Observations are reported of the effect of the buffer gases He, Ne, and CF4, in the pressure range of 0–30 torr, on the branching ratio HCl]/DCl] of the unimolecular decomposition The ratio R = kH/kD has been measured in high-pressure thermal decomposition (670–1100 K) and was shown to give a unique measure of the internal energy of the decomposing molecules and hence, with RRKM theory and pressure fall-off data, a time scale for their decomposition. Applying the thermal data to the photolysis leads to the conclusion that excitation and decomposition are produced by the laser spike (high intensity, 70 ns FWHM) and also at a slower rate by the larger, less intense tail (1.6 μs). Added buffer gases quench the latter, leaving the former which, from measurements of R, is shown to correspond to excitations of 115 ± 15 kcal/mol and lifetimes of ～30 ps. No bond breaking is seen despite the high energies, in accord with theoretical expectations. The results require an enhanced rate of photon absorption by the highly excited molecules, which are about hundredfold greater than that observed for 300 K molecules. Data are also reported for C2H2F2 and the secondary multiphoton photolysis of the ethylenes produced. Effects of beam geometry and wavelength are explored.
