Trajectory study of atomic recombination reactions. VIII. The energy transfer mechanism

Abstract

The energy transfer (ET) mechanism of atomic recombination reactions, X+X=X2*, X2*+M→X2+M, where X2* is a classically unstable orbiting quasidimer consisting of two X atoms and M is a third body, has been commonly used for the past forty years. Presently it is studied in some detail using the 3-D trajectory calculation technique and the Monte Carlo method of sampling. Since this mechanism approximates a pure three body problem by considering two independent and consecutive two body collisions, it introduces an error in trajectory calculations. In order to improve upon this approximation, a more reasonable modified method of trajectory sampling is made. It is found that even in one of the most favorable cases where the ET mechanism is predominant, i.e., for the 2I+He=I2+He reaction, the error introduced by the ET mechanism is of the order of two in the rate constant. It is concluded, in light of the present findings, that earlier as well as present trajectory studies for halogen atom recombination in helium are consistent with experimental data. The existing discrepancy between computed and experimental results can be accounted for by the approximations used in trajectory calculations. On the other hand, the combined Monte Carlo sampling and experimental errors cannot account for the existing discrepancies.