Langevin Dynamics Modeling of Gas-Phase Ion Recombination with Dilute Ion Concentration

Abstract

Ion recombination in the gas phase plays an important role in plasma science and chemistry that is closely related to various applications in combustion and materials processing. Due to the lack of robust theoretical models of three body ion recombination, this project intends to develop a Langevin dynamics-based ion recombination rate constant model that is applicable for a broad pressure and temperature range combination taking into account the effect of ionic structure (especially for polyatomic ions). At high pressures, Langevin Dynamics is commonly considered to be efficient model for ion-neutral gas interactions and the implementation of LD enables the trajectory analysis and obtaining the ensemble averaged estimates of the of ion recombination rate constant. Ion recombination or mutual neutralization is modeled within the framework of classical physics and thus does not explicitly model the quantum nature of electron transfer kinetics from the anion to the cation. We hypothesized that electron transfer takes place with near certainty when two atoms (one part of the cation and the other part of the anion) when their separation is less than or equal to σ - distance σ between two atoms at which the potential energy (due to van der Waals or polarization interaction) between the ions is zero. The comparison with experimental data 1 – 5 for several ion pairs reveals an agreement of ±50% with most of the tested ion pairs. At low pressure end, a good agreement is observed as well between ion pairs that consist of polyatomic ions.