Bound complex and triple collision mechanisms for diatom dissociation and recombination

Abstract

A method was devised for estimating relative contributions to atom recombination from the bound complex and triple collision mechanisms. The method is based on three assumptions: a collinear model is valid, recrossings of a suitably chosen transition state surface equally affect both contributions to the recombination rate, and the two contributions correspond to crossings of parts of the transition state surface which differ only in total energy. Ratios of the rate constants for recombination by the two mechanisms were calculated for a classical model of Br and I atom recombination in rare gases. The ratios had already been obtained for the model from three-dimensional trajectory calculations. The results disagreed by as much as a factor of ten for helium as the third body, but they were within 50% or better for xenon. It was argued that the discrepancies are largely due to a breakdown of the second of the foregoing assumptions. The formula for the rate constant for recombination by both mechanisms together was extended to three dimensions at the expense of introducing an adjustable parameter. The extended formula was found to give results in agreement with the trajectory calculation to within 30% or better.