Time Lags and V-V' Steady States in the Infrared Laser Induced Decomposition of CHClF2 and CDClF2 under Collisional Conditions

Article abstract of DOI:10.1021/ja00375a013

V-T relaxation times of IR laser excited CHClF2 are relatively long, being equivalent to ca. 400 gas kinetic collisions.IR laser induced decomposition of CHClF2 yields CF2+HCl as primary products, as in thermal activation, for which E_act=233 kJ/mol.Laser-induced decomposition at 15 and 50 torr is preceded by time lags that, when the absorbed energy is <70 kJ/mol, are several times longer than the 600 ns required for absorption of 95percent of E_absd.As E_absd increases, the time lags decrease, and when E_absd>ca.140 kJ/mol, they become <120 ns.Time lags for laser-induced decomposition of CDClF2 are nearly the same as those for CHClF2 decomposition at the same P and E_absd.These facts and others are shown to be consistent with the existence of V-V' steady-state equilibrium during V-T relaxation.A kinetic approach is developed in which the specific decomposition rate depends on the instantaneous kinetic temperature, which is expressed as a function of the vibrational-mode temperatures in the instantaneous V-V' steady state.The mode temperatures in turn depend on the dominant Δυ selection rules for collisional V-V' exchange at the given vibrational energy.When E_absdabsd(>ca.120 kJ/mol), time lags are reproduced by the V model, which relaxes the Δυ selection rules so that vibrational mode temperatures are all equal.Wider consequences for laser-induced nonthermal chemistry under collisional conditions are discussed in terms of this approach.