Calculation of ion-ion mutual neutralization rate constants using Landau-Zener theory coupled with trajectory simulations for Ar+-Cl-, Br-, I.

In this computational study, we self-consistently calculate the rate constants of mutual neutralization reactions by incorporating the electron transfer probability, using Landau-Zener state transition theory with inputs derived from ab initio quantum chemistry calculations, into classical trajectory simulations. Electronic structure calculations are done using correlation consistent basis sets with multi-reference configuration interaction to map all the molecular electronic states below the ion-dissociation limit as a function of the distance between the reacting species. Our electronic structure calculations have been significantly improved from our previous work [Liu et al., J. Chem. Phys. 159, 114111 (2023)] through improved selection of molecular electronic configurations maintaining a fine grid of 1a0 over a wide range of bond lengths and accurate treatment of spin-orbit couplings. Non-adiabatic coupling matrix elements are calculated with the three-point central difference method near each avoided crossing to estimate the exact crossing point Rx and coupling parameter Hif, which are inputs to the multi-channel Landau-Zener theory to calculate the electron transition probability. Our approach is applied to estimate the mutual neutralization rate constants for the following ion pairs: Ar+-Cl-, Ar+-Br-, Ar+-I- at ∼133 Pa. Our predictions are compared against the experimental data reported by Shuman et al. [J. Chem. Phys. 140, 044304 (2014)]. It is seen that the improvement in the electronic structure calculation results in excellent agreement between the simulation results and the available experimental data to within a factor of ∼2 or ∼±50%.