury and H. M. Lamberton Proc. Roy. Soc. 1968 A305 271. 2 y 3 I . M. Napier and R. G. W. Norrish Proc.Roy. Soc. 1967 A299 337. 2 9 4 A. B. Callear and P. M. Wood Truns. Farurfuy Soc. 1971 67 3399. 2 y 5 H. S. Johnston G. E. McGraw T. T. Paukert L. W. Richards and J. Van den Bogaerde, Proc. Nat. Acad. Sci. U . S . A . 1967 57 1146; H . Kijewski and J. Troe fnrernut. J. Chem. Kinetics 197 I 3 223. 296 H. W. Kroto Cunud. J . Phys. 1966 44 831; 1967 45 1439; H. W. Kroto T. F. Morgan and H. H. Sheena Truns. Furuduy Soc. 1970 66 2237. 2 9 7 N. Basco and K. K. Yee Chem. Cotnnz. 1968 150. 2 9 8 N. Basco and K. K. Yee Chem. Corntn. 1968 152. 2 9 y N. Basco and K. K. Yee Chetn. Cotnni. 1968 153. 3 0 " R. C. Mitchell and J. P. Simons J . Chetn. Soc. ( B ) 1968 1005. 30' C. W. Mathews Cunud. J. Phj,s. 1967 45 2355. 3 0 2 F. W. Dalby J. Chetn. Phys. 1964 41 2297. 3n3 (u) W.J. R. Tyerman Truns. Faruduy Soc. 1969 65 1188; ( 6 ) A. S. Lefohn and G . C. Pimentel J . Chetn. Phys. 1971 55 1213. 304 W. J. R. Tyerman Chetn. Corntn. 1968 392. ' O 5 R. N. Dixon and M. Halle J . Mol. Spectroscopy 1970 36 192. 3 0 h R. G . Cavell R. C. Dobbie and W. J. R. Tyerman Cunud. J. Chem. 1967 45 2849. jo7 S. E. Schwartz and H. S. Johnston J. Chem. Phys. 1969 51 1286. ' O M H. D. Mette J . Phys. Chern. 1969 73 1071. 3"y T. N. Rao and J. G. Calvert J . Phys. Chem. 1970 74 681. 3 1 0 K. Otsuka and J. G. Calvert J . Arner. Chem. Soc. 1971 93 2581. 3 1 1 H. W. Sidebottom C. C. Badcock J . G. Calvert G. W. Reinhardt B. R. Rabe and E. K. Damon J . Amer. Chetn. Soc. 1971 93 2587; G. E. Jackson and J. G. Calvert, ihid 2593 Reactions of Atoms and Small Molecules 169 Table 36 parentheses) Triatomic molecules and radicals which have been observed (references in BH2 (179b) CH (287,288) SiH (289) NH (287) PH (290,291) ASH, AsD (292) SbH2 (242b) HNO (293,294) HO,'? (295) HS,? (220) NCN (296 297) PCN (298) AsCN (299) CF2 (300-303) CFCl (304) SiF (305) PF (306) ClO (228) NO (307) SO2 (308-31 1) S3'? (220) 141.5 nm288a (the only known singlet system lying between 500 and 950 nm, has so far proved to be too weak for quanitative kinetic studies).Absolute rate data for quenching of singlet CH by He Ar and N have been reported. The most remarkable feature of these data is the relatively high efficiency for quenching by He. Any increase in spin-orbit coupling due to the proximity of the helium atom will be negligible and it has thus been suggested that the mechanism for relaxation involves a shift in the manifold of vibration-rotation states of the singlet relative to those for the triplet.Thus the collision acts as a perturbation bringing states of the singlet and triplet manifold into near coincidence.288a Transfer to the triplet surface is favoured due to its higher density of states and crossing is induced by spin-orbit coupling in the methylene molecule itself. The reaction of singlet methylene with H was also studied and the rates for reaction and quenching were determined. The methyl radical formed in this reaction was used as a 'spectroscopic marker' with which to obtain rate data for other insertion reactions. Reactions of triplet methylene are also reported, and the reaction of two triplet methylene radicals was shown to be highly effi-cient (collision efficiency of -a).288a The extinction coefficients and oscillator strengths of CH in the vacuum-u.v.have been measured,288b but will clearly need revision in view of recent comments on the spectrum in this region.288c CF .-The ground state of CF is a singlet state and although the production of triplet CF has been postulated in a number of photochemical systems no direct evidence for its formation has been reported. Simons et ~ 1 . ~ ~ have shown that ground-state CF is produced in the flash photolysis of a large number of fluorinated ketones fluoro-olefins and halogenated alkanes. Mathews301 has analysed the 2- z system and reported several new transitions in the vacuum-u.v.However several of these bands have been reassigned to CF (see section on CF,). The extinction coefficient of CF at 249 nm has been measured by Dalby302 and by T ~ e r r n a n . ~ ' ~ ~ The value given by T ~ e r r n a n ~ ' ~ ~ ( E ~ ~ ~ = 7620 -t 100 1 mol- cm-') is significantly greater than that of Dalby ; however Tyerman's determination of the f value for the x+- system (f = 0.028 & 10%) is in agreement with that reported from shock-tube studies and is thus probably the more reliable. The decay of CF is extremely slow and the radical has been observed to persist for several seconds, under favourable conditions. In general singlet CF is unreactive and the rat 170 R. J . Donovan and D. Husain of decay is not increased by the presence of molecules such as 0,.The dimeriza-tion of CF following the photolysis of C2F4 is reported to have a positive activation energy,303a as expected if CF2(Z1A1) is to be promoted to the 3 B , valence state for combination to occur. However the pre-exponential factor is low and thus the 'avoided crossing' between the two singlet surfaces [oce correlating with two CF,(Z'A,) and the other with two CF2(3B1)] appears to limit the combination rate. This will be the case if crossing does occur for some nuclear configurations of CzF4. This is expected on theoretical grounds and indeed must occur as singlet CF2 is observed in the photolysis of C,F,. The i.r. spectrum of CF has also been observed using rapid scan techniques, and rotational structure in the 1224 cm- ' band has allowed this to be assigned as v1 (another band at 11 12 cm- is assigned as v3).3036 HN0.-The formation of HNO in the mercury-photosensitized decomposition H in the presence of NO has been reported.294 A series of diffuse bands between 208 and 198 nm previously attributed to CH,O and nitroformalde-hyde have now been assigned to HN0.294 NO .-The phase-shift technique has been used to study the radiative lifetime of the state of Under collision-free conditions and after ensuring that diffusion out of the observation zone was negligible the lifetime was shown to vary with wavelength exhibiting a roughly saw-tooth behaviour.The life-time varied between 55 and 90ps in the spectral region 398400nm. For higher pressures data were obtained which gave values of the energy loss per effective collision and it was shown that the 2Bl state lost one quantum ( - 1230 cm-') per gas-kinetic collision.The results also suggested that the ,B1 state undergoes rapid internal conversion to high vibrational levels of the ground state.30 SO,.-Fluorescence from the first excited singlet state of SO has been employed to obtain quenching data for this state.308 .'. wide range of quenching mole-cules were studied and the quenching efficiency was found to correlate with polarizability. These data are also presented together with data for other similar excited species in a review by Stei11fe1d.l~~ It was suggested that collision-induced internal conversion occurs as with NO,. Calvert et have examined the fluorescence quantum yield for the first singlet state of SO2, and discuss the photochemistry of the molecule following excitation to this state.They conclude that the fluorescence quantum yield is less than unity in disagreement with Mette,308 and suggest that 'internal energy dissipation' occurs even at the limit of zero pressure. The phosphorescent decay of the lowest triplet state of SO2 following intersystem crossing from the singlet manifold has been observed using flash phot~lysis.~ Rate data for quenching of the triplet state and reaction to yield SO and SO have been obtained and Arrhenius parameters are given. Direct excitation of the triplet state using the Raman-shifted output from a frequency-doubled ruby laser has also been employed to examine these reactions and to measure the phosphorescenc Reactions of Atoms and Small Molecules 171 yield.31 It was suggested that the isolated triplet SO molecule is capable of undergoing some form of non-radiative decay.However further allowance for diffusion effects analogous to those discussed by Johnston and S~hwartz,~” may have to be considered. Previous suggestions that a thermal isomer of SOz is formed when the gas is subjected to adiabatic flash photolysis have been dis~ounted.~ The observed spectral effects have been shown to result from broadening of the banded structure following the temperature rise. A similar effect is reported for NOz. 5 Polyatomic Molecules Table 373’3-329 lists a few of the simpler polyatomic molecules and radicals which have recently been investigated using spectroscopic techniques in the visible and U.V.regions. The coverage is not comprehensive and it is worth emphasizing that the assignment of most spectra of polyatomic species is made on the basis of chemical evidence. In most cases a series of parent compounds Table 37 in parentheses). CH,(313-316) CF3( 3 17) CH2N (318) CH3CHN (318) (CHd2CN (318) CH2S (319) CF2N (320) HPCN (298) CH3S (319) ClCOOH (321) HCCCH (322a) CHOCHO (323) CH2C(CH3)CH2 CH3CHCHCH2 (324) CH2C(C2 H,)CH2 (324) cyclopentyl radical (324) cyclopentadienyl radical (326) phenylnitrene (326) benzyne (327) diphenylphosphine (328) PbMe (329) PbEt (329) PbCHO (329) Polyatomic molecules and radicals which have been observed (references CH2CHCHz (324,325) (324) 3 1 2 N. Basco and R. D. Morse Proc. Roy. Soc. 1971 A321 129.’ I 3 ) 1 4 H. E. van den Bergh A. B. Callear and R. J. Norstrom Chem. Phys. Letters 1969 4, 3 1 5 ( a ) A. B. Callear and H. E. van den Bergh Chem. Phys. Letters 1970 5 23; (6) Trans. N. Basco D. G. L. James and R. D. Suart Internat. J . Chem. Kinetics 1970 2 215. 101. Faruday Soc. 1971,67 2017. N. Basco D. G. L. James and F. C. James Chem. Phys. Letters 1971,8 265. N. Basco and F. G. M. Hathorn Chem. Phys. Letters 1971 8 291. 3 1 x ( a ) D. G. Horne and R. G. W. Norrish Proc. Roy. Soc. 1970 A315 301; (6) J. F. Ogilvie and D. G. Horne J . Chem. Phys. 1968 48 2248. *’ A. B. Callear J. Connor and D. R. Dickson Nature 1969 221,1238 ; ( h ) A. B. Callear and D. R. Dickson Truns. Faraday Soc. 1970 66 1987. R. N. Dixon J. P. Simons G. Duxbury and R. C. Mitchell Proc.Roy. Soc. 1967, A300,405. R. J. Jensen and G. C. Pimentel J . Phys. Chem. 1967,71 1803. ” 3 2 0 3 2 ’ 3 2 2 A. J. Merer Canad. J . Phys. 1967 46 4103. 3 2 3 J. I. Steinfeld G. W. Holleman and J. T. Yardley Chem. Phys. Letters 1971 10 266. 3 2 4 A. B. Callear and H. K . Lee Truns. Faraday Soc. 1968,64,308; A. B. Callear and H. E. 3 2 5 C. L. Currie and D. A. Ramsay J . Chem. Phys. 1966,45488. 3 2 6 G. Porter and B. Ward Proc. Roy. Soc. 1968 A303 139. 3 2 7 G. Porter and J. I . Steinfeld J . Chem. Soc. ( A ) 1968 877. 3 2 8 S. R. Wong W. Sytnyk and J. K. S. Wan Canad. J . Chem. 1971 49 994. 3 2 y 1. M. Napier Austral. J . Chem. 1971 24 179. van den Bergh ibid. 1970 66 268 1 172 R. J . Donovan and D. Husain containing a particular chemical group and which are thought to yield a particu-lar radical in low-intensity photochemical experiments are examined.If the same spectrum is observed with all of the compounds its assignment would appear reasonably safe. However in some cases even when end-product analysis is carried out the possibility that some minor product with a high extinction coefficient is responsible cannot be entirely ruled out. Data for CH, CF and glyoxal are discussed below. CH,.-The U.V. absorption spectrum of the methyl radical (pl t X) first reported by Herzberg and Sh~osmith,~~' has been employed by two independ-ent groups of workers to measure the absolute rate for recombination., The results are in excellent agreement [k = (4-04 0.4) x lo-" and (4.3 5 0.5) x lo-" cm3 molecule-'^-^ at 300 K ; refs.314 and 313 respectively]; however they are significantly higher than previous values determined using sector techniques and are thus of considerable importance to a large range of kinetic studies which have used the absolute value of the rate of methyl recom-bination as a basis for converting relative rate data to absolute data. The extinction coefficient for the 216 nm band was given yielding3'3.314 a value of f = 1.0 x The extinction coefficients and oscillator strengths for some of the Rydberg transitions in the vacuum-u.v. have also been reported.288b The recombination of methyl radicals was shown to be temperature-insensitive in the range 293-400 K314 and to be independent of pressure down to 3 Torr total pressure. 1 3 * l4 Rather surprisingly the rate of recombination was found to be the same even when 85% of the methyl radicals produced were vibrationally hot.50 Vibrationally excited methyl radicals are produced in the primary photochemical process with dimethylmercury and a significant time-lag in the production of CH in its vibrational ground-state has been observed at low pressures. By monitoring the formation of CH in its ground vibrational state in the presence of a number of added gases quenching efficien-cies have been established. The result for argon together with the Lambert-Salter empirical correlation for vibration-translation relaxation would indicate that a vibrational mode of wavenumber -450 cm- I corresponding to the out-of-plane bending mode is involved in the rate-determining step.However as the photolysis of dimethylmercury occurs in a banded region of the spectrum the excess photochemical energy over that required to break both C-Hg bonds (oiz. 357 kJ mol- I ) should be statistically distributed in the CH vibrational modes. Thus for most of the molecules investi-gated the observed relaxation rates probably represent the sum of multiple relaxation events. The highest efficiencies were observed for SF6 and C2H6 ( - 30 collisions) with helium requiring over 500 collisions for relaxation. 15a The rates of the gas-phase combination of methyl radicals with NO and 0, have been examined by the same two independent group^.^^^,^'^ The results for the high-pressure rate coefficients are not in agreement in this case. How-330 G . Herzberg and J.Shoosmith Canad. J . Phys. 1956 34 52 Reactions of Atoms and Small Molecules 173 ever they do show that the low-pressure fall-off region is accessible using flash photolysis. CF .-The CF3 radical has been observed using rapid-scan i.r. techniques,, ' and more recently the vacuum-u.v. spectrum has been reported.,, A complex spectrum in the region 1 6 6 1 4 6 nm was observed when CF,COCF,, CF3N,CF3 CF3COCOCF3 CF,NO or CF31 were flash-photolysed. A number of bands previously reported by Mathews,' as being due to CF have been reassigned to the CF radical. The decay of CF was monitored and the kinetics shown to be s e ~ o n d - o r d e r . ~ ~ ~ However the derived rate coefficient at 300 K in the presence of 100 Torr Ar ( k 21 5 x 10- l 2 cm3 molecule- s - ') appears to be almost a factor of two lower than that reported from rapid-scan i.r. ~pectrometry.~~ ' Both results show that recombination of CF is signifi-cantly slower than recombination of CH radicals and the work of Pimentel et indicates that this results from a small positive activation energy for CF combination. It has been suggested that the dipole moment present in the non-planar CF radical would provide a repulsive barrier to recombination of approximately the correct magnitude. The cross-combination of CF and CH radicals has been shown to result in the elimination of vibrationally excited HF and has been shown to give rise to laser action under appropriate conditions.333 Glyoxa1.-Resonance fluorescence from the first ' A state of glyoxal following excitation with a pulsed tunable dye laser has been reported.323 The lifetime was found to be 2.16 _+ 0.05 ps and the fluorescence to follow a simple exponen-tial decay. The importance of the three types of decay process (spontaneous emission internal conversion to the 'B and ' A ground-state and intersystem crossing) under collision-free conditions is discussed. Quenching by the gases glyoxal (3.9) He(26) Ar(10-5) Xe(11-6) 02(7.2) D2(15.5) and CH,F(3-1) was (the number of gas-kinetic collisions for quenching is given in parentheses). Similar studies by Rentzepis et a1.,334 for a range of large organic molecules in excited states have been carried out using picosecond flash photolysis. However this rapidly expanding field is unfortunately beyond the scope of this review. The Road goes ever on and on Down from the door where it began. Now far ahead the Road has gone, And I must follow if I can, Pursuing i t with eager feet, Until it joins some larger way, Where many paths and errands meet. And whither then? I cannot say. J . R. R. Tolkien, The Lord of the Rings. 3 3 1 T. Ogawa G. A. Carlson and G. C. Pimentel J . Phys. Chem. 1970,74 2090. 3 3 2 N. Basco and F. G. M. Hathorn Chem. Phys. Letters 1971 8 291. 3 3 3 G. C. Pimentel and M. J. Berry J . Chem. Phys. 1969 49 5190. 3 3 4 M. R. Topp P. M. Rentzepis and R. P. Jones Chem. Phys. Letters 1971 9 1 and references therein