Mutual Neutralization in Mono-Atomic Ion Pairs in Finite Gas Pressure Using Landau-Zener Theory Coupled to Trajectory Simulation*

In this computational study, we calculate the rate constants of mutual neutralization reactions self-consistently by incorporating the Landau-Zener state transition theory, which depends on ab initio quantum chemistry calculation, into classical trajectory simulations. Electronic structure calculations are done using diffuse correlation consistent basis sets to the Multi-Reference Configuration Interaction level with Davidson correction to deal with the size-extensivity of the atoms in the MOLPRO software package. Our electronic structure calculations have been significantly improved from our previous work [1] by explicitly treating the spin-orbit coupling between the transition states. Non-adiabatic coupling matrix elements (NACMEs) are calculated with the three-point central difference method close to each avoided crossing to estimate the exact crossing point and coupling parameter, fed into the multi-channel Landau-Zener theory to calculate the electron transition probability. Our approach is applied to estimate the rate constants for the following three ion pairs: Ar⁺-Cl^-, Ar⁺-Br^-, and Ar⁺-I^-  at 133 Pascal, and our results are validated against the data from the experimental study [2]. It has been seen that the improvement in the electronic structure calculation results in an excellent agreement between our simulation results and available experimental data.

1. Liu et al., J. Chem. Phys. 159, 114111 (2023)

2. Shuman et al., J. Chem. Phys. 140, 044304 (2014)