ACCOUNTS OF CHEXICAL RESEARCH V O L U M E 1 0
N U M B E R 3
M A R C H , 1 9 7 7
Single Collision Studies of Vibrational Energy Transfer Mechanisms George A. Fisk* Sandia Laboratory, Albuquerque, New Mexico 87115
F. Fleming Crim* Los Alamos Scientific Laboratory, University of California, Los Alamos, New Mexico 87545 Received November 4,1976
Chemists have appreciated that the rates of collisional production and deexcitation of highly energized molecules can have profound effects on the overall kinetics of chemical reactions since at least as early as Lindemann’s hyp0thesis.l Expressed in current terms, the Lindemann mechanism for a unimolecular reaction is2 A+M%A* + M
(1)
A* + M % A + M
(2)
In the past decade the development of infrared gas lasers has led to rapid growth in studies of energy transfer in small molecules? This has happened in part because lasers offer great flexibility and specificity in producing and monitoring excited-state populations and in part because intermolecular energy transfer plays an important role in the operation of gas laser^.^>^ Recently laser excitation of molecular vibrations has been used to enhance chemical reaction rates? and isotopically selective rate enhancement is under active consideration as a practical technique for isotope separation.1° A crucial limitation to the efficiency of most laser excitation rate enhancement schemes is the collisional transfer of energy out of the initially excited molecule.ll Despite the importance of intermolecular energy transfer in a great variety of chemical situations and the attention which the subject has consequently received, a good understanding of the detailed mechanisms involved in collisional shuttling of energy from one molecule or degree of freedom to another is lacking
4 B A* -+ (3) where A is the reactant species, B represents the products, and M is any species present in the system. A* is a reactant molecule with sufficiently high internal energy, predominantly vibrational, to undergo transformation 3, and k3 is the decomposition rate constant for the energized reactant. Steps 1and 2 represent the collisional production and deexcitation of A*. In steady state, the overall rate of reaction is
(1)F. A. Lindemann, Trans. Faraday Soc., 17,598(1921). (2)P.J. Robinson and K. A. Holbrook, “Unimolecular Reactions”, Wiley-Interscience, New York, N.Y., 1972. (3)For a good introduction to the field see B. Stevens, “Collisional Activation in Gases”, Pergamon, New York, N.Y., 1967. (4)(a) J. I. Steinfeld, MTP Int. Rev. Sci.: Phys. Chem.,Ser. One, 9, 247 (1972); (b) J. P. Toennies, Chem. SOC.Rev., 3,407 (1974). (5)D.Secrest, Annu. Reu. Phys. Chem., 24,379 (1973). (6)E. Weitz and G. Flynn, Annu. Rev. Phys. Chem., 25, 275 (1974). (7)J. L. Ah1 and T. A. Cool, J . Chem. Phys., 58, 5540 (1973). (8)C. B. Moore, R. E. Wood, B.-L. Hu, and J. T. Yardley, J. Chern. Phys., 46,4222 (1967). (9)(a) J. T. Knudtson and E. M. Eyring, Annu. Rev. Phys. Chem.,25, 255 (1974); (b) N. G. Basov, A. N. Oraevsky, and A. V. Pankratov in “Chemical and BiochemicalApplications of Lasers”, Vol. I, C. B. Moore, Ed., Academic Press, New York, N.Y., 1974; (c)N. V. Karlov, Appl. Opt., 13,301 (1974);(d) F. Klein, F. M. Lussier, and J. I. Steinfeld, Spectrosc. Lett., 8, 247 (1975). (10)V. S. Letokhov and C. B. Moore, Sou. J. Quantum Electron., (Engl. Traml.),6 , 129, 259 (1976). (11) J. P. Aldridge, J. H. Birely, C. D. Cantrell, and D. C. Cartwright in “Physics of Quantum Electronics”, Vol. IV, S. F. Jacobs, M. Sargent 111, M. D. Scully,and C. T. Walker, Ed., Addison-Wesley, Reading, Mass., 1976.
and is clearly sensitive to Itl and k2 as well as to K3. For low-pressure gas-phase reactions (kZ[M]
