2 Reactions of Atoms and Small Molecules By R. J. DONOVAN Department of Chemistry University of Edinburgh West Mains Road Edinburgh EH9 3JJ and D. HUSAIN and L. J. KIRSCH Department of Physical Chemistry University of Cambridge Lensfield Road, Cambridge CB2 7EP 1 Introduction Since the publication of the last Annual Report on this topic,’ which reviewed the kinetic study of atoms and small molecules by spectroscopic methods for the period 1966-1971 there has been a relatively large output of literature in this field. Also a number of very useful review articles on elementary processes, particularly relevant to the present Report have appeared. This Report will deal with the literature for 1971-1972 with the main emphasis on work em-ploying direct spectroscopic methods for monitoring kinetic species of interest.Of the recent review articles that of Troe and Wagner’ deals with a wide area of chemical kinetics and includes detailed coverage of dissociation and recom-bination reactions molecular beam studies (including ion-molecule collisions), unimolecular reactions and the thermal study of bimolecular reactions. Aspects of theory relevant to these areas are included in the article. An impressive set of review articles concerned with most modern aspects of chemical kinetics has been prepared under the editorship of P ~ l a n y i . ~ This book contains ten chapters and includes the consideration of atomic and bimolecular reactions chemi-luminescent reactions molecular beam reactions ion-molecule reactions and energy-transfer processes.A detailed review on the use of electron paramagnetic resonance for the quantitative determination of atomic and radical concentrations has been given by We~tenberg,~ and includes a bibliography of kinetic studies employing this technique. An article which is particularly useful for a consideration of processes involving metastable noble-gas atoms (not dealt with in the present Report) is that of R. J. Donovan and D. Husain Ann. Reports ( A ) 1971 68 124. ’ J . Troe and H. Gg. Wagner Ann. Rev. Phys. Chem. 1972 23 3 1 1. ‘MTP. International Review of Science Physical Chemistry Series 1 ’ Vol. 9 ‘Chemical Kinetics’ ed. J. C. Polanyi Butterworths London 1972. A. A. Westenberg Progr. Reaction Kinetics 1972 7 23. 1 20 R. J . Donovan D. Husain and L. J.Kirsch Rundel and Stebbings.’ The study of energy transfer from states pumped by laser transitions has been reviewed in a wide-ranging article on ‘Lasers in Chemistry’ by Moore.6 A detailed and critical review of the addition reactions of atoms and radicals with alkenes alkynes and aromatic molecules has been given by Kerr and Parsonage.’ This contains extensive tables of ‘preferred values’ of rate constants. Rate data for many of the reactions involving sulphur-containing radicals and molecules (including atomic sulphur) have been reviewed by Cullis and Mulcahy.8 A collection of papers highly representative of much present work on the reactions of atoms and molecules in excited states is that resulting from the recent Faraday Discussion on this ~ubject.~ Many of the investigations reported were concerned with direct spectroscopic methods for monitoring transient species in kinetic processes.A number of extensive compilations of kinetic data for elementary processes have also been published recently. The first volume in a series ‘Evaluated Kinetic Data for High Temperature Reactions’ based on the Leeds University reports on ‘High Temperature Reaction Rate Data’ and dealing with reactions in H2-0 systems has appeared.” The material from the original reports has been revised and updated. A compilation concerned with reviews of kinetic data, that have already been published and also those in preparation has also appeared recently.’ A survey of the kinetics of bimolecular and termolecular reactions up to the end of 1969 has been given by Kondratiev.” A bibliography on chemical kinetics in C-0-S and H-N-0-S systems listing published papers and reports on the gas-phase reactions of species containing C-0-S and H-N-O-S and complete up to June 1971 has also appeared.13 2 Monatomic Species Hydrogen.-The highly sensitive method of monitoring hydrogen atoms by means of the Lyman a transition (H22P1,,,3,2-12S1,2 ; 1 = 121.6 nm) either by absorp-tion attenuation or resonance fluorescence following the production of ground-state hydrogen atoms continues to be employed.Ahumada Michael and R. D. Rundel and R. F. Stebbings in ‘Case Studies in Atomic Collision Physics’ ed. E. W. McDaniel and M. R. C. McDowell North Holland Amsterdam 1972 v01.2. C. B. Moore Ann. Rev. Phys. Chem. 1971 22 387.’ J. A. Kerr and M. J. Parsonage ‘Evaluated Kinetic Data and Gas Phase Addition Reactions. Reactions of Atoms and Radicals with Alkenes Alkynes and Aromatic Compounds’ Butterworth London 1972. C. F. Cullis and M. F. R. Mulcahy Combustibn and Flame 1972 18 225. Papers in Faraday Discuss. Chem. Soc. 1972 no. 53. l o ‘Evaluated Kinetic Data for High Temperature Reactions’ Vol. 1 ‘Homogeneous Gas Phase Reactions of the H,-02 System’ by D. L. Baulch D. D. Drysdale D. G. Horne, and A. C. Lloyd Butterworth London 1972. CODATA Bulletin No. 3 ICSU CODATA Central Office 6 FrankfurtiMain Germany, 1971. V. N. Kondratiev ‘Rate Constants of Gas Phase Reactions Reference Book’ (trans-lated from the Russian) ed. R. M. Friston National Technical Information Service, Springfield Va.22151 U.S.A. (COM-72-10014). 1972. l 3 Nat. Bur. Stand. Special Publication 362 ‘Chemical Kinetics in the C-0-S and H-N-0-S Systems’ by F. Westley U.S. Government Printing Office Washington D.C., 20402 U.S.A. 1972. Reactions of Atoms and Small Molecules 21 O ~ b o r n e ' ~ have made an extensive study of the recombination of hydrogen atoms with 0 ,NO and CO. The atoms were generated by the transient mercury-photosensitized decomposition of molecular hydrogen and monitored in absorption by means of the Lyman a transition. These authors report the rate data shown in Table 1. Where a comparison can be made the data for re-Table 1 Rate constants (k) for hydrogen-atom-molecule recombination studied by absorption of Lyman a-radiation (room temperature) H + O + M - + H 0 2 + M M 1032k/~m6 molecule-' s-l H2 Kr He Ar Ne 1.2 * 0.1 1.1 & 0.1 0.75 f 0.33 0.60 & 0.04 0.15 & 0.03 H + N O + M + H N O + M M 1032k/~m6 molecule-2 s - l H2 Kr He Ar Ne 6.3 f 0.3 5.4 f 0.3 4.3 * 0.1 3.9 f 0.1 2.1 * 0.1 H + C O + M -+ HCO+M M 1032k/~m6 s - l H2 Kr Ar He Ne 0.8 & 0.04 0.69 f 0.03 0.62 0.07 0.60 +_ 0.07 0.48 f 0.05 combination with 0 and NO differ from those of Dorfmann and Bishop,' who employed pulsed radiolysis by about a factor of three.The data of Ahumada et a1.14 are however one to two orders of magnitude greater for recombination with CO. These authors14 suggest that energy transfer and intermediate complex mechanisms may be operative for H + NO and H + O2 with heavy monatomic third bodies.Kurylo' has employed Lyman ct resonance fluorescence following flash photolysis in order to investigate the temperature dependence of the reaction rate of H + O2 + M and has found that in the temperature range 2 0 3 4 0 4 K, 1.99 & 0.38 kJ mol-RT k, = 6.66( + 1.2 - 1) x exp ( l 4 J . J. Ahumada J . V. Michael and D. T. Osborne J . Chem. Phys. 1972 57. 3726. I s W. P. Bishop and L. Dorfmann J . Chem. Phys. 1970,52 3210. M. J . Kurylo J . Phys. Chem. 1972. 76 3518. 1 22 R. J. Donovan D. Husain and L. J. Kirsch Relative efficiencies for the deactivation of the upper state (22P1/2,3/2) are also reported namely, CH N He Ar = 15.7 3.4 1.0 1.0 at 298 K, N He = 4.5 1.0 at 226 K. The same technique has been employed by Welge et al.' ' to study the reaction of H + CH,O for which the rate constant k(297 K) = 5.4 _+ 0.5 x cm3 molecule- ' s- is found.The cross-section for Lyman a emission on the collision of H+ and H in the 1-25 keV energy range with CO and CO, and CH and NH, are reported by Birely and McNeal.18 These authors report qualitative results for the production efficiencies of H(2,P) and H(2,S) from H+ and H as a function of kinetic energy. Westenberg and de Haas" have also studied H atoms directly and also OH and 0 by e.p.r. in a flow system following which they report relative rate con-stants for the reactions of H and HO : H + HO -% H + 0 (2) H + H O * O H + O H (3) H + HO 5 H 2 0 + 0 H + 0 + M -% HO + M(M = Heor Ar) (4) k k k = 0.62 0.27 0.1 1.The rate of the reaction (1) is also measured yielding k = 1.9 cm' molecule-2 s- ' in good agreement with the recent work of Kurylo'' and the early work of Clyne and Thrush" (2.2 x Wagner and his co-workers have also employed e.p.r. spectroscopy for the measurement of hydrogen-atom concentrations as well as using mass spectro-metric and gas chromatographic analysis for product measurement. This work has recently resulted in the characterization of a number of rate processes for the hydrogen atom namely, 0.3 x cm6 molecule-' s-'). H ,C-C=CH,* kY H + H3C-C-CH kb H,C-CH=CH2* -8.4 f 0.8 kJ mol-' RT k = 1.1 0.02 x lo-l'exp ( ) cm3 molecule- 1 s- 1,'' *' B. A. Ridley J. A. Davenport L. J. Stief and K. H. Welge J . Chem. Phys. 1972 57, l 6 J.H. Birley and R. J. McNeal J . Chem. Phys. 1972,56 2189. l 9 A. A. Westenberg and N. de Haas J . Phys. Chem. 1972,76 1586. 2 o M. A. A. Clyne and B. A. Thrush Proc. Roy. SOC. 1963 A275 559. 2 1 H. Gg. Wagner and R. Zellner Ber. Bunsengesellschaft phys. Chem. 1972,76 518. 520 Reactions of Atoms and Small Molecules 23 H3C-C=CH2* (7) k9 k h 4 H + H,C=C=CH, cm3 molecule- s- ',, [ -8.4 f ";("T" mol-' k = 1.4 f 0.3 x lo-" exp cm3 molecule- ' s- 1,22 1 - 11.3 rf 1.7 kJ mol- ' k = 7 rf 3 x 10-12exp iso-C,H * (9) kP H + H3C-CH=CH2 k b n-C3H,* (10) k = 9.0 1.0 x 10-12exp cm3 molecule- s- 1,23 ( - 5.2 rf yTkJ mol-cm3 molecule- ' s- 1,23 1 - 11.5 rf 0.4 kJ mol- k = 7.3 rf 1.0 x 10- l 2 exp H + N2H3 2NH2 (12) cm3 molecule- s- 1,24 ( - 10.4~Tmo1- ' k = 2.2 x lO-"exp and k, = 2.7 x lo-" (3111 molecule-' s-1.24 Various methods of mass spectrometric analysis in some cases coupled with other analytical techniques have been employed to determine rate data for hydrogen-atom reactions.Teng and J ~ n e s ~ ' ~ ~ ~ have characterized rate constants for the reactions of H atoms with ethylene vinyl fluoride 1,l-difluoroethylene 2 2 H. Gg. Wagner and R. Zellner Ber. Bunsengesellschaff phys. Chem. 1972 76 667. 23 H. Gg. Wagner and R. Zellner Ber. Bunsengesellschaft phys. Chem. 1972,76,440. 2 4 M . Gehring K. Hoyermann H. Gg. Wagner and J. Woifrum Ber. Bunsengesellschafr " L. Teng and W. E. Jones J.C.S. Faraday I 1972,68 1267. '' L. Teng and W. E. Jones J.C.S. Faraday I 1972,68 189. phys. Chem. 1971,75 1287 24 R. J . Donovan D.Husain and L. J . Kirsch and various resulting intermediates. Rommel and Schiff2 have measured the reaction rates of H atoms in a flow system with H,S and COS and report kHzS = 3.8 x and k,, = 2.2 x 10-14cm3 molecule-'s-'. A fast flow system has been employed by Phillips et to investigate the process H + ICN -% HCN + I (13) These authors report k 1 3 = 3.2 k 1.0 x cm3 molecule-'s-' (296 K). has employed mass spectrometric analysis to measure the relative rates of isotopic exchange arising from the reactions of H with DCI. Studies of translationally energized H atoms by product analysis include the measurements on deuterium-atom abstraction from various ethanes and silanes by Hong and Mains,30 who found reaction with the silicon compounds more rapid than with the carbon analogues.A classical photochemical study of H atom abstraction from silanes by normal D atoms following the photolysis of C2D4 at 2 = 202.2 nm has been reported by Gunning et ~ l . ~ who measured absolute rate data. The reaction of 'hot' H atoms generated by flash photolysis of HI with COS has been reported by Oldershaw and Porter,32 who found that the probability of S atom abstraction was reduced by moderating collisions. Fink and Nicholas33 have measured the energy threshold (36 rf 3 kJ mol- l) for the reaction of translationally energized D atoms with cyclohexane. Some relative rates of the reaction of 'hot' H atoms with CO and N,O following the flash photolysis of HI are described by Tomalesky and S t ~ r m . ~ ~ Other kinetic investigations of hydrogen atoms include a cross-beam study of D + H by Krause et who report k(1400 K) = 1.2 & 0.6 x lo-'' cm3 molecule-'s-'.Gann and D ~ b r i n ~ ~ have computed the average value for the reaction cross-section of H + HBr .+ H + Br over the collision-energy range 0.35-1.7 eV from previous atom-recoil data37 and find Q = 1.6 & 0.3 A*. S ~ h o f i e l d ~ ~ has estimated the rate constant for the reaction H + O,(a'A,) -!% OH + 0 (14) from the analysis of atmospheric observations of O,(a'A,) current atmospheric models and established experimental rate constants and reports k < 3 x 10- cm3 molecule- s- '. This must be considered a crude estimate as k14 5 6.5 x 10- cm3 molecule- s- 1.39 Finally we mention the investigations 2 7 H. Rommel and H. I .Schiff Internat. J . Chem. Kinetics 1972 4 547. G . P. Horgan M. R. Dunn C. G. Freeman and L. F. Phillips J . Phys. Chem. 1972, 76 1392. 2 9 G. 0. Wood J . Chem. Phys. 1972 56 1723. 30 Kong-yi Hong and G. J. Mains J . Phys. Chem. 1972 76 3337. 3 1 3 2 G. A. Oldershaw and G. Porter J.C.S. Faraday I 1972,68,709. 3 3 R. D. Fink and J. E. Nicholas J.C.S. Faraday I 1972 68 1706. 3 4 R. E. Tomalesky and J. E. Sturm J.C.S. Faraduy IZ 1972 68 1241. 3 5 J. Geddes H. F. Krause and W. L. Fite J . Chem. Phys. 1972 56 3298. 36 R. G. Gann and J. Dubrin J . Phys. Chem. 1972,76 1321. 3 7 3 8 39 R. L. Brown J . Geophys. Res. 1970 75 3935. K . Obi H. S. Sandhu H. E. Gunning and 0. P. Strausz J . Phys. Chem. 1972 76,391 1. R. G. Gann W. M. Ollison and J. Dubrin J . Chem.Phys. 1971 54 2304. K. Schofield Internat. J . Chem. Kinetics 1972 4 255 Reactions of Atoms and Small Molecules 25 by Falconer and S ~ n d e r ~ ' ~ ' on the effect of deuterium substitution on the reaction of H(D) atoms with propene. These authors report the effects on relative rates of atom abstraction addition and disproportionation-recombina-tion ratios for deuteriated propyl radicals. Sodium and Potassium.-The emphasis of kinetic investigations on Group I atoms over the past year has been concerned with the excited state of sodium, (3,PJ). Krause Fricke and White4 have carried out a set of particularly detailed experiments on the observation of sodium D-line emission (3,PJ -+ 3,S1,,) following excitation of sodium atoms by internally excited H, D, or N in crossed molecular beams.As a result they report excitation cross-sections for N as a function of the vibrational level u and Av the most probable transition (Table 2). The alternative values presented in Table 2 arise from the use of a Table 2 Cross-sections (a) for the excitation of Na(3,Sl,,) -+ Na(32PJ) by vibrutionally excited molecular nitrogen Most probable transitions Av q>,,./A2 11-3 10-3 8-2 6- 1 4-0 271 190 3001640 751290 2011 70 13/78 particular chosen form for the kinetic energy dependence of the excitation cross-~ection.~~ Krause et compare their results with the ionic curve-crossing theory of Bauer et for the quenching of Na(3,PJ) by N, by invoking the principle of detailed balancing. The excitation of sodium by vibrationally excited N in a flow system has been studied by Sadowski Schiff and who found CT = 100 A' for the transition in closest resonance (1 1-3).This may be compared with the result of Krause et uL4 (Table 2) for this process. The collisional quenching of Na(3,P) by N and H,O over the temperature range 1500-2500 K has been studied by Linse and E l ~ e n a a r ~ ~ who employed atomic resonance fluorescence of the sodium d i n e s in a flame. The quenching cross-sections (0) for both gases were found to be independent of temperature over this range and given by 4 0 W. E. Falconer and W. A. Sunder Internat. J . Chem. Kinetics 1972 4 307. 4 ' W. E. Falconer and W. A. Sunder Internat. J . Chem. Kinetics 1972 4 315. 4 2 H. F. Krause J. Fricke and W. L. Fite J . Chem.Phys. 1972 56 4593. 4 3 E. Bauer E. R. Fisher and F. R. Gilmore J . Chem. Phys. 1969 51,4173. 4 4 C. M. Sadowski H. I . Schiff and G. K. Chow J . Photochem. 1972 1 23. P. L. Lime and R. J. Elsenaar J . Quant. Spectroscopy Radiative Transfer 1972 12, 1115. 4 26 R. J . Donovan D. Husain and L. J. Kirsch However the cross-section for N was also found to be ca. one half that at 400 K, derived from bulb experiments. This was attributed to either a strong dependence of the cross-section on velocity a t low velocity or differing cross-sections for different vibrational levels (see Table 2). Earl et have investigated the collisional quenching cross-section for Na(3,P) by various gases at different distributions of laboratory speeds for the sodium atom. This was achieved by the photodissociation of NaI in the wavelength range 190-250 nm with analysis of the d i n e emission in the presence of the added gases.Over the range 1.0-2.5 km s-’ the quenching cross-section (a) was found to vary as g-,’” where g = the relative collisional velocity and s is in the range 4-6. For a fixed value of g cr was found to vary as I > SO > C6H > CH3CN > CF3C1 > C2H4 > By reference to the experiments of Linse and EIsenaar,4’ Linse4’ has obtained an improved correlation between experiment for the quenching of Na(3,P’) by N and theory involving an ionic curve-crossing mode1.43,48,49 This was achieved by the inclusion of an attractive part in the interaction potential on collision. On the basis of this model Fisher and Smith49 conclude that cr is insensitive to the vibrational level in N and that the effect of N2(v) cannot account for the variation in the quenching cross-sections of alkali-metal atoms at low and high temperature.There would therefore appear to be some invariance between this conclusion49 and the results of Krause et aL4 (Table 2). Fontijn et al.” describe preliminary data for the reaction of Na + 0 + M, studied by atomic line absorption in a flow reactor at 1200K and report k x loh3 cm6 molecule-2 s - ’ for this three-body process. Bernstein and c o - w o r k e r ~ ~ ~ * ’ ~ have carried out a crossed molecular-beam study of K + CH31 over a range of collision energy. It was found that the total cross-section is ca. 50 A2 peaking at a collision energy of 0.18 eV. Ross et ~ 1 .’ ~ have reported on the effect of internal and translational energy on the nature of the collision of potas-sium atoms with SF, Ccl, and SnC1 in a crossed molecular-beam study. Calcium Strontium Barium and Thallium.-By contrast with previous years, there have been a number of kinetic investigations on Group 11 atoms especially barium. Particularly impressive are the beam experiments of Zare and his co-worker~.’~~’’ Zare et aLS4 have studied the electronic chemiluminescence from metal oxides in crossed molecular-beam studies of Ba Sr or Ca with N,O or NO in an attempt to ‘marry the techniques of molecular beams and molecular ~pectroscopy’.~~ These workers report metal oxide excitation cross-sections for CO,. 46 B. L. Earl R. R. Herm S.-M. Lin and C. A. Munns J .Chem. Phys. 1972,56 867. 4 7 P. L. Linse Chem. Phys. Letters 1973 18 73. 4 8 E. R. Fisher and G. K. Smith Appl. Optics 1971 10 1803. 4q E. R. Fisher and G. K. Smith Chem. Phys. Letters 1972 13 448. A. Fontijn S. C. Kurzius J. J. Houghton and J. A. Emerson Rev. Sci. Instr. 1972, 43 726. ’’ M. E. Gersch and R. B. Bernstein J . Chem. Phys. 1972,56 6131. 5 2 A. M. Rubis and R. B. Bernstein J . Chem. Phys. 1972,57 5497. 5 3 T. M. Ross S. Y. Young and J. Ross J . Chem. Phys. 1972,57 2745. 5 4 C. D. Jonah R. N. Zare and Ch. Ottinger J . Chem. Phys. 1972 56 263 Reactions of Atoms and Small Molecules Table 3 Metal oxide electronic excitation cross-sections (a) for collisions of Group II atoms 27 Collision OrdeP CIA2 aca,cIA2 Ba + N,O 1 27 Ba + NO 1 165 150 Sr + N,O 1 < 27 Ca + N,O 1 16 Ca + NO 2 93 70 Kinetic order in added gas pressure (first order in metal atom concentration).Calculated cross-sections employing an ‘electron jump’ model. Sr + NO 2 127 100 the atomic collision processes (Table 3). Schultz Cruse and %ress were also the first workers to employ laser-induced fluorescence for the observation of product u,J levels in a molecular-beam experiment. This method was applied to the study of the reaction Ba + 0 -+ BaO + 0. The fluorescence from BaO was excited by means of a tunable dye laser. The initial vibrational distribution in BaO was found to be Boltzmann in type and corresponded to a vibrational temperature of cu. 2500K. The results suggest the formation of a collision complex.” Lin et ul.s69s7 have reported beam studies of the reaction dynamics of Ca Sr or Ba with HI and Mg Ca Sr or Ba with Cl or Br,.Obenauf Hsu and Palmer5* have studied the reaction of Ba + 0 by observa-tion of the BaO emission from a diffusion flame. They propose a mechanism for electronic excitation of BaO via a BaO complex. These workers have further studied the process Ba(g) + N,O or NO -% BaO(A ‘C or X ‘C) + N or NO by measurement of the A ‘C-X ‘C emi~sion.~’ The photon yield for N,O was found to be 0.0039 and that for NO was 0.00033. This latter value for NO is consistent with previous data of Ottinger and which leads to the con-clusion that the major route on reaction is to the ground state of BaO. Using Zare’s data,60 Palmer et a1.” were able to place limits on the cross-sections for reaction (ure) to the BaO(A ‘C) state namely for N20,ure < 6-7A2 and for Naumann and Miche16’ have measured relaxation times for electronic excita-tion of barium and thallium atoms behind reflected shocks using atomic line absorption spectroscopy.From this they have obtained quenching cross-sections which were not found to be noticeably dependent on temperature (15) NO,,a, < 2-381,. 5 5 A. Schultz H. W. Cruse and R. N . Zare J . Chem. Phys. 1972,57 1354. ” C. A. Munns S.-M. Lin and R. R. Herm J . Chem. Phys. 1972,57 3099. 5 7 S.-M. Lin C. A. Munns and R. R. Herm J . Chem. Phys. 1972,58 327. 5 8 R. H. Obenauf C. J. Hsu and H. B. Palmer Chem. Phys. Letters 1972 17 455. 5 9 R. H. Obenauf C. J. Hsu and H. B. Palmer J . Chem. Phys. 1972 57 5607; 1973, 58 2674.‘O Ch. Ottinger and R. N. Zare Chem. Phys. Letters 1970,5 243. 6 ’ F. Naumann and K. W. Michel 2. Physik 1972,255 3480 28 R. J. Donovan D. Husain and L. J. Kirsch Table 4 Quenching cross-sections (G) for barium and thallium atoms Collision IS/@ Ba(6ID2) + Ar 0.12 T1(62P3,2) + Ar 4 10-3 Ba(63D,) + Ar 0.09 5 (Table 4). Using a Landau-Zener approach these authors concluded that the pseudo-crossing for the Ba-Ar collision takes place within 0.25 eV of the D levels of barium; further that spin-orbit coupling is not rate-controlling in the quenching of BzI(~~D,) to Ba(6'So). For Ba(6'D2) the collision deactivation constants for Ar and He were found to be in the ratio of ca. 3 and it was therefore suggested that polarizability may be important in the quenching process.The self-quenching of Tl(62P3,2) by Tl(62P1,,) was found to be characterized by a collision cross-section of 5 A2.61 Linse et a1.62 have studied the collisional quenching of electronically excited strontium atoms Sr(5'P1) in a flame using resonance fluorescence (51P,-51s0, ;1 = 460.7 nm). They present deactivation cross-sections which constitute the first reported kinetic data for this state (Table 5). Table 5 Collision quenching cross-sections (a) for Sr(5'P1) Collision partner I S p 21 * 2 67 6 22 * 5 400 80 Mercury Cadmium and Iron.-Extensive work continues to be carried out on the 63P state of the mercury atom especially the 3P0 and 3P1 levels. Particularly interesting is the experimental development of kinetic studies of H S ( ~ ~ P ~ ) , characterized by an emission lifetime of ca.lo3 greater than that of Hg(63Pl) + Hg(6'So) at 253.7 nm.63 The basis of the method is a crossed molecular-beam technique in which Hg(63P2) generated by electron bombardment undergoes spin-orbit quenching to the 63P state, Hg(63P2) + M -% Hg(63P1) + M (16) followed by measurement of the strong emission at 253.7 nm. This method was first described by Martin et who reported a number of relative quenching cross-sections for various gases. These authors have recently extended this work in considerable detail65 and report the relative cross-sections given in Table 6. " T. J. Hollander P. L. Linse L. P. L. Franken B. J. Jansen and P. J. Th. Zeegers J . b 3 P. Baltayan and J. Pebay-Peyroula Compt. rend.1965 260 6569. 64 L. J. Doemeny F. J. Van Itallie and R. M. Martin Chem. Phys. Letters 1969 4 302. 6 5 F. J. Van Itallie L. J. Doemeny and R. M. Martin J . Chem. Phys. 1972 56 3689. Quant. Spectroscopy Radiative Transfer 1972 12 1067 Reactions of Atoms and Small Molecules 29 Table 6 Relative collisional deactivation cross-sections (a) for intramultiplet quenching of Hg(63P2) -+ Hg(63P,) C*H, C2D6 C2H4 cis-2-C4H, n-C4H 10 n-C4D 10 C6H6 CyClO-C6H1, CYCIO-C,D~~ ‘ZLF6 He Xe HD H2 D2 C,,l 1.00 0.72 0.65 0.045 0.23 0.21 < 0.006 0.065 0.020 0.79 0.63 0.26 (0.22) (0.18) (0.08) 0.052 0.24 0.31 0.021 0.23 0.23 0.014 < 0.005 < 0.003 0.504 0.268 0.143 These data are compared with existing data for total and intramultiplet quenching of Hg(63P1).The results for the process Hg(63P2 + indicate (a) internal degrees of freedom are required when the quenching molecule lacks excited states below Hg(63P2); (b) energy resonance is not a major factor ; and (c) the large differences in cross-sections observed with some molecules can be accounted for in terms of competitive processes such as the observation of small intramultiplet quenching by chemically reactive species. Similar experiments were later reported by Krause et who employed velocity selection and whose results are compared with those of Martin et ~ l . ~ ~ where the relative cross-sections are normalized to unity for nitrogen (Table 7). These authors66 observed some indication of struc-ture in the curves for the velocity dependence of o for H and N .Quantum yield data for intramultiplet quenching Hg(63P -.o) is presented in a detailed kinetic spectroscopic investigation by Callear and McGurk6’ on all levels of Hg(63P0,,,,). By employing a quantum yield of unity for N2,68 these 6 6 H. F. Krause S. Datz and S. G. Johnson J . Chem. Phys. 1973 58 367. 67 A. B. Callear and J. C. McGurk J.C.S. Furuduy ZZ 1973 69 97. 68 A. B. Callear and R. E. M. Hedges Trans. Furuduy Soc. 1970 66 605 30 R. J . Donovan D. Husain and L. J. Kirsch Table 7 Comparison of velocity-selected and velocity-averaged relative collisional cross-sections (a) for Hg(63 Pz ~ 1) intramultiplet quenching a(0.050 eV velocity CT (velocity Quenching gas selected)66 ~veraged)~’ N2 1.00 1 .oo NO 0.92 0.90 CH4 0.78 0.87 D2 0.18 0.20 H2 0.58 0.70 authors report quantum yields for the process 63P -ro relative to the total relaxa-tion of Hg(63P,).These results are in good agreement with those of LeRoy et a1.,69 who employed a method based on product analysis of the mercury-photo-sensitized decomposition of ethylene which is enhanced by the long-lived state Hg(63P0) (Table 8). Total cross-sections for the quenching of Hg(63P,) by isotopic Table 8 Quantum yields (0) for intramultiplet quenching Hg(63P +,) relative to total quenching for Hg(63P1) 0 (Callear and Quenching gas McGurk6’) 0 (LeRoy et ~ 1 . ~ ~ ) NO < 0.10 co 0.90 0.88 & 0.07 < 0.03 --H2 0 2 co2 N2O NH3 ND3 H2O D2O CH4 0.10 - - 0.02 < 0.0 1 < 0.10 -0.64 1.05 0.05 - 1.00 f 0.08 0.76 f 0.18 0.50 - 1.06 & 0.26 0.11 -C2H6 0.67 0.64 f 0.16 0.21 0.14 & 0.01 - 0.47 f 0.10 - 0.51 f 0.07 - 0.05 & 0.01 iso-C4H CYC~O-C~H - 0.56 f 0.10 - 0.58 f 0.02 C3H6 C3D6 2,2-C3H6D2 n-C4H 10 0 .1 1 ~ ~ 0.11 f 0.01 -C(CH3)4 C2H4 <0.10 (H and D) aromatic molecules containing a single benzene ring have been reported by Mains and Tra~htrnan,~’ who employed a modified Stern-Volmer type of experiment. The resulting cross-sections all lay in the range 48-74A ’. 69 A. Vikis G. Torrie and D. L. LeRoy Canad. J . Chem. 1972 50 176. ‘ O G. J. Mains and M. Trachtman J . Phys. Chem. 1972,76 2665 Reactions of Atoms and Small Molecules Cross-sections for the production of Hg(63P) following energy transfer from metastable diatomic molecules have been reported.Van Itallie and Martin7' have studied the velocity dependence of the process 31 C0(a3ll) + Hg(6'So) A CO(X 'C+) + Hg(63P1) (17) by time-of-flight sensitized fluorescence in a molecular beam. CO(a311) was generated by electron bombardment and the time-of-flight distribution of CO and the Hg 253.7 nm emission were measured. It was found that c1 depended on velocity (u) in the form 017 K u - ~ * * * ~ . ' over the range u = 300-700 m s-'. Thrush and Wild72 have measured the rate constant for the excitation of Hg(6,PO) in active nitrogen from the process N,(A ,C,+) + Hg(6'So) J-% N,(X 'C,') + Hg(6,Po) (18) and report k18 = 3.4 _+ 1.0 x lo-'' cm3 molecule-'s-'. This is in good agreement with the kinetic spectroscopic measurement of this rate process reported by Callear and Wood7 of k = 2.9 & 0.15 x lo-'' cm3 molecule-' S-'.Callear and his c o - ~ o r k e r s ~ ~ ~ have further reported measurements on the lifetimes and stabilities of complexes following the attachment of NH molecules to Hg(6,PO). This was carried out by photoelectric measurement of the lumines-cence from such complexes in the range ca. 260-500nm following pulsed, monochromatic excitation (A = 253.7 nm) of Hg-NH mixtures up to pressures of 10atm.75 They report complexes of formula Hg(NH3),* [from Hg(6,PO) + NH,] where n = 1 4 . 7 5 These authors also report emission lifetimes for Hg(NH3)* and Hg(ND,)* of ca. 1.8 ps,75 in excellent agreement with previous results from the luminescence phase-lag measurements of Phillips et ~ 1 .~ ~ 9 ' Callear et ~ 1 . ~ ~ have further characterized (Table 9) the dissociation constants, K, for the reaction Hg(NH3),* 5 Hg(NH,):- + NH, Table 9 Dissociation constants K for Hg(NH3)n* (n -+ n - 1) Species KJmoIecule cm -Hg(NH3)2* Hg(NH,),* 5.05 0.70 x l O I 9 Hg"H3)4* 9.1 1.8 x 10'' 6.2 & 0.60 x 10" Hg(NH3)5* z 2 . 2 x lozo 7 ' F. J. Itallie and R. M. Martin Chem. Phys. Letters 1972 17 447. '' B. A. Thrush and A. M. Wild J.C.S. Faraday I I 1972 68 2023. '' A. B. Callear and P. M. Wood Trans. Faraday SOC. 1971 67. 272. '' J. Koskikallio A. B. Callear and J. H. Connor Chem. Phys. Letters 1971 8,467. 'Is A. B. Callear and J. H. Connor Chem. Phys. Letters 1972 13 245. C. G. Freeman M. J. McEwan R. F. C.Claridge and L. F. Phillips Chem. Phys. Letters 1970 5 5 5 5 . 7 7 R. H. Newman C. G. Freeman M. J. McEwan R. F. C. Claridge and L. F. Phillips, Trans. Faraday SOC. 1970,66 2827. 7 32 R. J . Donovan D. Husain and L. J . Kirsch Direct measurements on HgH(D) following abstraction from a hydrogen-containing molecule by Hg(63P1) have been continued. Callear and Hedges7' originally employed microwave pulsed excitation to generate the electronically excited mercury atoms. Callear and M ~ G u r k ~ ~ have now used monochromatic resonance flash excitation at 1 = 253.7 nm coupled with spectrographic detection of the electronic absorption spectrum of HgH(D) to measure the yield of this molecule. The results are given in Table 10. These yields are seen to be in sensible Table 10 Absolute yields of HgH(D) from the reaction of Hg(63Pl,o) with H, D, or HD Light's Yield' theorys0,* 0.67 5 0.04 0.76 0.05 0.13 k 0.02 0.70 & 0.19 1.00 k 0.08 0.88 0.08 0.175 _+ 0.02 0.82 f 0.08 0.52 0.60 0.17 0.43 0.62 0.75 0.17 0.47 To yield HgH.To yield HgD. agreement with Light's theory,80*81 in which it is assumed that a strongly coupled complex is formed initially which fragments distributing the energy statistically amongst the products. Callear and Wood' have photoelectrically monitored the molecule HNO following the mercury flash-sensitized decomposition of H,-NO and H,-NO-C,H mixtures. They report that for the pair of reactions Hg(6'S0) + 2H HgH + H (20) k20/(k,9 + k20) = 0.67 0.04 and that this ratio is 0.76 i- 0.05 for D,.Further, the quantum yield for the decomposition of H by Hg(63P,) is found to be 0.93 0.10. Vikis and LeRoyB3 have monitored the HgH-sensitized emission following the mercury-photosensitized decomposition of hydrogen-containing molecules. '' A. B. Callear and R. E. M. Hedges Trans. Faraday SOC. 1970,66 615. 7 9 A. B. Callear and J. C. McGurk J.C.S. Faraday II 1972 68 289. ' O J . C. Light J . Chem. Phys. 1964,40 3221. '' P. Pechukas and J. C. Light J . Chem. Phys. 1965 42 3281. '' A. B. Callear and P. M . Wood J.C.S. Faraday I I 1972 68 302. 8 3 A. C. Vikis and D. J. LeRoy Canad. J . Chem. 1972,50 595 Reactions of Atoms and Small Molecules 33 Phillips et dS4 have observed emission in the region 380-500 nm during the cadmium-photosensitized irradiation of Cd-NH mixtures.The emission was studied by a phase-shift method from which these authors report that for the process cd(53P0) + NH products (21) k = 1.7 f 0.3 x 10- l 2 cm3 molecule- ' s- ' (560 K). They further report the mean lifetime of the Cd(NH3)* complex as 4.9 f 0.2 x lO-'s. Two studies of the reaction of iron atoms with oxygen may be mentioned. Fontijn et a1.50,85 have described a fast-flow tubular reactor for use at high temperatures (up to 2000 K) and have employed this to study the reaction of Fe + 0 by atomic line absorption spectroscopy. They report that for the reaction Fe + 0 FeO + 0 (22) k = 3.6 & 1.4 x cm3 molecule-'s-' (1600 K). The same reaction has been studied by FeO emission spectroscopy in the region 10.4-16.3pm from shocked mixtures of Fe(C0,)-0 by Van Rosenberg and Wray.86 These latter authors report k 2 5 x lo-' cm3 molecule-' s-' (2400 K).Carbon and Lead.-A limited number of kinetic investigations have been reported for the Group IV atoms carbon and lead. Groth et ~ 1 . ~ ~ have extended to flow systems the use of atomic resonance absorption spectroscopy in the vacuum ultraviolet for the quantitative study of C(23PJ) reported hitherto for time-resolved static systems by Husain and K i r s ~ h . ~ ~ * ~ ~ Carbon atoms were generated in the flow by the reaction of active nitrogen with organic gases and it was demon-strated that the reaction of C + 0 provided a titration technique for carbon atoms. Hence these workers calibrated the line absorption of C(23P,) at 1 = 165.7 and 156.0 nm.It was found that the Beer-Lambert law was obeyed for up to 50% absorption for these transitions. Gosse et have monitored the C, Swann bands from a CO afterglow and shown that C,O is an intermediate in the formation of C,(A 3FIn). They report a rate constant for the reaction C + CO + M -% C,O + M (23) which is in sensible agreement with the previous result of Husain and Rose9' has described a study of 'hot' carbon atoms with H usinga recoil-product-analysis technique and concludes that the reaction between these species yields 8 4 P. D. Morten C. G. Freeman M. J. McEwan R. F. C. Claridge and L. F. Phillips, 8 5 A. Fontijn and S. C. Kurzius Chem. Phys. Letters 1972 13 507. 86 C. W. Von Rosenberg and K. L. Wray J. Quant. Spectroscopy Radiative Transfer, 1972 12 531.'' D. Kley N. Washida K. H. Becker and W. Groth Chem. Phys. Letters 1972 15 45. '' D. Husain and L. J. Kirsch Chem. Phys. Letters 1971 8 543. 89 D. Husain and L. J. Kirsch Trans. Faraday SOC. 1971,9 412. 90 F. Gosse N. Sadegi and J. C. Pebay-Peyroula Chem. Phys. Letters 1972 13 557. 9' Chem. Phys. Letters 1972 16 148. T. L. Rose J . Phys. Chem. 1972,76 1389 34 R. J. Donovan D . Husain and L. J. Kirsch 3CH2 almost entirely. Tschuikov-Roux et ~ 1 . ~ ~ have carried out a mechanistic study of the photolysis of C,02 at 1 = 147 nm in the presence of CH4 and Ar by chromatographic analysis of products. These workers describe evidence for the quenching of C(21D2) by Ar CO or CH,. In particular they argue that the data of Braun et al.93 for the interaction of C(2'D2) with CH4imply somewhat surprisingly that a high proportion (12/13) of the collisions are inelastic.The greater overall removal rate observed by Husain and approaching unit collisional efficiency would not lead to this conclusion. Rebbert and A u ~ l o o s ~ ~ have carried out analyses on the products of the photolysis of methane at I = 104.8 106.7 and 123.6 nm and report quantum yield data for the formation of CH and C(2'D2). The principal recent contribution to the detailed kinetic study of Group IV atoms in defined electronic states arises from the measurements of Husain and Littlerg6-" on the optically metastable electronically excited states Pb(61D2) and Pb(6'So) respectively 2.66 and 3.65 eV above the 63P0 ground state ofatomic lead.'" These atomic states were generated by the pulsed irradiation of tetra-ethyl-lead at very low pressures in the presence of added gases and monitored photoelectrically in absorption by time-resolved attenuation of atomic resonance Table 11 Second-order rate constants (k/cm3 molecule- s - ' 300 K) for the collisional remo*ial of Pb(6'So) and Pb(6lD2) by various gases (M) M He Xe H2 0 2 N2 co NO co2 N2O CH4 C2H2 CF4 C2H4 SF, PbEt, Pb( 6 ' S o ) -<2 x 10-15 < 10-14 1.2 k 0.3 x 10-lo 1.6 f 1.6 x 6.3 f 0.4 x lo-'' 2.1 * 0.2 x 10-'O 3.5 f 0.5 x 10-15 4.2 & 0.3 x 2.3 0.3 x lo-'' 2.3 f 0.3 x -= 10-14 < 5 x 10-l6 1.7 k 0.2 x 10-15 3.7 f 0.4 x lo-" Ref: 98 98 98 99 99 99 99 99 99 99 99 99 99 98 Pb(6 4) < 2 x lo-', <10-15 < 10- 14 < 10- 15 < 10- 1 4 1.1 k 0.3 x lo-'' 9.3 f 0.2 x 10-l2 9.2 & 0.8 x lo-" 4.5 k 0.8 x 10-l2 2.6 & 0.4 x lo-" 2.0 f 0.6 x lo-" < 10- l 4 < 10- 1 4 < 10- 1 5 5.6 f 1.2 x lo-" Ref: 96 96 96 96 97 97 97 97 97 97 97 97 97 97 96 92 E.Tschuikov-Roux Y. Ind. S. Kodama and A. W. Kirk J . Chem. Phys. 1972 56, 9 3 W. Braun A. M. Bass D. D. Davis and J. D. Simmons Proc. Roy. Soc. 1969 A312, 94 D. Husain and L. J. Kirsch Trans. Furuduy. Soc. 1971 67 3166. 9 5 R. E. Rebbert and P. Ausloos J. Photochem. 1972 1 171. 9 6 D. Husain and J. G. F. Littler Chem. Phys. Letters 1972 16 145. 9 7 D. Husain and J. G. F. Littler J.C.S. Furuduy 11 1972 68 2110.9 9 D. Husain and J. G. F. Littler J.C.S. Furuduy 11 1973,69 842. 3238. 417. D. Husain and J. G. F. Littler J . Photochem. 1973,1 327. l o o C. E. Moore Nat. Bur. Stand. Circular 467 'Atomic Energy Levels' Vols. 1-111 U.S. Government Printing Office Washington D.C. 1958 Reactions of Atoms and Small Molecules 35 radiationatA = 374.0 nm ( P ~ [ ~ s ( ~ P ; ) + 6p2('D2)]) and 500.5 nm (Pb[7s('P:)+ 6p2('So)]). This has led to a large amount of data for the collisional quenching of these two atomic states by various gases and these are summarized in Table 11. The main conclusion of this work is that the large spin-orbit coupling in the lead atom prevents the use of spin and orbital correlations for defined states of reactants and products normally employed for light atoms,"' and which were found to be successful in accounting for the collisionalprocesses that C(23PJ 2'D2, and 2lSO) undergo with molecules.94 Nitrogen and Phosphorus.-A number of reactions of nitrogen atoms with molecules have been studied by molecular emission.Golde and Thrush'02 have reported kinetic data on electronically excited nitrogen atoms N(22PJ) produced by the reaction N(24S,,2) + N2(A 3C,f) + N(22PJ) + N2(X 'Cg+) (24) in a flowing nitrogen afterglow by monitoring the atomic emission N(22PJ)--) N(24S3,2) at 346.6 nm the first positive emission of N2(B 311,-A 3Cu') and the NO 6 emission (C 2C+-X 211). By employing the absolute rate data for the col-lisional quenching of N(22PJ) by N 2 0 and O2 of Husain Kirsch and Wiesen-feld,lo3 Golde and Thrushlo2 describe absolute data for the quenching of this state by 0(23PJ) and N(24S3/2) namely k = 7 x 10- l1 and k = 1 x 10- l1 cm3 molecule- s- '.It was further found that k, < 5 x 10- ' cm3 molecule- ' s- '. Campbell and Nea11°4 report the rate constant for the removal of NO(B 211) by N24S3/2) of k - 2.5 x lo-'' cm3 molecule-' s-' by monitoring the NO /3 chemiluminescence (B 211-X 211) in a discharge flow system. Provencher and McKenney'" have described measurements on the emission from CN in a discharge flow system which yield for the process N + CN + M NCN* (electronically excited) (25) k 2 5 = 3 x cm6 molecule-2 s-'. Felder and Younglo6 have also studied the chemiluminescence of NO in a flow system and discuss the factors governing the relative yields of O(2lD2) and N2(u) from the reaction of N + NO.Groth et allo7 have described a detailed investigation of nitrogen-atom recombination in a slow-decaying Lewis-Rayleigh afterglow in a static system. Radiative recombination via the transitions N2(B 311g u' = 13 J'-+ A 3Cu+, u" J") and N2(a 'I& u' = 6 J' = 13 -+ X 'Cg+ u" J") was found to be charac-terizedbyabsoluterateconstantsof1.6 x 10- lgand 1.8 x cm3 molecule-' s-' respectively. A rate constant of 2.4 x cm6 molecule-2 s-' was found l o ' l o * R. J. Donovan and D. Husain Chem. Rev. 1970 70,489. M. F. Golde and B. A. Thrush Faraday Discuss. Chem. SOC 1972 no. 5 3 p. 233. D. Husain L. J. Kirsch and J. R. Wiesenfeld Faraday Discuss. Chem. SOC. 1972, no. 53 p. 201. I o 4 I. M. Campbell and S.B. Neal Faraday Discuss. Chem. SOC. 1972 no. 53 p. 72. I o s G. M. Provencher and D. J. McKenney Canad. J . Chem. 1972 50 2527. I o 6 W. Felder and R. A. Young J . Chem. Phys. 1972 57 572. O 7 K. H. Becker E. M. Fink W. Groth W. Jud and D. Kley Faraday Discuss. Chem. SOC., 1972 no. 53 p. 35 36 R. J . Donovan D. Husain and L. J. Kirsch for the three-body recombination (M = N2) into the levels v' = 9-12 of N2(B ,TIg). Total chemiluminescence from the B ,IIg state formed from col-lisional recombination was found to be characterized by a rate constant of k - 1 x cm6 molecule-2 s- '. Mandlemann Carrington and Younglo8 have employed the radiation of an N + 0 afterglow as a specific light source to photodissociate NO. From this they report a rate coefficient for the radiative recombination N + 0 NO(C 213 V' = 0) + NO(X 211 U" = 0) + h v (26) k y e = 2.9 x 10- l 8 cm3 molecule-' s-' (300 K).Further summed over all v", k26 = 1.5 Jacob and Winkler"' have employed the NO titration technique in a flow discharge for N atom determination coupled with cold trapping of products to measure the rate of the process 0.4 x 10- l 7 cm3 molecule-' s-'. N + SO -% so2 + . . a (27) where k was found to be the order of 5 x cm3 molecule-' s-' (300 K). Hand and Obenauf"' have also used the NO titration method together with time-of-flight mass spectrometry to measure the rate of the reaction N + C4N2 NC,N2* (28) for which it is reported that k 2 8 = 5.3 f 4.3 x lo-'' cm3 molecule-' s-'. An extensive body of data on the collisional removal of electronically excited phosphorus atoms P(32D,) and P(32PJ) respectively 1.40 and 2.32 eV above the P(34S3,2) ground state,'00 has been reported by Husain et al.'11-113 The electronically excited atoms were generated by the pulsed irradiation of PC1, at low pressures and monitored photoelectrically in absorption by attenuation of atomic resonance radiation at the wavelengths shown in Table 12a.The resulting Table 12a L/nm Transit ion 213.55 42P3j2 + 32D0 2 13.62 42P312 +- 32D$: 253.40 4=P312 +- 32PYi2 253.57 42P3j2 * 32P:12 rate data are presented in Table 12b. In general the J levels of the 2D state were not optically resolved during kinetic experiments ; one investigation' did result in the separation of J = 1/2 and 3/2 of P(32PJ) and this demonstrated equal M.Mandelmann T. Carrington and R. A. Young J. Chem. Phys. 1973,58 8 5 . A. Jacob and C. A. Winkler J.C.S. Faraday I 1972 68 2077. A. U. Acuna D. Husain and J. R. Wiesenfeld J . Chem. Phys. 1973 58 494. A. U. Acuna D. Husain and J. R. Wiesenfeld J. Chem. Phys. 1973 in the press. A. U. Acuna and D. Husain J.C.S. Furuduy 11 1973,69 5 8 5 . ' l o C . W. Hand and R. H. Obenauf jun. J . Phys. Chem. 1972.76 2643. 'I Reactions of Atoms and Small Molecules 37 Table 12b Rate constants (k/cm3 molecule- s- ' 300 K) for the removal of P(3'DJ) and P(32PJ) on collision with various atoms and molecules Collision partner He Ar Kr Xe H2 N2 0 2 co NO HCl C'2 co2 N2O PC1, p(32DJ) <5 x 10-l6 <5 x 10-l6 1.7 f 0.3 x lo-" 4.0 f 0.7 x 1.4 k 0.2 x lo-" 1.5 f 0.4 x lo-" 5.5 f 0.6 x lo-" 2.4 f 0.2 x lo-" 1.8 0.2 x 10- l 1 2.6 f 1.1 10-15 <5 x 10-l6 3.3 f 1.0 x 10-l2 1.2 f 0.2 x lo-" 9.7 f 0.9 x lo-" 1.1 * 0.1 x lo-" 7.3 5 0.8 x 10-15 3.9 5 0.7 x 10-13 1.5 f 0.1 x lo-'' CH4 C2H4 CF4 CF,H CF,CI SF 1.5 f 0.3 x C2H6 1.5 f 0.1 x lo-" 6.0 k 0.7 x lo-" 1.3 f 0.1 x lo-'' 8.7 f 0.7 x lo-" CH3-CH=CH2 C2H2 Ref: 112 112 112 112 111 112 111 112 112 113 113 112 112 111 112 112 113 113 113 113 113 113 113 p(32pJ) <5 x 10-l6 <5 x 10-l6 <5 x 10-l6 2.0 f 0.3 x 3.1 f 0.8 x <5 x 10-l6 2.6 & 0.2 x lo-" 7 +_ 6 x 3.0 f 0.5 x lo-" 6.0 f 0.3 x 2.9 f 0.4 x lo-" 7.3 5 1.9 x 3.1 f 0.6 x 1.1 f 0.1 x 10-l0 (2pJ) 1.1 5 0.2 x lo-'' ( J = 1/2) 1.1 0.3 x lo-'' ( J = 3/2) 2.8 f 0.5 x 4.2 f 1.2 x lo-" 1.9 f 0.2 x 2.0 f 0.3 x 2.4 f 0.5 x 2.7 f 0.5 x lo-" 5 x 10-15 1.4 f 0.2 x lo-'' 3.6 f 0.4 x lo-" Ref: 112 112 112 112 111 112 111 112 112 113 113 112 112 111 112 112 112 112 113 113 113 113 113 113 113 rates for the removal of the two J levels which appears to indicate that these close-lying levels are maintained in a Boltzmann distribution throughout reaction.'' For diatomic molecules and the linear triatomic molecules N,O and CO, it was found that the removal rates for the 2D and ,P states were in accord with the correlation of the spin and orbital symmetry of reactants and products assuming weak spin-orbit coupling and a collision complex of C , symmetry.For polyatomic molecules,' l 3 the removal rate constants for both P(3*DJ) and P(3,PJ) exhibited a good correlation with the ionization potential of the quenching gas over a wide range of potentials (9-19eV). The principal gap at present in our knowledge of rate data for the low-lying levels arising from the configuration P(3p3) is that of the 4S ground state. Oxygen Sulphur and Tellurium.-The large body of investigations on atomic kinetics in Group VI has been concerned primarily with oxygen atoms in the three low-lying states of the 2p4 configuration O(23PJ) 0(2'D,) and 0(2'5,), as expected for this aeronomically important species. There have also bee 38 R. J . Donovan D. Husain and L.J. Kirsch developments in methods of studying sulphur atoms both by resonance fluores-cence and spectroscopic marker methods which indicate a future development in detailed studies on S atoms in defined states for a wide range of fundamental processes. 0(2'S0). The forbidden emission at A = 557.7 nm from 0(2'S0)-+ O(2'0,) + hv continues to constitute the basis of kinetic methods for monitoring this optically metastable excited state. The temperature dependence of collisional quenching by simple molecules is the main advance that has taken place for this state. Welge and co-workers' ' 4*1 have employed the pulsed photolysis of CO,, followed by time-resolved atomic emission from 0(2'S0) whereas Slanger and co-workers'16 have used N,O as the parent molecule.The results from these investigations for quenching by molecular oxygen show very good agreement (Table 13). Further Welge and Atkinson" found enhancement of the emission by N and Ar to be independent of temperature. These authors report k, < 5 x lo-'' and k, < 5 x cm3 molecule-' s-'. Table 13 Rate constants (k) for the temperature dependence of the collisional quenching of 0(2lSO) Quenching gas k/cm3 molecule- ' s- ' Re.$ 114,115 1 - 11.0 0.8 kJ mol-' co2 3.3 x l o - ' ' exp 0 2 1 -7.1 0.8 kJ mol-' 4.9 x exp -7.2 1.2 kJ mol-' 4.0 x 10-"exp 115 116 Felder and Young' l 7 have employed the emission at 557.7 nm to study the quenching of 0(2'S0) generated from the pulsed photolysis of N,O at 147 nm in a flow system by ground-state oxygen atoms.Using the N + NO titration method as the source of O(Z3PJ) these authors report koJP = 7.5 x lo-' cm3 molecule-'s-' which is ca. 50% greater than the previously reported value.' 18,119 Felder and Young also conclude that kNeS < lo-' cm3 mole-cule- ' s- '.' l 7 They argue that the above rates for quenching by 0(23PJ) imply that the processes 0 + 0 + 0 -+ 0 + 0(2'S0) N + 0 + 0 + NO + 0(21S0) (29) (30) and N + N + 0 -+ N + 0(2'S0) (31) K. H. Welge A. Zin E. Vietzke and S. V. Filseth Chem. Phys. Letters 1971 10 13. 'Is P. Atkinson and K. H. Welge J . Chem. Phys. 1972 57 3689. 'I6 T. G. Slanger B. J. Wood and G. Black Chem. Phys. Letters 1972 17,401. W. Felder and R. A. Young J . Chem. Phys. 1972 56 6028 Reactions of Atoms and Small Molecules 39 are characterized by rate constants of 4.8 x (29) 2.56 x (30) and 1.3 x (31) respectively.Welge et al.'" have reported corrected values for quenching by CO and NO namely k, = 1.0 x and k = 8.0 x lo-'' cm3 molecule-' s-'. Lawrence12' reports the quantum yields (Q) for the production of 0(2'S,) from the photolysis of C02 as a function of wavelength and finds a( - 110 nm) = 1.0 (D(90 nm) = 0.3 and Q(121.6 nm) = 0.15. 0(2'D2). The principal development for the direct kinetic investigation of 0(2'D2) has been the design of a method by Heidner Husain and Wiesenfeld'22 for monitoring this state in absorption by time-resolved attenuation of atomic resonance radiation at A = 115.2 nm [0(3'D! + 2'D2)]. O(2'0,) was generated by the repetitively pulsed irradiation of ozone at low pressure and monitored by signal-averaging of the photoelectric pulses resulting from line absorption.This technique is a factor of ca. lo2-lo3 more sensitive than the previous main method for studying this state directly and which employed the highly forbidden emission, 0(2'D2)-+ O(23P2) a t A = 630nm.'23 Heidner et ~ 1 . ' ~ ~ ~ ' ~ ~ ' ~ ~ have obtained absolute quenching data for the removal of this state with a range of collision partners (Table 14). Where a comparison can be made with the absolute data of Table 14 Rate data for the collisional quenching of O(2'0,) by various gases studied by attenuation of atomic resonance radiation at A = 115.2 nm [0(3'D:) + 0(2'D,)] and at 300 K k/cm3 molecule- ' s- ' 7.0 k 0.5 x lo-'' 6.9 0.6 x lo-" 7.3 _+ 0.7 x lo-'' 3.0 & 0.3 x lo-'' 2.7 f 0.2 x lo-'' 2.7 0.3 x lo-'' 1.8 & 0.2 x lo-'' 9.4 k 0.8 x lo-" 2.3 0.2 x lo-'' 3.1 & 0.4 x lo-'' 4.0 0.4 x lo-'' G 1.5 x 10-15 2.1 * 0.2 x lo-'' 2.2 & 0.2 x lo-'' Ref.122 124,125 124,125 124,125 124,125 124,125 122,124,125 126 126 1 26 126 126 126 126 R. A. Young and G . Black Planet. Space Sci. 1966 14 113. ' I 9 R. A. Young and G. Black J . Chem. Phys. 1966 44 3741. "O S. V. Filseth F. Stuhl and K. H. Welge J . Chem. Phys. 1972 57 4064. I * ' G . M. Lawrence J . Chem. Phys. 1972 57 5616. R. F. Heidner tert. D. Husain and J. R. Wiesenfeld Chem. Phys. Letters 1972 16, 530. J. Noxon Canad. J . Chem. 1969,47 1873; J . Chem. Phys. 1970 52 1852. R. F. Heidner tert. and D.R. Husain Nature 1972 241 10. R. F. Heidner tert. D. Husain and J . R. Wiesenfeld J.C.S. Faraday IZ 1973 in the press. I z 6 R. F. Heidner tert. and D. Husain Internat. J . Ckem. Kinetics 1973 in the press 40 R. J . Donovan D . Husain and L. J. Kirsch N ~ x o n ' ~ ~ (O, N, CO and CO,) there is reasonable agreement with the exception of the rate for CO,. Noxon' 2 3 originally obtained an anomalously low value for CO (k,, = 3.0 x 10- l 2 cm3 molecule- s- ') which unfortunately, was the basis of later extensive discussion of an intermediate CO in the atmos-pheric chemistry of Mars and Venus. More recently Clark and Noxon12' have modified their experimental method for monitoring O(2l0,) in emission and their resulting relative-rate data now confirm the rapid quenching by CO .Thus they have found k, k, kC0 = 0.55 0.45 1.00 (O'D emission) and k, k, kcOz k, = 0.55 0.4 1.00 0.2 [from O,(b 'Cg+) emission]. Steady-state photolysis coupled with final-product analysis also continues to be widely employed for the determination of relative-rate data for the collisional removal of O(2'0,). Greenberg and Heicklen'28 have quantified the relative extents to which the different pathways contribute to the overall process O(2l0,) + CH products (32) following the photochemical production of O(2l0,) from N,O (A = 231.9 nm). They have further found that for the processes O(2l0,) + N,O 5 N + 0, O(2l0,) + N,O A 2N0, (33) and (34) k,,/(k33 + k3,) = 2.28 f 0.20 and that this ratio is reduced to the value of 1.35 0.3 on the removal of excess translational energy by collisions with helium.Heicklen et ~ 1 . ' ~ ~ further report k,,/k, = 0.69 0.05 which is in-creased to 0.86 & 0.06 on moderation with helium. This latter value is in better agreement with that of Scott et ~ 1 . ' ~ ' (k3,/k3 = 1.01). The molecular dynamics of reactions (33) and (34) are discussed by Simons et who measured the distribution of vibrational energy in NO following the flash photolysis of N,O. Simonaitis and Heicklen13 have also studied the steady photolysis of N,O in the presence of H,O CO and He to investigate the relative rates of the reactions 0(2102) + H,O -% 2 0 H (35) 1 2 ' I. D. Clark and J. F. Noxon J . Chem. Phys. 1972 57 1033. 1 2 9 R. Simonaitis R. I . Greenberg and J . Heicklen Internat.J . Chem. Kinetics 1972 4, 130 P. M. Scott K. F. Preston R. J . Anderson and L. M. Quicla Canad. J . Chem. 1971, '" C . Boxall J. P. Simons and P. W. Tasker Faraday Discuss. Chem. Sac. 1972 no. 5 3 , ' 3 2 R. Simonaitis and J . Heicklen Internat. J . Chem. Kinetics 1972 4 529. R. I . Greenberg and J. Heicklen Internat. J . Chem. Kinetics 1972 4 417. 497. 49 1808. p. 182 Reactions of Atoms and Small Molecules and 41 o(210,) + co o(2313,) + c o , for which they report ( - 5.0 kRJTmol- ' k3&6 = 2.6exp This ratio at 300 K (0.35) may be compared with that derived from the absolute data of Heidner et al.124*125 (Table 14) for the overall quenching by these two molecules where k,,o/kco = 4.1 and where rapid overall removal by H,O is clearly established.The results of the two investigations may indicate a contribu-tion to the collisional relaxation of O(2'0,) to the ground state by H,O. Schumacher and c o - w ~ r k e r s ' ~ ~ ~ ~ ~ have employed ozone as the photo-chemical source of O(2'0,) and report relative rates for removal of the excited atom of 0 0 N Xe Ar He = 1.0 0.23 0.19 0.18 0.043 0.012 and 1.0 0.23 0.20 0.17 0.039 0.01 for steady photolysis at 313 and 253.7 nm respectively. The rate for helium is clearly high by ca. three orders of magnitude when compared with the direct absolute data of Heidner et al. (Table 14).12, Loucks and Cvetanovic' 35 have studied the photolysis of C0,-N,O mixtures at A = 163.3 nm. This work yielded kCO2/kNzO = 0.80 for the removal of 0(2l0,), in agreement with both previous work and the absolute data of Heidner et a1.(Table 14),' ,4-' 26 and indicates that O(2'0,) undergoes physical quenching with CO and chemical reaction with N,O. These authors'36 have investigated the photolysis of CO at 163.3 nm in detail especially with regard to a material balance. This work is a very useful basis for considering various inconsistencies in the earlier literature. Cvetanovic and co-workers' 3 7 further report kN,,/ kneopentane = 0.145 0.010 following the photolysis of N,O and have shown that O(2 '0,) undergoes reaction with these two molecules. Michaud and Cvetano-vic13* have quantified the main reaction pathways that 0(2'D,) generated in this system undergoes with cyclopropane. Lissi and Heicklen' 39 have presented a detailed investigation of the steady photolysis of 0 in the region 228.8-285.0 nm and have concluded that O(2'0,) is not generated in the chain step.Giachardi and Wayne'40 have studied reactions of O(2'0,) from the photolysis of ozone (A = 253.7 nm) by monitoring O(23PJ) using atomic resonance fluores-cence. The system was sampled after both 0(2'D2) and 0 2 ( b lEg+) had reacted. Details of quantum yield measurements are given. Pathways for chemical ' 3 3 E. Castellano and H. J. Schumacher Z . ph-vs. Chem. (Frankfurt) 1972 76 258. 1 3 4 G. Von Ellerieder E. Castellano and H . J . Schumacher Z . phys. Chem. (Frankfitrr), 1 3 ' 1 3 6 '" G. Paraskevopoulos V. B. Symonds and R. J. Cvetanovic Canad. J . Chem. 1972,50, 1 3 ' P. Michaud and R. J. Cvetanovic J . Phys. Chem.1972 76 1375. 1 3 9 E. Lissi and J. Heicklen J . Photochem. 1972 I 39. I4O b. J. Giachardi and R. P. Wayne Proc. Roy. SOC. 1972 A330 131. 1972 76 240. L. F. Loucks and R. J . Cvetanovic J . Chem. Phys. 1972 56 1682. L. F. Loucks and R. J. Cvetanovic J . Chem. Phys. 1972 56 321. 1838 42 reaction of O(2l0,) + 0 are described and it is concluded that the process R. J. Donovan D. Husain and L. J. Kirsch O(2l0,) + 0 -+ O(23P') + O,(b lZg+) (37) takes place on 50-60% of the quenching collisions. In general Wayne's141 article in the Faraday Discussion provides a useful review of the reactions of excited species in the photolysis of ozone. McCullough and McGrath14 have measured the vibrational distribution in O,(X ?Zg-) following the ultraviolet flash photolysis of ozone and conclude that the levels Y" = 13-15 are formed in the deactivation of O(2l0,) with 0 with an efficiency (a) of 0.3 < a < 0.5.A detailed measurement on the vibrational distribution of 0 in this same system has been given by Webster and Blair,'43 following the reaction of O(2lD2) + 0 , who discuss the results in terms of a model for the statistical break-up of an 0, complex. Fisher and B a ~ e r ' ~ ~ have published an excellent theoretical paper on the quenching of O(2l0,) by N . The model involves curve-crossing of covalent states and can also be applied to the vibrational relaxation of N by O(2,PJ) and to the unimolecular decomposition of N20. The principal result for O(2l0,) is that less than 5 % of the electronic energy should appear as vibration in N, but that the molecule is likely to be rotationally excited.This paper'44 is of especial use in the consideration of a number of aspects of atmospheric chemistry. 24 Finally Lin and co-workers have developed a number of pulsed lasers arising from the reactions of 0(2'0,) generated from the flash photolysis of ozone with various molecules. HCl and HF lasers have been constructed from the reactions of O(2l0,) with ha loge no me thane^.'^^-'^^ It is envisaged that the vibrationally energized diatomic molecule is eliminated from the highly vibrationally excited halogenomethanol initially formed on insertion of O(2'0,). Similarly Brus and Lin'48 have observed CO laser emission from the reaction of O(2l0,) + C302 and also from O(2'0,) + XCN (X = C1 Br I or CN).14' O(2,PJ).Resonance fluorescence at 3 = 130 nm [0(3jS1) + O(23P,)] especially in the time-resolved mode following flash photolysis has continued to be employed as a powerful tool for direct kinetic investigations of O(23PJ). Davis et a1.''' have used this technique to study the recombination reaction 0 + 0 + M -L 0 + M (38) over the temperature range 20@-346 K and obtained the result 4.24 & 0.19 kJ mol-' k$; = 6.57 5 0.59 x 10-35exp ( RT ) cm6 molecule-2 s-1 l 4 I 1 4 2 D. W. McCullough and W. D. McGrath Chem. Phys. Letters 1971 12 98; J . Phoro-1 4 3 H. Webster tert. and E. J. Blair J . Chem. Phys. 1972,57 3802. 144 E. R. Fisher and E. Bauer J . Chem. Phys. 1972,57 1966. 1 4 5 L. E. Brus and M. C. Lin J . Phys. Chem. 1971 75 2546. 146 M. C. Lin J .Phys. Chem. 1972,76 811. 14' M. C. Lin J . Phys. Chem. 1972,76 1425. 14* M. C. Lin and L. E. Brus J . Chem. Phys. 1971 54 5423. 149 L. E. Brus and M. C. Lin J . Phys. Chem. 1972,76 1429. R. P. Wayne Faraday Discuss. Chem. SOC. 1972 no. 53 p. 172. chem. 1973,1 241. 5 0 R. E. Huie J. T. Herron and D. D. Davis J . Phys. Chem. 1972,76,2653 Reactions of Atoms and Small Molecules 43 They have further found the relative rates for process (38) with M = He Ar or N of 0.92 1.0 1.6 at 298 K and for Ar and N, of 1.0 1.7 at 218 K. Slanger and co-~orkers''~ have employed this method for a careful re-examination of the recombination reaction O + C O + M L C O + M (39) This slow process is particularly prone to effects of impurities. Slanger et al.' report cm6 molecule-2s-' ( - 18.1 & i;kJ mol-k$ = 6.5 x 10-33exp Rate constants at T = 296 K of 2.3 x are also given for M = N and CO, respectively.'51 These slow rates should in the opinion of the authors be considered as the most definitive yet obtained.Davis et ~ 1 . ' ' ~ and Kury10"~ have studied the addition of O(23PJ) to ethylene by resonance fluorescence and have compared the results experimentally with those derived from mass spectrometry and NO + 0 chemiluminescence. The value of the resulting rate constant to emerge from these techniques is given by and 6.2 x cm3 molecule- ' s- -4.72 f 0.13 kJmol-' k = 5.42 & 0.3 x exp Davis et al.' 54 have also employed resonance fluorescence to quantify the rate of addition of O(23PJ) to but-1-ene in the temperature range 190491 K and have found that the data could not be satisfied by a linear Arrhenius plot.Concurrent abstraction and addition by the atom was assumed following which they report 1 ( - 0.2 & yTkJ mol- ' kadd = 3.7 & 1.8 x 10-12exp and cm3 molecule- s- ',' 54 1 [ -8.24 & y kJ mol- ' kabs = 1.6 & 0.9 x lO-"exp Clyne and Cruse"' have also employed resonance fluorescence on O(23PJ) in a discharge flow system and report that k, = 6.1 & 0.6 x 10- l 2 cm3 molecule-' s- (298 K) for the reaction O+NO,*O,+NO (40) in good agreement with previous data. Very recently Davis et ~ 1 . ' ~ ~ have investigated the kinetics of reaction (40) using atomic resonance fluorescence 1 5 * D. D. Davis R. E. Huie J. T. Herron M. J. Kurylo and W. Braun J . Chem.Phys., l S 3 M. J. Kurylo Chem. Phys. Letters 1972 14 117. T. G. Slanger B. J. Wood and G. Black J . Chem. Phys. 1972,57 233. 1972,56,4868. R. E. Huie J. T. Herron and D. D. Davis J . Phys. Chem. 1972 76 331 1. M. A. A. Clyne and H. W. Cruse J.C.S. Faraday 11 1972,68 1281. l S 6 D. D. Davis J. T. Herron and R. E. Huie J . Chem. Phys. 1973 58 530. 5 44 R. J . Donovan D . Husain and L. J. Kirsch following flash photolysis and obtained k, = 9.12 f 0.44 x cm3 mole-cule-' s-' which was found to be independent of temperature in the range 23&339 K. The result is in sensible agreement with that of Clyne et al.'" Chemiluminescence from 0 + NO has also been employed recently to obtain kinetic data for O(23P,). Stuhl and Niki'57,'58 have obtained rate constants for a number of second- and third-order reactions using this chemiluminescence as well as that from 0 + COY in the time-resolved mode following pulsed vacuum-U.V.photolysis. Their results are presented in Table 15. These ~ o r k e r s ' ' ~ have Table 15 Rate data for Op3 P,) derived from time-resolved chemiluminescence at 300 K from 0 + NO and 0 + CO following vacuum-ultraviolet pulsed photolysis Third-order reactions' 5 7 Reactants k/cm6 molecules-2 s-' O + N O + N O 1.5 x 10-3l k 10% O + N O + H e 6.65 x 10-32 f 10% 0 + 0 + 0 6.4 x 10-34 f 25% 0 + 0 + N 5.4 10-34 _+ 25% 0 + 0 + co 6.7 10-34 25% O + C O + N 2.2 x 10-36 f 25% 0 + CO + co 0 + N + N, 0 + co + co 3.2 x f 25% 0 + CO + He 1.7 x k 25% 9 x 10-36 5 x 10-38 Second-order reactions' 5 8 Reactants k/cm3 molecule- s- ' 0 + C,H, 0 + C,D, 0 + C2H4 1.31 x 10-13 + 5 -10% 1.31 x 10-13 + 5 -10% 6.3 x 10-13 + 5 -10% 0 + C3H6 3.6 x + 5 -10% also re-investigated the rates for the addition of O(z3PJ) + C2H or C2D4 using the same technique and have found kC2H4 = 6.25 f 0.63 x 10- '' and kCZD4 = 5.6 f 0.56 x cm3 molecule-' s-'.This confirmed {heir earlier measure-ments for thesequantities(Tab1e 15) which are slightly smaller than those reported by Davis et a1.ls3 Atkinson and Cvetanovic'60 have employed phase-shift measurements of the NO2 afterglow following modulated generation of O(23PJ) in a flow system by the mercury-photosensitized decomposition of N20. They report Arrhenius parameters (Table 16) for the addition of oxygen atoms to olefins.The value for 0 + C,H4 at 300 K (5.1 x cm' molecule-' s- ') is in sensible agreement with that of Stuhl and Niki'59 at this temperature. Atkinson and Cvetanovic'60 have also obtained a rate constant for the recombination of Is' F. Stuhl and H. Niki J . Chem. Phys. 1971,55 3943. l s 8 F. Stuhl and H. Niki J . Chem. Phys. 1972 55 3954. l S 9 F. Stuhl and H. Niki J . Chem. Phys. 1972 57 5403. I6O R. Atkinson and R. J. Cvetanovic J . Chem. Phys. 1972 56 432 Reactions of Atoms and Small Molecules 45 Arrhenius parameters for the addition of O(23PJ) to olejns from phase- Table 16 sh@ measurements of the NO, OleJn 10' ' ~ / c r n ~ molecule- ' s - ' E/kJ mol-' Ethylene Propene But- 1 -ene Isobutene 1.3 1.1 1.0 1.0, 8.1 k 0.4 4.3 k 0.4 3.4 & 0.4 0.0 & 1.7 0 + NO + M of cm6 molecule- s- ' (6.7 f 0.; mol- ' k = 7.2 x 10-33exp McCrumb and Kaufman' 61 have studied the reaction of 0 + O3 in the tempera-ture range 2 6 9 4 0 9 K from the thermal decomposition of O3 in a flow system using the NO afterglow.They report a rate constant of k = 1.05 & 0.18 x lo-'' exp - 17.9 f 0.42 kJ mol- ' cm3 molecule- s-for reaction. This value is in good agreement with recent data of Heicklen et al. - 18.0 kJ mol- ' k = 1.2 x lo-" exp ( RT ) cm3molecule-1 s-1 A number of kinetic processes that O(23P,) undergoes have been studied by employing the e.p.r. atomic spectral intensity often coupled with other analytical methods such as mass spectrometry. Bonanno et have investigated the reaction of O(23P,) with benzene in the temperature range 255-305 K using a fast-flow system and report a rate constant of cm3 molecule- ' s- ' ( - 18.4 1- i k k J mol- ' k = 3.8 1- 1.5 x 10-13exp They found no significant kinetic isotope effect with C,D,.Timmons et have investigated the kinetics of the processes 0 + (CH3),0 -% OH + CH20CH3 (41) and 0 + CH30H -% OH + CH,OH and report [ - 11.9 & :LkJ mol- ' k, = 5.0 & 1.0 x lo-' exp ''I J. J. McCrumb and F. Kaufman J . Chem. Phys. 1972,57 1270. 1 6 ' D. C. Krezenski R. Simonaitis and J . Heicklen Inrernat. J . Chem. Kinetics 1972 3, 467. 1 6 3 R. A. Bonanno P. Kim J.-H. Lee and R. B. Timmons J . Chem. Phys. 1972,57 1377. H. F. Lefevre J. E. Meagner and R. B. Timmons Znternar. J . Chem. Kinetics 1972, 4 103 46 R. J. Donovan D.Husain and L. J. Kirsch and cm3 molecule- ' s- ' ( -9.5 &:; kJ mo1-l k42 = 1.70 & 0.66 x 10- l 2 exp Mack and Thrush' have employed e.p.r. spectroscopy chemiluminescence, and product analysis for a study of the reaction of 0 + CH,O in a flow system. Atomic abstraction, 0 + CH,O 5 HCO + OH (43) is found to proceed with a rate characterized by k, = 1.5 0.15 x molecule-' s-'. Relative rates for the subsequent reactions of HCO, cm3 0 + HCO -+ OH + CO (4) 0 + HCO -+ H + CO, (45) and H + HCO -+ H + CO, were found to be given by k, k, k, = 0.54 0.46 0.40. Herbrechtsmeir and Wagner'66 have characterized the rate of the 0 + allene reaction in the tem-perature range 275-375 K and have found (46) cm3 molecule- ' s- ' ( - 6.7 :Tmol- ' k z 7.8 x lo-' exp Davis and c o - ~ o r k e r s ' ~ ~ report rate constants for the addition of oxygen atoms generated in a flow discharge to halogenoethylenes.Mass spectrometry was employed for analysis and the results are given in Table 17. Hand and Table 17 Rate constants for the addition O ~ O ( ~ ~ P ) to halogenoethylenes at 307 K Halogenoeth ylene k/cm3 molecule- * s-' C2H3F 3.95 _+ 0.63 x C2H3CI 8.68 _+ 0.40 x C,H,Br 8.15 f 0.57 x 1,l -C,H,F 3.64 _+ 0.30 x l,%-C,H,F 4.48 +_ 0.57 x Obenauf' have employed a similar method involving time-of-fligh t mass spectrometry to study the reaction of 0 + dicyanoacetylene and report a rate constant of k = 1.1 & 0.3 x 10- l 5 cm3 molecule-' s-' for this process. Jacob ' 6 5 G. P. R. Mack and B. A. Thrush J.C.S.Faraday I 1973 69 208. P. Herbrechtsmeir and H. Gg. Wagner Ber. Bunsengesellschaft phys. Chem. 1972, 76 5 17. R. E. Huie J. T. Herron and D. D. Davis Infernat. J. Chem. Kinetics 1972 4 521. "' 1 6 ' C. W. Hand and R. H. Obenauf jun. J . Phys. Chem. 1972 76 269 Reactions of Atoms and Small Molecules 47 and Wink1e1-l~~ have investigated the reaction of 0 + SO3 in a flow discharge. Oxygen atoms were generated by the N + NO titration and products were analysed following trapping at low temperatures. The results may be sum-marized by the rate constant for reaction of cm3 molecule- ' s- ' ( -4.2 yTmol-' k = 3 x 10-'5exp in the temperature range 300-500 K. McNeal et ~ 1 . ' ~ ' have employed the photoionization of molecular nitrogen to investigate the quenching process for which they report k47 = 3.5 & 1.4 x lo-'' cm3 molecule-' s-' which may be considered within the context of Fisher and B a u e r ' ~ ' ~ ~ theoretical calculations.Bevan and Johnson' 71 have investigated the kinetics of ozone formation following the pulsed radiolysis of molecular oxygen using the 0 absorption intensity for analysis. They postulate the involvement of two states of O, the first formed from the recombination of 0 + 0 + 0 which then relaxes to the second form on collision. Deactivation rates for this process are reported. The details of the vibrational product distribution in CO following the reaction of 0 + CS have been further investigated by Smith and c o - w o r k e r ~ ' ~ ~ and this has been extended to the reaction of 0 + CSe.'73 0 Atoms were generated in a flow discharge and vibrationally excited CO was monitored in emission in the infrared.With CS levels up to u" = 15 were populated reaction into u" = 13 being most probable;17 with CSe CO(u" < 20) was produced the relative rate into u" = 18 being greatest. Smith et ~ 2 1 . ' ~ ~ also report a rate coefficient for the reaction o + C S ~ SeO + C S ~ (48) of k4* 2 2 x lo-' cm3 molecule-' s- ' from flash photolysis experiments. The vibrational product distribution in CO from 0 + CS has also been investigated by F o ~ t e r ' ' ~ using a CW laser and he has found that the level 0'' = 12 f 1 was most highly produced in clear agreement with Smith et ~ 1 . l ~ ~ Felder Morrow and Young' 75 have obtained quantitative kinetic data for the quenching of CO(a 311) by O(23PJ).The oxygen atoms were generated by the 0 + NO titration and CO(a 313) by a Tesla discharge the (0,O) and (1,O) transitions of the forbidden Cameron system [CO(a 311-X 'Z')] being monitored in emission. These experiments yielded quenching constants of k,.= = 1.9 x 10- l o and k,,= = 2.1 x cm3 molecule- ' s- '. Shackleford et ~ 1 . ' ~ ~ have A. Jacob and C. A. Winkler J.C.S. Faraday Z 1972,68,2077. 1 7 0 R. J. McNeal M. E. Whitson jun. and G. R. Cook Chem. Phys. Letters 1972,16,507. 1 7 ' P. L. T. Bevan and G. R. A. Johnson J.C.S. Furaday I 1973,69 216. G. Hancock B. A. Ridley and I. W. M. Smith J.C.S. Faraday II 1972 68 2117. 1 7 3 C. Morely B. A. Ridley and I. W. M. Smith J.C.S. Faraday II 1972 68 2127. 1 7 ' K. D. Foster J .Chem. Phys. 1972 57 2451. '" W. Felder W. Morrow and R. A. Young Chem. Phys. Letters 1972 15 100. W. L. Shackleford F. N. Mastrup and W. C. Krege J . Chem. Phys. 1972 57 3933 48 R. J. Donovan D. Husain and L. J. Kirsch monitored the vacuum-u.v. chemiluminescence emission of CO(A ' rI-X 'C+) from the reaction of 0 + C,O in a flow system. C,O was generated by the flash photolysis of C302 in this environment. An overall rate for the process 0 + c 2 0 ~ 2 c 0 (49) of k, = 9.5:;: x 10- '' cm3 molecule-' s-' was found and that for 0 + C,O CO(A 'rI) + co (50) estimated as 10- "-10- cm3 molecule- s- '. These authors also report collisional quenching rates for CO(A Ill). Matsuda et a1.177-'79 have investi-gated the oxidation of carbon monoxide by oxygen atoms behind shock waves in the presence of metal carbonyls.CO was monitored by i.r. emission and metal oxides by mass spectrometry. It was found that the 0 + CO recombination was catalysed by the metal atoms. Simonaitis and Heicklen'80 have also studied the 0 + CO recombination following the mercury-photosensitized decomposi-tion of N,O. The recombination was studied by competition with 2-trifluoro-methylpropene in the temperature range 2 9 8 4 7 2 K. Kinetics intermediate between second and third order were found and the results interpreted in terms of surface crossing of the initially formed COJ3B,) with CO,('B,). Similar kinetics for this reaction have been found by DeMore,18' who has studied this process in competition with the recombination of 0 + 0 + M light absorption by 0 being measured.The third-order recombination rate constant for 0 + CO (kM=C02 = 5.0 x cm6 molecule-2 s-') was found to be in sensible agreement with the data of Slanger Wood and B1a~k.l~' DeMore'8','82 has employed a similar method to determine relative rates for 0 atom addition to olefins and reports kCzHs kC3H8 kbut-l-ene = 1 20 20. S and Te. Davis et al. 183*184 have employed time-resolved resonance fluorescence in the vacuum-u.v. to monitor S(33PJ) generated from the flash photolysis of COS-CO mixtures. The rate of the reaction with 0 was investigated in the temperature range 2 5 2 4 2 3 K yielding cm3 molecule- s - ( -0.0 & yTkJ mol- ' k = 2.24 & 0.27 x 10- l 2 exp This study also included the monitoring of product O(23PJ) by resonance fluorescence.The same method'84 was applied to the reaction of S(33P,) with 17' T. P. J. Izod G. B. Kistiakowsky and S. Matsuda J . Chem. Phys. 1972 56 1083. 1 7 8 S. Matsuda J . Chem. Phys. 1972 57 807. 17' S. Matsuda J . Phys. Chem. 1972 76 2833. R. Simonaitis and J . Heicklen J . Chem. Phys. 1972,56 2004. W. B. DeMore J . Phys. Chem. 1972 76 3527. W. B. DeMore Chem. Phys. Letters 1972 16 608. 1 8 3 D. D. Davis R. B. Klemm and M. J. Pilling Internat. J . Chem. Kinetics 1972,4 367. l S 4 D . D. Davis R. B. Klemm W. Braun and M. J. Pilling Internat. J . Chem. Kinetics, 1972 4 383 Reactions of Atoms and Small Molecules ethylene in the temperature range 2 1 8 4 2 K yielding 49 -6.6 & 0.3 kJ mol- k = 7.13 & 0.74 x lo-' exp ( RT ) cm3 molecule-1s-1. Donovan and Little185 have also studied the reaction S + 0 by time-resolved atomic absorption of resonance radiation following flash photolysis.This work yields k = 1.7 & 0.3 x lo-' (295 K)cm3 molecule-' s-l in good agreement with the results of Davis et aI.'84 Donovan et a1.ls6 have applied the NS spectroscopic marker method de-veloped by Donovan and Bre~kenridge,'~~ to measure relative quenching rates of S(31D2) with a range of collision partners. The method is very sensitive on account of the intense C + X transition of NS at 230 nm,'87 and the resulting kinetic plots obtained following the flash photolysis of COS-N,O-added-gas mixtures demonstrate the reliability of the method. The rate data resulting from this investigation are presented in Table 18. Table 18 collision partners derived from the NS spectroscopic marker technique Relative rate data for the collisional removal of S(3'D2) by various Collision partner (M) c H4 C2H6 CH4 co2 N2 H2 NO co Ar Kr Xe I 0.17 0.076 0.24 0.68 0.19 0.062 0.22 0.0097 0.028 0.16 Donovan and co-workers'88 have observed Te(SID,) in absorption by plate photometry at 175.8 nm following the flash photolysis of TeD .Further, Donovan and Little189 have carried out a detailed kinetic investigation of the spin-orbit states Te(53P,) and Te(s3Po) respectively 4751 and 4707 cm- above the S3p2 ground state.'" The tellurium atoms were generated by the flash photolysis of H,Te and monitored in absorption by time-resolved attenuation of resonance radiation at the wavelengths 238.40 nm [5~~6s(~S:) + 5p4(3P0)] and R.J. Donovan and D. J. Little Chem. Phys. Letters 1972 13 488. no. 53 p. 21 1. R. J. Donovan and W. H. Breckenridge Chem. Phys. Letters 1971,11 520. R. J. Donovan D. J. Little and J. Konstantatos J. Photochem. 1972 1 86. l a 6 D. J. Little A. Dalgeish and R. J. Donovan Faraday Discuss. Chem. Society 1972, l S 9 R. J. Donovan and D. J. Little J.C.S. Furaduy 11 1973 in the press 50 R. J. Donouan D. Husain and L. J. Kirsch 238.65 nm [5~~6s(~S:) t- 5p4(3Pl)J in order to monitor the J = 0 and J = 1 levels. The experiments indicate that these levels which are only separated by 44 cm- I are maintained in an equilibrium distribution throughout reaction. The resulting rate data are presented in Table 19. The quenching cross-sections Table 19 Rate constants (k) and collisional quenching cross-sections (0) for Te(s3P1) and Te(53p0) Quenching gas k/cm3 molecule- s- O/A2 H2 1.03 & 0.15 x lo-" 0.58 D2 8.8 _+ 6.5 x lo-'' 6.9 x 10-4 0 2 1.28 & 0.55 x 2.6 x 10-3 He 3.0 x lo-'' ~ 2 .4 x 10-4 Ar 1.38 f 0.28 x lo-'' 3.0 x 10-4 Xe 62.7 x 10-15 G8.6 x refer to the sum for these quantities for both spin-orbit states on collision with a particular gas. Particularly striking is the large isotope effect for H and D,, which is accounted for in terms of an energetically more favourable electronic-to-vibration energy-transfer process for H (u" = 1) compared with D (u" = 1). Fluorine Chlorine Bromine and Iodine.-A limited amount of kinetic data has been described for fluorine atoms.Kolb and Kaufman'" have employed a flow discharge system with molecular beam-mass spectrometric analysis of intermediates including the fluorine atom. They report a rate constant of k = 4.0 x 10- l 6 cm3 molecule- s- ' for the reaction of this atom with CCl,. Kompa and Wanner' have analysed the shape of the time-resolved laser pulse in the i.r. from HF following the reaction of fluorine atoms with hydrogen-containing molecules. F Atoms were generated from the flash photolysis of WF . These authors report the rate constants for the systems F + H,, F + HCl, k = 6.3 x lo-" cm3 molecule-'^-^ k = 2.5 x lo-" cm3 molecule-' s-' and F + CH, k = 2.5 x lo-'' cm3 molecule-'^-^ Czarnowski and Schumacher ' 92 have investigated the thermal decomposition of F,O in a static system and have obtained a rate constant of cm3 molecule- ' s- ' ( - 57.3 & i;kJ mol- ' k = 8.6 x 10-'4exp for the reaction F + F,O -% F + FO (51) C.E. Kolb and M. Kaufman J . Phys. Chem. 1972,76,947. K. L. Kompa and J. Wanner Chem. Phys. Letters 1972 12 560. 1 9 ' l g 2 J. Czarnowski and H. J. Schumacher Chem. Phys. Letters 1972 17 235 Reactions of Atoms and Small Molecules 51 Clyne et a1.1g3 have observed the states B 311 (0') in emission formed from the recombination of F(22P3/2) with iodine and bromine atoms in the presence of 0 2 ( a 'Ag) and O,(b 'Xgf) in a flow system. A three-body recombination mechan-ism followed by energy transfer on collision is discussed. The major recent contribution to the kinetics of chlorine and bromine atoms is the work of Clyne and Cruse'94 on the study of C1 and Br atoms in a flow discharge system using atomic resonance fluorescence in the vacuum-u.v.This work is highly detailed from both the spectroscopic and kinetic points of view. The resulting kinetic data are given in Table 20. Bernard and Clyne"' have also Table 20 Rate data for chlorine and bromine atoms derived from atomic resonance Jluorescence spectroscopy in the vacuum ultraviolet at 298 K Reaction C1 + ClNO -+ NO + C1, Br + ClNO -+ NO + BrCl C1 + BrCl -* C1 + Br C1 + Br -P BrCl + Br Br + ICl -* BrCl + I Br + IBr -* Br + I c1+ ICl -* c1 + I k/cm3 molecule-' s-' 3.0 If 1.5 x lo-" 1.0 & 0.2 x lo-" 1.45 f 0.20 x lo-'' 1.20 0.15 x lo-'' 8.0 1.0 x lo-', 3.0 & 0.8 x 3.5 & 0.6 x lo-" studied the emission from Br(5s4PS12) and Br(5s4P3,,) following the selective vacuum-u.v.photolysis of Br in a flow system using atomic emission photolytic sources. The quenching of the emission was investigated and collisional quench-ing cross-sections for the 4P5/2 state are reported (Table 21) and compared with Table 21 Rate data for the collisional deactivation of Br(5s4PSi2) Collision partner (M) Ar He *2 H2 Br* SF6 co k&m3 molecule- ' s- Qd8.' <2 10-13 c4.0 10-4 <2 x 10-13 < i s 10-4 5.0 x lo-'' 2.7 10-3 1.0 x lo-" 1.8 x 1.6 x lo-" 5.1 x lo-'' 1.46 1.0 x 10-'O 0.284 2.9 x lo-' Q JQ, 4 0 - 3 <5 x 10-4 6.7 x 10-3 3.6 x lo-, 5.8 x lo-, 2.5 0.38 the hard-sphere collision cross-sections (cc).The rate of quenching with in-dividual gases is somewhat more rapid than that observed hitherto by Donovan and H ~ s a i n " ~ for the lower spin-orbit state Br(42P1/2) but the trend with collision partners is similar. 1 9 3 M. A. A. Clyne J. A. Coxon and L. W. Townsend J.C.S. Faraday II 1972,68 2134. ' 9 4 M. A. A. Clyne and H. W. Cruse J.C.S. Faraday II 1972 68 1377. 19' P. P. Bernard and M. A. A. Clyne J.C.S. Faraday II 1972. 68 1758. R. J. Donovan and D. Husain Trans. Faraday SOC. 1966 62 2987 52 R. J. Donovan D. Husain and L. J. Kirsch Clyne and his c o - w ~ r k e r s ' ~ ~ have also studied the recombination of bromine atoms in a discharge flow system using optical absorption by Br, titration with CINO and measurement of the A 3n(l,) -+ X 'C,' emission intensity following the recombination kY* Br + Br + M * Br,[A 3n(l,)] + M (52) These authors have characterized recombination rate constants for formation of the A state with different third bodies (M) of k:r,2 = 8.7 4.4 x 10-34cm6 molecule-' s- ' kFy/k$ = 22 and kyz/k$; = 1.1.Activation energies of -9.6 & 3.0 and -7.9 f 3.0 kJ mol-' are reported for k:? and k$i respec-tively. Burns et a1,198*199 have carried out trajectory calculations on bromine-atom recombination which include the effects of radical complex formation, energy transfer and rigid-rotor relaxation. Ferguson and Whittle200 have described a competitive study of bromine-atom abstraction from fluoroalkyl bromides by photochemically generated bromine atoms. Chromatographic analysis of products was employed and relative-rate data are reported."' Deakin and Husain201,202 have extended their kinetic studies on 1(5'P,,,), involving time-resolved attenuation of atomic resonance radiation following flash photolysis to a detailed investigation of the effects of temperature on both physical relaxation and chemical reaction.The physical processes of spin-orbit relaxation to I (52P3,2) on collision were studied by these authors over a wide range of temperature (180-410 K) and the resulting Arrhenius rate data are presented in Table 22. Especially striking are the small negative temperature coefficients Table 22 collisional deactivation of 1(52 Pl12) by various molecules Arrhenius parameters and the mean transition probability (P) for the Quenching gas CH4 C2H6 C3H8 n-C4H 10 CH2=CH2 CH3-CH=CH2 CHECH D2 H2 N' 0, 12 log ,4/crn3 molecule- s -- 13.40 f 0.17 - 13.43 & 0.11 - 13.24 f 0.14 - 12.73 f 0.11 - 13.09 0.1 - 12.42 f 0.1 - 13.45 f 0.12 -15.56 f 0.01 - 11.65 f 0.14 -15.19 f 0.29 -10.45 & 0.18 - 13.41 f 0.67 E/kJ mol-' -2.1 f 0.8 -2.1 0.4 -2.9 f 0.4 - 1.3 f 0.4 -2.5 f 0.4 -0.4 f 0.4 -5.0 f 2.5 0.0 & 0.4 7.1 0.8 6.3 f 0.4 0.8 f 0.8 -13.8 f 4.6 P = AfZyibK 1.2 x 10-4 1.3 x 1 0 - ~ 2.0 x 10-4 7.2 x 10-4 2.9 x 10-4 1.3 10-3 1.2 x 10-4 2.3 x 10-3 1.3 x 10-7 2.2 x 10-4 5.2 x 1.2 x lo-' observed for a number of gases.These could equally be due to a temperature dependence of ca. T - in the pre-exponential factor.The relatively large 'negative 19' M. A. A. Clyne J. A. Coxon and A. R. Woon-Fat Faraday Discuss. Chem. SOC., 1972 No. 53 p. 82. 19* A. Gelb R. Kapral and G. Burns J . Chem. Phys. 1972 56,4631. A. G. Clark and G. Burns J . Chem. Phys. 1972,56,4636. * O 0 K. C. Ferguson and E. Whittle J.C.S. Faraday I 1972 68 64. J. J. Deakin and D. Husain J.C.S. Faraday II 1972 68 1603. '02 J. J. Deakin and D. Husain J . Photochem. 1973,1 353 Reactions of Atoms and Small Molecules 53 activation energy' for I (- 3.3 kcal mol- ') is in accord with the stability of I formed from ground-state atoms.203 The quenching data for N and CO cm3 molecule- s- 1 ,01 ( -3.3 yTkJ mol- ' p = 4.7 x 10-16 exp the latter having been determined by time-resolved emission in the i.r. 1(5P1/,) + I(5'P3,,) + hv(1.315 pm) are in quantitative accord with the theory of Andreev and Nikitin,,04 who postulate the production of d' = 3 following quenching by both these molecules.Arrhenius data for some chemical reactions of I(52P1/2), obtained by Deakin and Husain202 using this attenuation method are presented in Table 23. Table 23 Arrhenius parameters for chemical reactions of I(5 PI,,) with various molecules log, A/cm3 React ion molecule-'s-' E/kJ mol-' AHlkJ P= A/ZydbK 1(52P1,2) + c1 * IcI + c1 - 11.6 0.2 6.7 k 1.3 -59.8 1.0 x lo-' I(5'P1,,) + Br * IBr + Br - 10.4 k 0.2 1.7 & 1.3 -75.2 0.3 I(52p1,2) + IcI --+ 1 + c1 -10.8 0.1 0 -32.2 0.1 1(5'P1,,) + IBr -+ I + Br -10.7 & 0.3 0 -64.4 0.1 I(52p1,2) + NOCl -* ICI + NO -10.6 k 0.2 2.5 1.3 -141.3 0.2 Two groups of experiments on iodine-atom recombination may be mentioned, both involving photoelectric monitoring of the reappearance of I following flash photolysis.Ip and Burns205 have made an extensive study of the recombina-tion over a wide temperature range 300-1173 K for a number of third bodies. The recombination rate constants which have been determined with considerable accuracy are presented in an empirical form as a function of temperature. This work essentially supplements previous measurements by Blake and Burns.206 Troe et aL207 have employed conventional and giant-pulse laser photolysis to study the recombination of iodine atomsat total pressures of up to ca. 10o0 atm. They conclude that this method yields a separation of the contributions of the intermediate-complex and energy-transfer recombination mechanisms and that, in the limiting low-pressure range the energy-transfer mechanism accounts for ca.one-third of the overall recombination rate. The authors present limiting second-order rate constants at high pressure for recombination by each of these mechanisms namely, kgT = 3.3 & 1.7 x lO-l4cm3 molecule-' s-l and k;! = 3.3 & 0.3 x lo-" cm3 molecule-'s-' 20' N . Davidson in 'Fundamental data obtained from shock tube measurements' ed. A. '04 E. A. Andreev and E. E. Nikitin Theor. Chim. Acta 1970 17 171. ' O S J. K. K. Ip and G. Burns J . Chem. Phys. 1972,56 3155. ' 0 6 J. A. Blake and G. Burns J . Chem. Phys. 1971 54 1480. * 0 7 H. Hippler K. Luther and J. Troe Chem. Phys. Letters 1972 16 174.Ferri Agardograph KO. 41 Pergamon Press Oxford 1961 p. 138 54 Further the radical complex equilibrium R. J . Donovan D. Husain and L. J. Kirsch I + Ar 5 IAr (53) is found to be characterized by an equilibrium constant of K z 3 x 102cm3 mol-'. Broadbent and Callear"' have stu'died the quantum yields for the collisional dissociation process 12(B3110+,) + M -+ I(S2Pl,,) + I(5'P3,,) + M (54) (M = He or Ar) by means of steady-photolysis measurements. The results are discussed in relation to fluorescence-quenching measurements and non-equili-brium effects in the recombination of atoms. 3 Diatomic Species H2(a3 Eg+).-The collision-free lifetime of H2(a3 CB+ u' = 0) has been deter-mined as 26 f 2ns following observation of the U.V. continuum emission produced by high-energy electron impact on H2.'09 The cross-section for energy transfer to a state (tentatively identified as the log2pn'rI state) at resonance with the a 3Xg+ state was found to be (6 f 2) x 10- l4 cm' being -20 times larger than the gas kinetic collision cross-section.209 It was further shown that quenching of these coupled states to the ground state by molecular hydrogen has a cross-section of 4.8 x 10-'6cm2.This is in agreement with the measurement by Center (Center's value'" is based on an earlier measurement of the lifetime; however the value based on the above lifetime still gives reasonable agreement), although no account was taken of an intermediary state in this work.210 He2(3p311g).-Rotational relaxation of He2(3p3n,) has been monitored following optical pumping of the (2s3C,') state (formed in a flowing helium afterglow), with a tunable dye laser.'" Relaxation by ground-state He atoms via the AJ = 1 channel was found to have a large rate coefficient -2 x lo-" cm3 molecule-'^-^ contrary to expectations from first-order theory.The AJ = 1 channel accounts for about half of the total relaxation rate for the 3 = 8 state of He2(3p3ng u' = 0). Ar,(4~'*~ Xu+) and Kr,(5~'*~ Xu+).-Energy transfer from these excited states to ground-state Xe and from Ar2(4s'93Zu+) to ground-state Kr has been in-vestigated.' '' The well-known vacuum-u.v. emission continua which arise from these states were monitored (radiative lifetimes for the upper states - s); however it was not possible to distinguish between the 'singlet' and 'triplet' state decay rates.The total cross-sections for relaxation of both states were found to be large (- 10- l 4 cm') and a theory involving long-range dipole-dipole 'On T. W. Broadbent and A. B. Callear J.C.S. Faraday II 1972,68 1367. ' 0 9 R. T. Thompson and R. G. Fowler J . Quant. Spectroscopy Radiative Transfer 1972, 12 117. ' I o R. E. Center J . Chem. Phys. 1971,54 3499. 2 1 1 C. B. Collins and B. W. Johnson J . Chem. Phys. 1972,57 5317. ' 1 2 A. Gedanken J. Jortner B. Razard and A. Szoke J . Chem. Phys. 1972 57 3456 Reactions of Atoms and Small Molecules 55 coupling was proposed to account for the The results were also compared with similar data on the relaxation of excited noble-gas atoms and it was shown that while the excited atoms undergo near-resonant transfer the excited molecules tend to populate the lowest excited states of the quenching atom provided these lie in the same energy range as the molecular continuum e.g.CH(A*A).-Rotational relaxation of CH(A2A) produced by the pulse radiolysis of CH and C2H has been monitored via the emission CH(A2A+ X 211).213 Fluorescence from the A2A and B2C states of CH (and from excited states of CH, and the H atom) has been observed following vacuum-u.v. photolysis (55.5-124.2 nm) of CH4.214 Absolute cross-sections for a number of photodissociation channels were determined.2 CN.-Further work on the reactions of CN(X2C) produced via the pulse radiolysis of cyanogen in the presence of argon has been reported.,' The rate constant for reaction with 0 was given as 1.1 x lo-'' cm' molecule-' s-' in agreement with previous On the basis of studies at two temperatures (300 and 377 K) it was tentatively suggested that reaction with 0 had a small negative activation energy.However it was also shown that the total rate for removal of CN in the presence of 0 increases with vibrational energy content of CN (the rates for v" = 0 2 and 4 were presented and showed a systematic increase in rate with increasing vibrational energy) contrary to previous work.2' The formation of the product NCO was also monitored and the rate shown to be faster than the decay of CN(X ,X v" = 0) strongly suggesting that vibrationally excited CN radicals contribute to the formation of NCO. Thus the reaction appears to have a positive activation energy with respect to vibrational This was rationalized in terms of a 'collision complex' NC-0-0 with vibra-tional energy assisting the fracture of the 0-0 bond as the complex forms products.Quite how this can be reconciled with an overall negative activation energy is not clear. The reaction of CN with C,N2 was also studied; however the rate data presented2" (log k = - 12.03 - [13.l(kJ mol-')/2.3 RT]) are only in moderate agreement with previous work.,' Curvature in kinetic plots (attributed partly to vibrational relaxation of CN) and changes in reaction order with concentration of CN were observed and thus further work will be required to characterize fully the reactions involved.21 Work on the reactions of CN with alkanes has been extended' l 7 and Arrhenius parameters have been presented for reaction with methane and ethane {log kCHs = - 10.67 - [8& & 0.8 (kJ mol- ')/2.3 RT] ; log kCZH6 = - 10.61 E x 0).The activation energy for reaction with methane therefore lies between those for ' 1 3 M. Clerc and M. Schmidt Faraday Discuss. Chern. SOC. 1972 no. 53 p. 217. 'I4 A. R. Welch and D. L. Judge J . Chem. Phys. 1972,57,286. 'I5 G. E. Bullock and R. Cooper J.C.S. Faraday I 1972,68 2175. 'I6 J. C. Boden and B. A. Thrush Proc. Roy. SOC. 1968 A305 107. '" G. E. Bullock and R. Cooper J.C.S. Faraday I 1972,68 2185 56 R. J . Donovan D. Husain and L. J. Kirsch chlorine- and fluorine-atom reactions. It should be pointed out however that these Arrhenius parameters" ' conflict with previous relative rate data and with the work of Boden and Thrush.216 The rate constants for reaction of CN with propane and CD, a t 300K were also determined [kCJHs = (5.3 & 0.8) x lo-'' cm3 molecule- ' s- ' ; kCD4 = (4.0 f 0.6) x lo-' cm3 molecule-' s-'; this latter value refers to reaction for u'' = 01.The rate constant for CD is thus approximately half of that observed for CH at 300K and demonstrates the importance of a relatively large tunnelling effect. The removal of vibrationally excited CN(v" = 4) by CD was found to be significantly faster than CN(u" = 0) [k = (5.8 0.3) x lo-' cm3 molecule- ' s-'I. This supports the data for CH,, where a similar but less significant effect was observed. It should be emphasized, however that the increased rate of removal for vibrationally excited CN was attributed to physical relaxation effects,'17 and not increased reactivity as observed for 0 .' The reaction of CN with NH has also been reinvestigated,"' CN + NH -% HCN + NH (56) The decay of CN and the formation of NH were monitored photoelectrically, the rates being in agreement within the experimental error. The rate constant k was determined as 2.1 x lo-" cm3 molecule-' s - l at 300 K and as 1.8 x 10- '' cm3 molecule- ' s- ' at 375 K indicating a small negative activation energy.218 The results are shown to be in agreement with previous flow-tube work.,' Faster removal of CN(u" = 4) relative to CN(d' = 0) was attributed to vibrational relaxation by NH, and a small negative temperature dependence for relaxation similar to that observed for quenching of vibrationally excited NO and CO was reported."' Rotational relaxation of CN(B 'Z +) formed by dissociative energy transfer from metastable argon atoms to CNBr has been studied and it was shown that relaxation by ground-state argon involved multiquantum transitions.Cook and have measured the lifetimes of CN(A 'll uf = lo) z A = 137 & 45 ns, and CN(B ,C u' = 0) zB = 39.4 9.3 ns from an analysis of the zero electric field limit linewidth of the level anticrossing spectrum. The lifetime of the B state was found to be in reasonable agreement with the most recent phase-shift measure-ments and that for the A state was in essential agreement with a value previously determined by microwave intensity measurements on the perturbed u' = 10 level. The lifetime of CN(A 211 u' = 10) is therefore almost two orders of magni-tude shorter than for the u' = 1-3 states.'" Further work on the excitation of the CN(B 'Z+ --* X 'Z+) violet emission in active nitrogen supports the earlier proposal that the B state is formed by energy transfer from vibrationally excited Nz molecules.1 0 5 7 2 2 En ergy transfer from N2(A ,CU+) to radiative levels of CN was shown to be of minor imp~rtance."~ 2 1 8 G. E. Bullock R. Cooper S. Gordon and W. A. Mulac J . Phys. Chem. 1972 76, 2 1 9 W. H. Duewer J. A. Coxon and D. W. Setser J . Chem. Phys. 1972,56 4355. 2 2 0 T. J. Cook and D. H. Levy J. Chem. Phys. 1972 57 5059. 2 2 1 G. M. Provencher and D. J. McKenney Chem. Phys. Letters 1971 10 365. 1931 Reactions of Atoms and Small Molecules 57 Quenching of the CN(B ,Cf + X ,C+) fluorescence (produced by vacuum-U.V.photolysis of ICN) has been studied for the collision partners CH, H, Kr, Ar and ICN.,, The variation in cross-section for quenching by ICN was studied as a function of relative collision velocity and found to be constant. C0.-The observation of stimulated emission (181-197nm) on Q- and R-branch lines of the CO fourth positive system ( A 'lT+ X 'C) should generate considerable interest in the A state.223 Optical excitation of CO(A 'lT) leading to resonance fluorescence has been used to obtain rate data for quenching of the u' = 0 and u' = 1 levels by He Ne Ar Kr H, D, and N,.224 Large cross-sections with values approaching gas-kinetic were observed. Further data are given by Shackleford et al. l 7 The rotational dependence of the lifetime of CO(a31T) has been investigated using a time-of-flight technique., The results were compared with those predicted by theory226,227 and show that rotational-level lifetimes vary from 3 ms to several hundred ms.The rotationally averaged lifetime (for a Boltzmann distribution at 4500 K) has been given228 as (70 & 4.0) x CO(a311) is formed by dissociative recombination of CO,+ and has been studied in a flowing He afterglow.228 The forbidden emission via the Cameron bands CO(a31T + X ' 2 ) has been employed to measure quenching rates by CO, N NO and He (Table 24). Emission from the A and B states of NO was s. Table 24 Quenching data for CO(a311) Quenching species 10' 'k/cm3 molecule- s- ' co2 N2 NO He < 10- l4 4.8 k 2.4 (u' = 0 4 ) 3.8 k 1.8 (u' = 0) 7.3 k 3.6 (u' = 1 4 ) 32 f 16 (u' = 0 4 ) observed following quenching of CO(a 311) and emission yields were estimated as ( A x 8 %; B x 2 %).228 Transfer to N,(A '2) is possible from CO(a311 u' = 1) but not from the u' = 0 level and accounts for the increased rate of relaxation for the levels u' = 1 - 4 .The CO(d3A) and (d3C) states are also populated228 by dissociative recombination of CO +. CO(a31T) is also formed by vacuum-u.v. photolysis of CO (109-85 nm).229 The emission yield from CO(a311 + X 'C) rises smoothly from a zero value at 109 nm to -60 % at 90 nm and indicates that the surface crossings leading to CO(a311) lie outside the Franck-Condon region for the initial excitation.229 2 2 2 W. M. Jackson and J.L. Faris J . Chem. Phys. 1972,56,95. 2 2 3 R. T. Hodgson J . Chem. Phys. 1972 55 5378. 2 2 4 F. J. Comes and E. H. Fink Chem. Phys. Letters 1972 14 433. 2 2 5 C. E. Johnson and R. S. Van Dyck J . Chem. Phys. 1972,56 1506. 2 2 6 T. C. James J . Chem. Phys. 1971 55 4118. 2 2 7 T. C. James personal communication in ref. 225. 2 2 8 T. S. Wauchop and H. P. Broida J . Chem. Phys. 1972 56 330. 2 2 9 G. M. Lawrence J . Chem. Phys. 1972 56 3435 58 R. J. Donovan D. Husain and L. J. Kirsch C§.-The radiative lifetime of CS(a3 n) formed by the dissociative excitation reaction between Ar(43P,)and CS has been measured using flow-tube techniques (z x 3 x s).,~' Quenching of CS(a311) by CS and Ar were also studied. A more detailed spectroscopic investigation of the emission a311+ X IC+, analogous to the CO Cameron bands has been reported.231 Fluorescence from CS(A 'IJ -+ X 'C) has been observed following the vacuum-U.V.photolysis of CS .232 The spin-forbidden nature of the overall process cs2 5 CS(A 'n) + s(33P2) (57) was accounted for in terms of predissociation from the initially populated Rydberg states of CS . CSe.-The formation of CSe following the flash photolysis of CSe with light restricted to A > 320nm (for which direct photodissociation is energetically impossible) has been attributed to the reactions173 and 2 CSe,* -+ 2 CSe + Se, (59) which is similar to the mechanism previously proposed to explain analogous results with CS,. The formation of Se simultaneously with CSe appears to rule out an alternative mechanism involving absorption of a second photon by CSe,*.The reaction of oxygen atoms produced by photolysis of NO, with CSe, was also shown ' to produce CSe via o + CS~ * SeO + CSe (60) By monitoring the growth of CSe a lower limit for k60 of > 2 x cm3 molecule- s- ' was estimated. The SeO radical was not observed ; however, SeO formed via SeO + NO -+ SeO + NO (61) was observed and an approximate value for k6 given as k6 x 1.5 x 10- cm3 molecule- ' s- I estimated from the rate of growth of SeO .' 7 3 The NO + CSe, system is thus similar to the NO + CS system. Furthermore laser action resulting from the reaction 0 + CSe + Cot + Se (where Cot represents vibrationally excited CO molecules) has been observed.23 (62) 230 L. G. Piper W. C. Richardson G.W. Taylor and D. W. Setser Faraday Discuss. 2 3 1 G. W. Taylor D. W. Setser and J. A. Coxon J . Mol. Spectroscopy 1972 44 108. 232 H. Okabe J . Chem. Phys. 1972,56,4381. 233 1. W. M. Smith and C. Wittig Appl. Phys. Letters 1972 21 536. Chem. SOC. 1972 no. 53 p. 100 Reactions of Atoms and Small Molecules 59 NH.-Previous work on the reactions of NH(X3C) produced in the pulse radiolysis of NH and monitored via the A ,lI + X 3C transition (336.0 nm),234 has been extended to include reaction with NO(kN,+No = 3.8 x lo-" cm3 molecule- ' s- 1).235 N,.-Energy transfer from N,(A C,') to mercury atoms to yield Hg(6,P,) has been studied in a flow system:72 The value determined for k63 [(3.4 f 1.0) x lo-'' cm3 molecule-' s- '3 is in agreement with the value given by Callear and and strongly suggests that earlier work in which the mercury concentration was not monitored gave values which were too low.Rate data for quenching of N,(A ,Z,,+) by NH, SO,, C,N, CH,CN CH,OH CO (0'' = 0 and u" = l) N,O O H, CO, CH,, and NO have been given and are compared with previous data.236 Most of the data are in agreement where overlap occurs. N,(A3Z,+) has also been ob-served' Freund has presented a model calculation on the intrasystem cascading between the a'n, a' 'Xu- and wlAu states of N, and has resolved a number of apparently conflicting reports on the lifetime of the a'n state.238 Energy transfer from N,(a'll,) to CO to yield CO(A 'll) has been investigated in detail and the vibrational dependence of this process e s t a b l i ~ h e d .~ ~ ~ ~ ~ ' Quench-ing of N,(B313,) to N2(u'rIg) by ground-state nitrogen atoms has been ob-served;241 emission from N,(B' ,Xu-) has also been observed and the B 31-Ig state identified as the precursor.242 The mechanism for the recombination of N atoms into excited states of N continues to be a controversial problem. 107,243*244 Rate coefficients for the relaxation of vibrationally excited N (0'' = 1) by O(z3PJ) [k = (3.5 & 1.4) x loei5 cm3 molecule- ' s-'I CO [k = (2 & 1) x lo-' cm3 molecule-' s-'1 and N,O [ k = (9 f 4) x 1O-l4cm3 molecule-' s- '3 have been measured using a photoionization techniq~e.'~',~~' Relaxation by CO and N,O proceeds by near-resonant vibrational exchange."' Nikitin has presented a theoretical treatment for the vibrational relaxation of N by oxygen atoms together with a number of other interesting atom + diatom systems.The theory shows agreement with the experimental results within an order of magnitude.246 The theoretical treatment by Fisher and B a ~ e r ' ~ ~ is also relevant to these data. 23* G. M. Meaburn and S. Gordon J . Phys. Chem. 1968,72 1592. 235 S. Gordon W. Mulac and P. Nangia J . Phys. Chem. 1971,75 2087. 236 J. A. Meyer D. W. Setser and W. G. Clark J . Phys. Chem. 1972 76 1. "' 238 R. S. Freund J . Chem. Phys. 1972 56 4344. 2 3 9 ' * O 2 4 1 2 4 2 2 4 3 M. F. Golde and B. A. Thrush Faraday Discuss. Chem. SOC. 1972 no. 5 3 p. 52. '** See also the general discussion following refs. 107 and 243. 2 4 s R. J. McNeal M. E. Whitson and G. R. Cook J . Chem. Phys.1972,57,4752. 246 E. E. Nikitin and S. Ya. Umanski Faraday Discuss. Chem. SOC. 1972 no. 53 p. 7. N,(A 3Cu+) + Hg(6'So) -% N,(X 'Cg+) + Hg(63P1) (63) directly ; however kinetic studies have not been made. Personal communication in ref. 236. M. F. Golde and B. A. Thrush Proc. Roy. SOC. 1972 A330,97. M. F. Golde and B. A. Thrush Proc. Roy. SOC. 1972 A330 109. M. F. Golde and B. A. Thrush Proc. Roy. SOC. 1972 A330.79. M. F. Golde and B. A. Thrush Proc. Roy. SOC. 1972 A330 121 60 R. J. Donovan D. Husain and L. J. Kirsch N,+.-The direct observation of N2’(X2 Zgf u” = 0) using kinetic absorption spectroscopy with pulse radiolysis has been reported.2474 The decay of N2+ was observed to be third order up to pressures of 160Torr although slight curvature in a plot of the second-order rate coefficients against pressure of N, could be observed from the data presented and suggests that the reaction to yield N,+ is in the fall-off region.The reactions of N,’ with Xe O, CO NO, H, H,O NH, CH, C,H, buta-1,3-diene and cyclopropane (charge transfer being dominant in many cases) were also and demonstrate that this is a useful technique for obtaining total cross-sections for ion-molecule reactions. NO.-Vibrational relaxation of NO(X2 n 0’’ = 2 and v” = 1) has been in-vestigated using a sensitive ‘split beam’ kinetic spectroph~tometer.~~~~ The vibrationally excited nitric oxide was formed via the reaction between O(2’0,) and N,O the yield of vibrational energy being -20% of the total available energy. Vibrational relaxation by N,O CH, and Ar was investigated and it was shown that the rate for vibrational exchange with N,O decreased with vibrational quantum number owing to the increasing energy discrepancy.The quenching and enhancement of NO /3-emission by N(2,S)and 0(23PJ) has been discussed.247c OH.-An excellent review of gas-phase reactions involving the hydroxyl radical has been given by Drysdale and Lloyd.248 A second more recent review has been given by Wilson,249 with emphasis on reactions involving CO H CH,, and self-disproportionat ion. Resonance fluorescence (A ,C+ u’ = O-+ X ,lI D” = 0) has been used to monitor the reactions of OH(X ’n) with H, D, and C0.250 The OH radicals were produced by vacuum-u.v. photolysis of H,O and concentrations as low as 10” molecules cmb3 could be detected thus reducing the possibility of radical-radical side-reactions.Rate data at 298 K were determined as (overall uncertainty OH + co a CO + H (k64 = 1.35 x 10-13 cm3 molecule-’ s-1) (64) It 15 %), OH + H H,O + H (k65 = 7.1 x 10-15 cm3 molecule-’ s-1) (65) OH + D -% HDO + D (k66 = 2.05 X lo-’’ cm3 molecule-’ S - l ) (66) A brief review of previous work on these reactions was given together with an extensive table of previous data. The resonance fluorescence technique gives values for the above rate constants which are in very close agreement with the results of Greiner (reviewed last year) and with most of the other direct deter-minations.* 5 0 2 4 7 ( a ) J. W. Dreyer and D. Perner Chem. Phys. Letters 1971,12,299; (b) C. R. Boxall and J. P. Simons Proc.Roy. SOC. 1972 A328 515; (c) I. M. Campbell and S. B. Neal, Faraday Discuss. Chem. SOC. 1972 no. 53 p. 72. 14’ D. D. Drysdale and A. C. Lloyd Oxidation and Combustion Rev. 1970 4 157. 249 W. E. Wilson J . Phys. and Chem. Ref. Data 1972 1,535. 1 5 0 F. Stuhl and H. Niki J. Chem. Phys. 1972 57 3671 Reactions of Atoms and Small Molecules 61 Reactions of OH with the oxides of nitrogen have received considerable atten-tion and their importance in atmospheric reactions has been empha~ized.~’ 1-254 The reaction OH +NO + M -% HONO + M (67) has been studied by a number of techniques including resonance fluores-cence,251*252 resonance absorption,253 and e.s.r. ;”‘ rate data for this reaction are collected in Table 25. Morley and Smith253 studied reaction (67) using flash Table 25 Rate data for the reaction OH + NO + M + HNO + M k/cm6 molecule-2 s-(4.1 & 0.6) x (1.9 & 0.3) x 3.3 x 10-3’ 5.6 10-31 T/K 300 416 273 298 397 297 300 300 M He He Torr) Ar (8 Torr) He (30 Torr) He (5 Torr) Comments Re$ 253 He was added to gas mixtures to yield a total pressure of 30 Torr Studied over the pressure range 1 254 0.5-5 Torr Preliminary 1 252 values Rate constants taken from graph in ref.251 at 30Torr 251 Data are pre-sented for the range 5-82 Torr and a high-pressure limiting value of k = (2 k 1) x 10-l2 cm3 molecule-s-’ was given H was shown to be 2.3 k 0.9 times more efficient than He as third body. photolysis and resonance absorption techniques. Hydroxyl radicals were produced by flash-photolysing NO + H mixtures : NO NO + 0(210,) (68) (69) 0 ( 2 ’ D ) + H + OH + H H + NO _* OH + NO 2 5 2 5 2 F.Kaufman and J. C. Anderson Chem. Phys. Letters 1972 16 375. 2 5 3 C. MorleyandI. W. M. Smith J.C.S. Furaduy ZI 1972,68 1016. 2 5 4 A. A. Westenberg and N. De H a s J . Chem. Phys. 1972,57 5375. F. Stuhl and H. Niki J . Chem. Phys. 1972 57 3677 62 R. J. Donovan D. Husain and L. J. Kirsch The technique proved to be very sensitive and concentrations of OH of less than lo1 molecules cm- were used. Reaction (67) was assumed to be in its third-order region (total pressure 30Torr) by comparison with data on reaction (71) and k67 given as (4.1 f 0.6) x cm6 molecule-2 s-’ (300 K).253 The smaller value for k, observed at 416 K demonstrates that the reaction has a ‘negative activation energy’ as expected [ x - 6.7 (& 2.0) kJ mol- The data of Westen-berg and De H ~ s s ~ ~ obtained using a fast-flow reactor and e.s.r.detection of OH (OH produced via H + NO,* OH + NO) show a similar temperature dependence (Table 25)’ but differ by a factor of two from the data of Morley and Smith indicating that reaction (67) is not in its third-order region at 30Torr (He) as assumed. This point is confirmed by the work of Stuhl and Niki,251 who employed the resonance fluorescence technique to follow the reaction over the pressure range 5-82Torr. The data of Morley et al.253 and Westenberg et al.254 are thus seen to be in agreement within experimental error. The e.s.r. data254 also show Ar to be less efficient than He as third body and thus indicate that the preliminary results of Kaufinan and Anderson252 are in agreement with the other rate data for reaction (67).The only anomalous feature in these kinetic studies appears to be the observation with the e.s.r. technique that reaction (67) becomes pressure-independent above 3 T ~ r r . ~ ~ The reaction OH +NO + M HNO + M (71) has also been studied by resonance absorption, 53 resonance fl~orescence,~ 5 2 and e.~.r.,’~ techniques. The data are collected in Table 26. Morley and Smith253 Table 26 Rate data for the reaction OH + NO + M -+ HNO + M k/cm6 molecule-2 s- T / K M Comments Ref } 252 (1.0 0.3) x 10-30 297 Ar (3 Torr) Data given for (2.0 0.5) 10-30 297 N (8 Torr) the range 0.5-10 Torr 2.0 x 10-30 273 He Studied over 1.6 x 10-30 298 He the range } 254 5.8 10-31 395 He 0.5-5 Torr (I Data are presented253 over the pressure range 20-300 Torr and for temperatures of 300 and 416 K.studied the reaction over the pressure range 20-300 Torr and at temperatures of 300 and 416 K. In this pressure range the reaction was clearly observed to be in the transition region between second- and third-order kinetics. Unfortunately, the high-pressure limit was not accessible; however the variation of the rate with total pressure was compared with the behaviour predicted by RRKM theory.253 The e . ~ . r . ~ ~ data show that Ar is a factor of two less efficient than He as a third body in reaction (71) and are thus in agreement (for p < 3 Torr) with the data of Kaufman and Anderson.252 The rate constant for reaction (71) also shows a negative temperature coefficient as expected.Reactions of Atoms and Small Molecules The reactions 63 OH + HN03 -& H,O + NO3 0 + HNO -% OH + NO, (72) (73) were also investigated and k72 was determined as (1.3 & 0.5) x lo-' cm3 mole-cule-' s- at 300 K. The rate constant k, was found to be < 1.3 x cm3 molecule- ' s-' at this temperat~re.'~~ Hydroxyl radicals have also been observed in absorption following the reaction of hot hydrogen atoms with CO 2 5 5 H + CO -+ OH + CO AH = 102kJmol-' (74) It was shown that only a small proportion (- 10 %) of the hot atoms lead to OH formation the major process being loss of kinetic energy on collision with CO . Overtone emission from the U" = 9 level of OH(X ,lI) produced via the reaction of H atoms with O, has been utilized to monitor the reactions of this vibrationally excited state with 0,.256 The mean radiative lifetime for U" = 9 [r = (6.4 & 1.4) x lo- s] and the rates for vibrational relaxation by the quenching gases 0, N, NO N,O CH, CO, SO, H2S or H,O have been given.2579258 The rates for reaction of levels U" = 2-9 with ozone have also been examined.," Chemiluminescence resulting from the reaction of 0 with a number of small olefins and with acetaldehyde has been shown to arise (in part) from the forma-tion of vibrationally excited OH radicals.,,' SH.-The formation and decay of SH radicals following the flash photo-dissociation of H,S has been re-inve~tigated.~~~ It was proposed that the reaction S + SH + S + H AH = -8lkJmol-' (75) is rapid and that the rate-determining step for removal of SH under the conditions used was either H + SH -% H + S AH = -92kJmol-' (76) or SH + SH -% H,S + S AH = -43kJmol-' (77) where k, = 1 x lo-'' or k, = 3 x lo-'' cm3 molecule-'s-'.H,S has been monitored in time-resolved experiments.261 2 5 6 A. E. Potter R. N. Calthorp and S. D. Worley J . Chem. Phys. 1971,54 992. 2 s 7 A. E. Potter R. N. Calthorp and S. D. Worley J . Chem. Phys. 1971,55 2608. 2 s 8 A. E. Potter R. N. Calthorp and S. D. Worley J . Phys. Chem. 1972,76 151 1. 2 5 9 A. E. Potter R. N. Calthorp and S. D. Worley Appl. Optics 1971 10 1786. 2 6 0 B. J. Finlayson J. N. Pitts and H. Akimoto Chem. Phys. Letters 1972 12 495. 2 6 1 Fluorescence from SH(A 2Z+ U' = 0) formed in the vacuum-u.v.photolysis of Values for the 5 5 R. B. Langford and G. A. Oldershaw J.C.S. Furuduy 11 1972,68 1550. K. H. Becker and D. Haaks J . Photochem. 1972.1 177 64 R. J. Donovan D. Husain and L. J . Kirsch radiative lifetime [ z ~ ~ = (0.55 & 0.14) x lop6 s ; T~, = (0.37 & 0.07) x s] and cross-sections for electronic quenching by H,S D,S H N He Ar or Ne, are reported. Cross-sections for quenching of SH by H,S and SD by D,S (-200 x 10- l6 cm2) indicate that long-range resonant transfer is involved ; cross-sections for the other gases are very much smaller (0.1 x 10- l 6 cm2).261 The oscillator strength for the transition SH(A ,C+ u' = 0- X ,II u" = 0) was calculated as foo = (1.45 k 0.4) x lo- [foo(SD) = (2.2 k 0.4) x 10-3].261 HSe and me.-Vacuum-u.v.spectra of these transient species have been observed following the isothermal flash photolysis of H,Se and H,Te.262 A number of new Rydberg transitions were reported and values for the ionization potentials given as I.P.(HSe) = 9.8 eV and I.P.(HTe) = 9.1 eV. The kinetics for the formation and decay of HSe and HTe were not investigated. However it was noted that electronically excited HTe ( X ,111,,) was formed ;262 spin-orbit relaxation of this state to the ground-state HTe(X ,II3/2) should be of interest as HTe is isoelectronic with the iodine atom. However unlike the iodine atom, it is now possible for the electronic energy to be partitioned between vibration and rotation states during collisions with monatomic quenching partners.0 2 ( X 3 C,-).-The vibrational relaxation of 0 formed by flash photolysis of 0, in levels u" = 9-14 has been monitored uia the Schumann-Runge ~ y s t e r n . ' ~ ~ ~ ~ ~ Relaxation by OQ3P,) was shown to be efficient and the results strongly suggest that multiquantum transitions are important.' 43*263 O,(db,).-A large number of new absorption bands arising from 02(u'Ag) have been observed in the vacuum ultraviolet following the flash photolysis of 0 in the presence of He.264 Over twenty new electronic states (Rydberg) of 0 have thus been located and Rydberg series leading to the first ionization potential identified. This new spectrum was further used to examine quenching of O,(u'Ag) by the gases He Ar Kr Xe N, H, and CO (Table 27).264-274 The rate constant for reaction with 0 was also determined264 as (4.4 f.1.3) x 10- ' cm3 molecule- s- ' (300 K). The Arrhenius parameters for reaction with O3 have been reported by several workers and there is now general agreement over their v a l ~ e s . ~ ~ ~ ~ ~ ~ Us ing labelled oxygen Jones and B a y e ~ ~ ~ have shown that '" R. J. Donovan D . J. Little and J. Konstantatos J.C.S. Faraday 11 1972 68 1812. 263 H. Webster and E. J. Bair J . Chem. Phys. 1972,56 6104. 264 R. J. Collins D . Husain and R. J. Donovan J.C.S. Faraduy 11 1973 69 145. 2 6 5 K. H. Becker W. Groth and U. Schurath Chem. Phys. Letters 1971 8 259. 2 6 6 F. D. Findlay and D. R. Snelling J . Chem. Phys. 1971 55 545. 2 6 7 R. P. Wayne d d v . Photochem. 1969,7 31 1 . 2 6 8 I. D. Clark and R.P. Wayne Chem. Phys. Letters 1969 3 405. 269 I. D. Clark and R. P. Wayne Proc. Roy. SOC. 1969 A314 1 1 1 . 2 7 0 K. H. Becker W. Groth and U. Schurath Chem. Phys. Letters 1972 14 489. 2 7 1 "' I. D. Clark I. T. N. Jones and R. P. Wayne Proc. Roy. SOC. 1970 A317 407. 2 7 2 7 4 W. H. Breckenridge and T. A. Miller J . Chem. Phys. 1972 56 465. 2 7 5 W. Groth and R. P. Wayne Faraday Discuss. Chem. SOC. 1972 no. 53 p. 232. 276 I. T. N. Jones and K. D. Baynes J . Chem. Phys. 1972,57 1003. F. D. Findlay and D . R. Snelling J . Chem. Phys. 1971 54 2750. F. D. Findlay C. J. Fortin and D. R. Snelling Chem. Phys. Letters 1969 3 204 Reactions of Atoms and Small Molecules Table 27 Rate constant (k,)for the collisional quenching of02(a1A,) at 300 K 65 Quenching gas He Ar Kr Xe 0 3 N2 H2 co k,/cm3 molecule- s - ' < 10-20 8.0 f 2.6 x < 8 x lop2' 9.0 f 1.2 x < 8 x < 7.9 * 5.4 x 3.4 f 0.9 x < 2 10-19 4.4 1.3 x 10-15 4.4 10-15 3.8 x 10-15 3.5 1.0 10-15 ~ 1 .1 x 10-19 < 10-20 1.4 k 0.2 x < 3 x 5.3 _+ 0.9 x lo-'* 4.53 k 0.19 x lo-" 3.7 f 0.3 x lo-'* <3.3 x 10-l6 <7 x 10-17 Method" v. u. v. a.e. i.r.a.e. i.r.a.e. i. r.a. e. a.e. i.r.a.e. k.p.i. V.U.V. V.U.V. V.U.V. V.U.V. a. e. i.r.a.e.b k.p.i.' k.p.i. a.e. i.r.a.e. i. r. a.e. i.r.a.e. a.c. i.r.a.e. e.p.r. V.U. v. V.U.V. V.U.V. Ref: 264 26 5 266 264 266 265 264 264 264 270 27 1 267,272 264 268,269 26 5 273 264 266 265 264 274 267-269 v.u.v.kinetic spectroscopy in the vacuum ultraviolet; a.e. atmospheric band emission following energy pooling ; i.r.a.e. infrared atmospheric band emission ; k.p.i. kinetic photoionization. Rate constant determined as a function of temperature. resonant energy transfer from 02(a'A,) to another oxygen molecule is extremely efficient. An important paper by Merkel and K e a r n ~ ~ ~ ~ on the quenching of 02(a1Ag) has recently appeared and although the experimental results were obtained from studies in solution they will certainly be relevant to gas-phase studies and the theoretical treatment given should be of fundamental importance. The efficiencies for quenching by the various solvents studied showed marked differences (H20 was - 300 times more efficient than CCl,).Furthermore marked differences between deuteriated and non-deuteriated solvents were observed H,O being ten times more efficient than D,0.277 According to the theory developed by Merkel and K e a r n ~ ~ ~ the electronic energy is coupled to the vibrational degrees of freedom in the quenching molecule and thus the quenching efficiency is related to the intensities of infrared overtone and combination bands of the quenching molecule in the energy regions appropriate for the transitions O,(a'A,)-+ O,(X 3Cg- u" = n) (with n = 0 being dominant in most cases). The theory was also extended to include quenching of 0 2 ( b 1 Z + ) and gave results in quantitative agreement277 with gas-phase experimental data. However, 2 7 7 P. B. Merkel and D. R. Kearns J .Amer. Chem. SOC. 1972,94 7244 66 R. J. Donovan D. Husain and L. J. Kirsch quenching of O,(a'A,) by amines and species with low ionization potentials may occur via a mechanism involving charge transfer and is thus not expected to follow the predictions made by the above theory. The rate data for quenching of O,(a'A,) by aliphatic amines have recently been revised in view of the realization that even very low concentrations of atomic oxygen in a flow system may lead to erroneous rate data278 (0 atoms were reduced to very low concentration levels by titration with NO,). Similarly the rate constant for quenching by tetra-methylethylene has been revised.," An increase in the rate coefficient at low pressures for quenching of O2(a1A,) by TME has been attributed to redissocia-tion of the initially formed addition adduct.280 It would appear that the presence of oxygen atoms might also account for these results (the data of Ogryzlo et ~ 1 .~ " show a decrease in rate coefficient with decreasing pressure; however if 0 atoms are not rigorously excluded the system is expected to be complex). Efficient relaxation of O,(a'A,) by the triplet states of quinoxaline and naph-thalene has also been reported.281 Direct excitation of the ' A state via the forbidden (a'A + X 'Z,-) transition has been achieved using the output from a Nd-YAG laser;282 however pressures of 130atm were required to obtain sufficient light absorption. The excitation of I by singlet oxygen has received further detailed study and a number of rate constants for energy transfer between these species and iodine atoms have been e s t a b l i ~ h e d .~ ~ ~ ~ ~ ~ Th e strong chemiluminescence observed from this system has been shown to arise from the stepwise excitation I,(X 1zg+) + o,(m,+) -+ I,[A 3n(i,)l + o,(x 3 ~ - ) (78) with the first process also producing atomic iodine. This work also demonstrates that resonant energy-transfer processes may be several orders of magnitude faster than analogous but non-resonant spin-allowed processes. Thus the process (80) 02(a1Ag) + I(5,P3,,) -+ I(52P1,2) + O,(X 'Xg-) 02(a'Ag) + I(52P,p) -+ 1(s2P3/,) + O,(X 3Zg-) is more than 5 x 10 times faster than the non-resonant (81) Similarly the energy transfer reaction I,[B 3n(0,+)] + O,(X 'X,-) -+ I,(X 'Cg+) + 0 2 ( ' A g 'C,+?) (82) has been shown to be highly inefficient (experiments carried out in hydrocarbon 278 K.Furukawa and E. A. Ogryzlo J . Photochem. 1972 1 163. 2 7 9 K. Furukawa and E. A. Ogryzlo Chem. Phys. Letters 1971 12 370. 2 8 0 R. A. Ackerman J. N. Pitts and R. P. Steer Chem. Phys. Letters 1972 12 526. 2 8 1 C. K. Duncan and D. R. Kearns Chem. Phys. Letters 1971 12 306. 2 8 2 I. B. C. Matheson and J. Lee Chem. Phys. Letters 1972 14 350; ibid. 1970 7 475. 283 R. G. Derwent and B. A. Thrush J.C.S. Faraday I I 1972,68 720. 284 R. G. Derwent and B. A. Thrush Faraday Discuss. Chem. SOC. 1972 no. 53 p. 162. 2 8 5 J. Olmsted and G . Koral J . Amer. Chem. SOC. 1972 94 3305 Reactions of Atoms and Small Molecules 67 solution). The absorption of U.V. radiation leading to the simultaneous excitation of O,(a'A,) and the lowest triplet state of several aromatic hydrocarbons has been observed286 (solutions containing the aromatic hydrocarbon and oxygen under pressure were used).The reaction of hydrogen atoms with O,(a'A,), H(l2S1,,) + O,(ulAg) -+ OH(X 'll) + O(23PJ) (83) has been shown to be anomalously k83 < 3 x 10- l 3 cm3 molecule- s- ' (see also earlier section on H atoms p. 24). Both reactants and products correlate adiabatically via surfaces of ,A' and ,A'' symmetry the surface of ,A' symmetry being attractive according to recent 'non-empirical LCAO-MO-SCF and CI SO.-Fluoresence from SO[(A311) and B(3 C)] has been observed288 following the vacuum-u.v. photolysis (Kr lines 116.5 and 123.6 nm) of SOCl, and has been used to determine the heat of formation of ground-state SO; 5.4 > AGo(SO) > 3.3 kJ mol- '.and should therefore provide an efficient path for fast reaction. The rate constant for the energy-transfer process O,(a'A,) + SO(X 3C-) .-) SO(a'A) + O,(X 3C,-) (84) has been determined as 3.5 x 10- l 3 cm3 molecule-' s-l using e.p.r. with flow-tube techniques.' 74 Br2.-The radiative lifetimes for the vibrational levels u' = 1 to u' = 31 of Br2[311(OU+)] have been determined using a tunable dye laser.289 The observed variations in the lifetimes (0.15-1.2 ps) were attributed to spontaneous pre-dissociation. Despite the use of a laser excitation to a wide range of rotational levels must certainly have been involved (the width of the exciting pulse was 0.1-0.8 nm) and thus the reported lifetimes will not necessarily be the same as those determined using other 'broad band' excitation sources ; considerably higher resolution will be required before the lifetimes of individual vibration-rotation states can be determined (see 1 below).Cross-sections for self-quenching of these vibrational levels were also reported.289 BrF and IF.-Emission from the B311(O+) states of BrF and I F [B311(O+) --* X 'C+] following combination of the atoms in the presence of O,(a'A,) has been reported.lg3 Numerous new band systems have been observed and vibra-tional constants for the ground states of these unstable interhalogens have been given. The role of energy transfer from O,(u'A,) to the states of these inter-halogens populated by the atomic combination processes is discussed.193 12.-Steinfeld290 has recently given a useful review of the various processes important in the non-radiative depopulation of the [B 311(Ou')] state of I,, 2 8 6 D. F. Evans and J. M. Tucker J.C.S. Faraday IZ 1972,68 174. 2 8 7 J. L. Cole and E. F. Hayes J . Chem. Phys. 1972 57 360. 2 8 9 G. Capelle K. Sakurai and H. P. Broida J . Chem. Phys. 1971,54 1728. 2 9 0 J. I. Steinfeld Faraday Discuss. Chem. SOC. 1972 no. 53 155. H. Okabe J . Chem. Phys. 1972 56 3378 68 R. J. Donouan D. Husain and L. J. Kirsch including spontaneous magnetically-induced and collision-induced predissocia-tion. However several points made in that review require some modification in view of the more recent work discussed below. Work on the lifetime of 12[B 3n(0,')] has been reviewed in detail by Telling-h u i ~ e n ~ ~ l (including the recent work by Shotton and Chapman292).The location of the 'lI(1,) state deduced from data on the continuous absorption spectrum, was shown to be consistent with the data on spontaneous predissociation and explains the main features in the variation in the lifetime for the B state vibrational levels. Discrepancies in some of reported data for specified u' levels are attributed to a J(J + 1) rotational dependence of the predissociation (different lifetimes resulting from different J distributions formed in the 'broad band' excitation process). 'Narrow band' excitation of the u' = 11 J = 128 and u' = 6 J = 32 using a modulated He-Ne laser has allowed a very precise determination of the lifetimes of these states.292 A previous proposal293 that quenching of 12[B31T(0,+) v' = 431 proceeds in part uia collision-induced predissociation yielding two I(52P3,2) atoms (in addition to quenching to the ground electronic state and vibrational relaxation), has been re-interpreted208 as due to 'collisional release' to yield I(52P,,2) + I(52P3,,).Thus collisional release is observed from levels 3 kT below the dis-sociation limit with an efficiency of 0.5; the efficiency rises to approximately unity within 2 kT of the dissociation limit.208 These surprisingly high efficiencies clearly have important implications in several areas of photochemistry and chemical kinetics and particularly for theories of atomic recombination. The observation that He and Xe have closely similar efficiencies for collisional release at all energies is another surprising feature208 and cannot apparently be reconciled with available theoretical models ; it also rules out the stabilized atom-molecule-complex mechanism at least for the noble gases.A shortening of the fluorescence lifetime of 12[B 311(0,')] in the presence of a strong magnetic field has been and attributed to mixing of the B state with the repulsive 0,- state induced by the field. The lifetime was ob-served to decrease by up to a factor of two for fields of - 15 kG for some vibra-tional levels. A consideration of the variation in lifetime has allowed the approxi-mate position at which the 0,- state crosses the B state to be located.294 The vibrational relaxation of 12[B 311(0,')] has been discussed in terms of an optical and the influence of open reactive channels (collision-induced predissociation in this case) considered.Vibrational transition probabilities are only affected when the predissociation probability is larger than 0.3. The total cross-section for quenching of 12[B 31T(0,') u' = 61 by I has been determined297 2 9 1 J. B. Tellinghuisen J . Chem. Phys. 1972 57 2397. 2 9 2 K. C. Shotten and G. D. Chapman J . Chem. Phys. 1972,56 1012. 293 R. B. Kurzel and J. I. Steinfeld J . Chem. Phys. 1970 53 3293. 294 G. D. Chapman and P. R. Bunker J . Chem. Phys. 1972,57 2951. 2 9 5 H. P. Broida and G. A. Capelle J . Chem. Phys. 1972,57 5027. 2 9 6 L. L. Poulsen J. Ross and J. I. Steinfeld J . Chem. Phys. 1972 57 1592. 2 9 7 R. B. Kurzel E.0. Degenkolb and J. I. Steinfeld J . Chem. Phys. 1972 56 1784 Reactions of Atoms and Small Molecules 69 as 53 x 10- l 6 cm’ in agreement with the value of Shotten et ~ 1 . ~ ’ ~ [(64 f 3) x 10- l 6 cm2]. Vibrationally inelastic collisions with Ne for Av = f 1 have a c r o ~ ~ - ~ e c t i o n ~ ~ ~ of 1.3 x cm’. Collisional depolarimtion of I,[B 3n(0,’)] has been in~estigated,~” and it was shown that reorientation of angular momentum is only important when accom-panying rotational or vibrational energy changes takes place. Depolarization cross-sections for H He and Ne are reported (typically of the order 10- ’ cm2). Ba0.-Probably one of the most important recent developments in molecular-beam scattering is the reported use of laser-induced fluoresence for observing the vibrational-rotational distribution of reaction products.The product molecules (BaO) resulting from the reaction of a beam of Ba atoms with O,, were excited with a tunable dye laser and the resonance fluorescence spectrum was recorded.55 From a knowledge of the Franck-Condon factors for the transitions involved the relative populations of the Id’ J ” ) states could be deduced from the relative fluorescence intensities. Further developments in the technique will be required to obtain ‘unrelaxed’ rotational distributions ; however the preliminary resultss5 clearly indicate that reliable vibrational distributions may be obtained. The production of electronically excited BaO via the reactions Ba + N,O -+ BaO(A ‘Z) + N, Ba + NO -+ BaO(A ‘Z) + NO (85) (86) has also been rep~rted.~’ Emission from the A ‘C state was observed and it was shown that -0.4% of the reactive events lead to this state for reaction (85).59 Reaction (86) gives a significantly lower yield (-0.03 %) of BaO(A ‘Z) in agree-ment with previous molecular-beam work.Chemiluminescence has also been observed’ using semi-molecular-beam techniques following the reaction of Ba Sr and Ca with NO and N,O. The total cross-sections for beam attenuation with NO lie in the range (90-170) x 10l6 cm2 (see Table 3 for more details) indicating that an electron-jump mechanism is involved and (15-30) x 10- l6 cm2 for N,O. The chemilumines-cence was found to be weakly polarizeds4 in some cases and a model involving the partitioning of angular momentum between rotation and recoil angular momentum of the products was proposed to account for this.HgH.-Vacuum-u.v. absorption spectra of HgH and HgD formed by reaction of Hg(63P0,1) with H and D have been recently reported,’ but no electronic assignments were made. Emission from HgH(’lI + X ,Z’) at 350 nm following reaction between Hg(63P0) and H D, CH, C,H6 C3H8 or n-C,H has been observed. ’ 4 Triatomic Species NH2.-The reactions of NH produced in the pulse radiolysis of NH have been studied by monitoring the ,A p-+ X 2 B transition (597.6 nm) in ab~orption.~’’ 2 9 8 R. B. Kurzel and J. I. Steinfeld J . Chem. Phys. 1972 56 5188 299 S. Gordon W. Muiac and P. Nangia J . Phys. Chem. 1971,75 2087 70 R. J. Donovan D. Husain and L. J. Kirsch The rate constants for reaction with NO (k = 2.7 x 10- '' cm3 molecule- ' s- '), recombination with hydrogen atoms [k"H2+H+M) = 6.1 x cm6 molecule-2 s- ' ; high-pressure limiting rate for M = NH,] and reaction with a second NH, radical [k(M12+NH2) = 1.03 x lo-'' cm3 molecule-' s-'1 were reported.,'' Fluorescence from NH,(A2A ') produced by vacuum-u.v.photolysis of NH , has been employed3" to measure the total cross-sections for removal by He, Ne Ar Kr Xe H, N, NH, and CH (the cross-sections are all close to the gas-kinetic values but with He being least efficient). NO2 .-The fluorescence lifetime of NO continues to receive much attention, and has led to a number of reports which apparently conflict. The present status of this work has been reviewed in considerable detail by Sackett and Yardle~,~" who also present results obtained by narrow band (- 0.08 nm) excitation with a tunable dye laser.Excitation in the region 451.5-460.5nm gave rise to non-exponential decays and 'lifetimes' following excitation in different wavelength regions were found to vary between 62 and 75 p . Anomalies in some previous lifetime measurements were shown to arise from restrictive observation geo-metries (i.e. owing to diffusion of the longer-lived states out of the observation zone). The observation of a short-lived fluore~cence~'~ demonstrates that the states responsible for the relatively high oscillator strength of NO in the visible and ultraviolet can contribute to the fluorescence although it is rather weak. The long-lived fluorescence (z > 80 p s ) was attributed to states associated with the quasi-continuous ab~orption.~' ' It was concluded that transitions to both the ,B1 and ' B electronic states occur over a large portion of the visible spectrum, with the ,B transition being the stronger (both states are also perturbed).," In view of the stimulus provided to theoretical treatments it is to be hoped that further work in this area using even narrower excitation sources will be forth-coming in the near future.The relative fluorescence efficiency for NO, excited in the wavelength range 380-520 nm (with 0.01-0.1 nm resolution) has been examined and shown to be constant in the region 415-520nm.303 Below 415nm the fluorescence efficiency declines rapidly but indicates that thermal rotational energy of NO, can be used to reach the threshold for predissociation even for electronic excita-tion just below threshold (Thrush304 has pointed out that this argues against predissociation by rotation being important).Results on the quantum yield for photodissociation in the threshold r e g i ~ n ~ " . ~ ' ~ support the fluorescence data, and the relevance of these results to the two-body radiative recombination of 0 + NO has been disc~ssed.~'~ 3 0 0 M. Lenzi J. R. McNesby A. Mele and C. N. Xuan J . Chem. Phys. 1972,57 319. 301 P. B. Sackett and J. T. Yardley J. Chem. Phys. 1972,57 152. 302 P. B. Sackett and J. T. Yardley Chem. Phys. Letters 1971 9 612. 3 0 3 E. R. C. Lee and W. Uselman Faraday Discuss. Chem. SOC. 1972 no. 5 3 p. 125. 304 B. A. Thrush Faraday Discuss. Chem. SOC. 1972 no. 53 p. 142.3 0 5 J. Troe I. T. N. Jones and K. D. Bayes Faraday Discuss. Chem. SOC. 1972 no. 53 3 0 6 H. Gaedtke H. Hippler and J. Troe Chem. Phys. Letters 1972 16 177. 307 D. Kley Faraday Discuss. Chem. SOC. 1972 no. 53 p. 150. p. 148 Reactions of Atoms and Small Molecules 71 The 'photofragment spectrum' of NO resulting from excitation at 347.1 nm, indicates that 60 % of the available energy appears as translational motion of the recoiling fragments.,08 However the possibility that large numbers of slowly recoiling fragments are formed and which would be undetected by the apparatus employed could not be ruled out. Energy transfer from NO, excited by visible light to 0 uiu NO2(,Bl?) + O,(X 'Xg-) -+ N0,(w2A1) + O,(a'Ag) has been observed.309 The efficiency for transfer is low varying with the wave-length for NO excitation from 3 % at 400 nm to 0.5 % at 600 nm.However this would be sufficient to provide a significant source of 02(u1Ag) in some polluted atmospheres. ,09 NF .-The absorption bands of NF centred at 260 nm have been employed to monitor the kinetics of this radical produced by thermal dissociation of N,F4 in a low-pressure flow ~ystem.~ ' Recombination in the presence of He Ar 0 , NO or SF was investigated : (87) NF + NF + M A N2F4 + M (88) Values of k were given as k, = (1.26 f 0.15) x k, = (0.76 f 0.15) x 0.18) x cm6 molecule-2 s - ' at 293 K. The equilibrium constant was also measured for M = Ar by observing the concentration of NF in equilibrium with N2F4 following reaction in both directions (i.e. recombination of NF and dissociation of N2F4 at 'infinite' reaction time).A comparison of the forward and back reactions (shock-tube studies) with the equilibrium constant shows agree-ment within the combined uncertainties (i.e. within a factor of two).310 Further comparison with shock-tube data indicates that the temperature coefficients for recombination by the third bodies studied are significantly different. The reactions of NF with C1(3,P3,,) Br(4,P3/,) N(24S,i2) and O(2,PJ) were also studied and it was shown that removal of C1 and Br proceeds via the sequence lo-,, kSFa = (2.9 f 0.3,) X kNo % 0.6 X and ko = (0.98 & X + NF + M NF,X + M NF,X + X 5 NF + X, with kk = (2.7 & 0.4) x lo-,' and (1.0 f 0.2) x cm6 molecule-2 s - l for C1 and Br respecti~ely.~'' Removal of NF by N atoms produces fluorine atoms and the stoicheiometry for the reaction suggests that either or N + NF2 --* N2 + F2 N + NF -+ 2NF 2NF -+ N + F, are involved.lo 3 0 8 G. E. Busch and K. R. Wilson J . Chem. Phys. 1972,56 3626 3638. 3 0 9 I. T. N. Jones and K. D. Bayes Chem. Phys. Letters 1971 11 163. 310 M. A. A. Clyne and J. Connor J.C.S. Faraday II 1972,68 1220 72 R. J. Donovan D. Husain and L. J. Kirsch HO .-The molecular modulation technique has been employed to study the infrared and ultraviolet absorption spectra of the HO radical (absorption cross-sections are given as 4.5 x 10- '* cm2 at 210 nm and 5 x lo-,' cm2 at 1395 cm- 1).3 ' ' HO was produced from a number of sources including the photolysis of H,O (253.7nm) photolysis of 0 (253.7nm) in the presence of H,O, and photolysis of C1 (350 nm) also in the presence of hydrogen peroxide.The structure of the radical was deduced from infrared data (H-0-0 angle = 108" ; 0-0 distance 0.13 nm ; 0-H distance 0.096 nm).31 ' The rate constant for the disproportionation reaction HO + HO -% H,O + 0 (94) was found3 ' ' to be k, = (3.6 f 0.5) x 10- ' cm3 molecule- ' s- in agreement with the value given previously312 by Foner and Hudson (3 x 10- l 2 cm3 mole-cule- ' s- ') but differing significantly from the recent value3 l3 by Hochanadel et al. r(9.5 & 0.9) x cm3 molecule-' s-'1. The latter authors produced HO by the flash photolysis of water vapour in the presence of 0 (plus a 100-fold excess of inert gas) and report an extinction coefficient for HO2 at 210 nm which is - 32 % lower than that given by Paukert and John~ton.~ '' The rate constant for the reaction HO + OH -+ H,O + 0 (95) has been determined313 as (2.0 & 0.3) x lo-'' cm3 molecule-' s-'.SO .-The fluorescence of SQ from the and d states has been studied using single-photon counting technique^.^ l4 A decrease in the lifetime and fluorescence quantum yield was observed for 2 < 220.6 nm resulting from the opening of the dissociative channel leading to SO + 0 which becomes accessible at this energy (in agreement with Okabe3 "). For excitation above the dissocia-tion limit the lifetime was observed to decrease relatively slowly3 l4 (see Table 28), Table 28 Fluorescence data for SO, 229.75 227.7 1 226.09 225.8 224.3 222.41 220.63 218.78 216.93 215.24 z/ns 31.82 32.77 35.28 33.39 3 8.70 41.27 45.41 28.11 9.63 7.81 @,/(no rmaltzed) 0.907 1.0 0.98 0.956 0.919 0.745 0.664 0.151 0.0714 0.0244 3 1 1 T.T. Paukert and H. S. Johnston J . Chern. Phys. 1972,56,2824. 3 1 2 S . N. Foner and R. L. Hudson Adv. Chern. Ser. 1962 no. 36 p. 42. 3 1 3 C. J. Hochanadel J. A. Ghormley and P. J. Ogren J . Chern. Phys. 1972,56,4426. 'I4 Man-Him Hui and S. A. Rice Chern. Phys. Letters 1972 17,474. 3 1 5 H. Okabe J . Arner. Chem. SOC. 1971,93 7095 Reactions of Atoms and Small Molecules 73 indicating that the internal redistribution of energy is inefficient in small molecules, in agreement with theoretical predictions. Coupling between the c and d states and states belonging to other electronic manifolds was shown to be weak in contrast to the SO,('B,) state.314 The role of excited states of SO in the photo-chemical reaction with CO yielding CO has been disc~ssed.~' Collision-induced intersystem crossing from the singlet- to triplet-state manifold occurs with an efficiency of 3-9 % withN cyclo-C6H ,, or so as collision partner^.^^ CO ' ( A 'ill and B2 C,+).-Rate data for quenching of C02+[B2 Xu+ (O,O,O)] and CO,+[A ,nu (3,0,0)] have been reported.318 These states were formed by photolysis of CO with the He(1) line at 58.4 nm ; however emission from the C state which should also be populated at this wavelength (photoelectron data), could not be detected suggesting that spontaneous predissociation is significantly faster than radiative processes for this state.Stern-Volmer plots of the A -P X and B -P X emission following the addition of various quenching gases yielded relative rate data which were converted into absolute data using independently determined values for the radiative lifetimes of these states (zA = 1.5 x s; zB = 1.4 x s). Quenching by the molecules CO, H, N, O, CO N,O, C& Ar Ne or He was inve~tigated.~'~ Charge transfer to the quenching species is possible for all but He and Ne which are observed to be least efficient. The high quenching efficiencies observed for the other molecules (typically 3 0 - 4 times the gas kinetic collision rates) strongly suggest that charge transfer is an important channel although this could not be checked directly.318 quenching of CO,+[A 2nu(0,0,0)] was reported together with data for quenching of N,+(B ,Xu+) and N,O+(A 'Xu+). The quenching gases studied were NO C2H6 O, N,O CO, CO H, N,, Ar and He. A comparison of the results given for the different ions suggests that effects due to 'chemical interactions' are superimposed on the basic charge-transfer mechanism. In later work by Phillips et 5 Tetra-atomic Species CH .-The combination reaction between methyl radicals and molecular oxygen has been re-examined in detail using flash spectroscopy to monitor the CH,(P *- x) transition.320 The order of the reaction was found to lie between 2 and 3 for pressures of 25-380 Torr (at 295 K) and the results were shown to be consistent with the simple reaction sequence a CH + 0 'T; CH302* CH302* + M --* CH302 + M (97) 3 1 6 F. Wampler A. Horowitz and J. G. Calvert J . Amer. Chem. Soc. 1972,94 5523. 317 A. Horowitz and J. G. Calvert Znrernar. J . Chem. Kinetics 1972 4 191. C. A. Winkler J. B. Tellinghuisen and L. F. Philips J.C.S. Faraday ZI 1972 68 121. 3 1 9 J. B. Tellinghuisen C. A. Winkler C. G. Freeman M. J. McEwan and L. F. Phillips, J.C.S. Faraday ZZ 1972 68 833. 320 N. Basco F. C. James and D. G. L. James Znrernar. J . Chcm. Kinetics 1972 4 129 74 R. J. Donovan D. Husain and L. J. Kirsch The limiting value for the second-order rate coefficient at high pressure was given320 as (5.2 0.5) x 10- '' cm3 molecule-' s- ' which is a factor of three lower than the value determined by Callear et a1.321 using the same technique. However the limiting values given for the third-order rate coefficients at low pressures320 [(1.0 0.1) x lo-,' cm6 molecule-2 s-' for M = neopentane; (2.6 & 0.1) x cm6 molecule-2 s-' for M = N2] appear to be con-sistant with the data of Callear et ~ 1 . ~ ~ ~ (-6.1 x lo-,' cm6 molecule-2 s-' for M = propane). It was also shown that the rate constant for the reaction CH + 02+ CH20 + OH (98) has a low value (-3 x cm3 molecule-'^-^) and that this channel was therefore unimportant under the conditions used.320 A brief review of the pre-vious work on these reactions was given and the data were presented in con-venient graphical form ;320 this emphasizes the widely divergent results which have been presented for this ~ y s t e r n . ~ * ' ~ ~ ~ 32' A. B. Callear and H. E. van den Bergh Trans. Furuduy SOC. 1971,67,2107. 322 G. R. MacMillan and J. G. Calvert Oxidation and Combustion Rev. 1965 1 83