Mechanism of Atom Recombination by Consecutive Vibrational Deactivations

Abstract

The process of atom recombination in the presence of foreign gases, M, not forming complexes with the atoms, is considered in terms of a cascade model in which deactivation occurs by small increments, essentially by one quantum at a time. This leads to an equation for the termolecular recombination rate constant: k_r=λZK^*G(T), where K^* is the equilibrium constant for species X₂^* in equilibrium with X atoms, Z is the collision frequency of these species with M, and λ, assumed to be unity, is the probability that such species will be de‐excited in a collision with M to the top vibrational level of X₂, the diatomic molecule. The quantity G(T) is essentially the reciprocal of the number of vibrational states within a range of kT of the dissociation threshold. Methods are given for the calculation of K^*, G(T), and Z from physical data for the X₂ and M molecules without any undetermined parameters. The use of Morse functions yields values of k_r in excellent agreement with available experimental data for H₂, N₂, O₂, Br₂, and I₂ over the range 300° to 2000°K (6000°K for O₂) with argon as M. The temperature dependence of k_r is not simple and varies between light and heavy molecules. On the average and almost independently of the potential function used, it is in the range of T^—1. It does not appear as though contributions from other electronic states beyond the ground state are significant in the recombination process.