A laser flash photolysis-resonance fluorescence kinetics study of the reaction Cl(2P) + CH4→CH3 + HCl

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Abstract
The technique of laser flash photolysis‐resonance fluorescence is employed to study the kinetics of the reaction Cl(2P) + CH4→CH3 + HCl over the temperature range 221–375 K. Chlorine atoms are produced by photolysis of Cl2 at 355 nm. At temperatures ? 241 K the apparent bimolecular rate constant is found to be dependent upon the identity of the chemically inert gases in the reaction mixture. For Cl2/CH4/He reaction mixtures (total pressure =50 Torr) different bimolecular rate constants are measured at low (k1L) and high (k1H) methane concentrations. For Cl2/CH4/CCl4/He and Cl2/CH4/Ar reaction mixtures, the bimolecular rate constant (k1) is independent of methane concentration with k1?k1L. k1 and k1L are in good agreement with previous results obtained using the flash photolysis‐resonance fluorescence technique while k1H is in good agreement with previous results obtained using the discharge flow‐resonance fluorescence and competitive chlorination techniques. At 298 K the measured bimolecular rate constant is independent of the identity of the chemically inert gases in the reaction mixture and in good agreement with all previous investigations. The low temperature results obtained in this investigation and all previous investigations can be rationalized in terms of a model which assumes that the Cl(2P1/2) state reacts with CH4 much faster than the Cl(2P3/2) state. Extrapolation of this model to higher temperatures, however, is not straightforward.