Capturing the dependence of gas-phase recombination or mutual neutralization rate constants on neutral gas pressure using Landau-Zener theory for electron transfer coupled with trajectory simulations and comparisons with measurements*

In this computational study, we have extended Olson’s Absorbing Sphere model ¹, that invokes the Landau-Zener (LZ) transition state theory ² to estimate the probability of electron transfer between two ions, to include the effect of neutral gas pressure on ion-ion mutual neutralization (MN) reactions. We carried out electronic structure calculations using MOLPRO® to derive inputs to the LZ theory for the subsequent self-consistent calculation of electron transfer probability. The key advance of our trajectory simulation approach is the inclusion of the effect of ion-neutral interactions on MN. For monoatomic ion pairs such as H⁺-H^- and Li⁺-H(D)^-, we showed that our approach quantitatively agrees with corresponding experimental data. For the ion pair Ne⁺-Cl^-, our predictions of the MN rate constant at ~1 torr are a factor of ~2 – 3 higher than experimentally measured value. Similarly, for Xe⁺-F^- in the pressure range of ~20000 – 80000 Pa, our predictions of the MN rate constant are a factor ~0.5 – 3 lower and are in qualitative agreement with experimental data. The current study sets up a paradigm for modeling MN reactions to self-consistently capture the effect of gas pressure while offering scope for the inclusion of complexities such as de-excitation kinetics, the effect of external electric and magnetic fields, and chemical systems with multiple state interactions as opposed to the two-state interaction treatment applied here. While considering additional factors that may affect the quantum mechanics of electron transfer, a more sophisticated methodology of capturing the probability of electron transfer may be necessary, but we consider that the core approach of incorporating the same into classical trajectory simulations remains valid.

¹ R. E. Olson, The Journal of Chemical Physics 56 (1972) 2985.

² C. Zener, and R. H. Fowler, Proceedings of the Royal Society of London. Series A, Containing Papers of a Mathematical and Physical Character 137 (1932) 696.