Mechanism of Atom Recombination by Consecutive Vibrational Deactivations

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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: kr=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 kr 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 kr 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.

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