Monte Carlo modeling of gas phase ion transport under thermal gradients and external fields

A null collision Monte Carlo method is proposed to calculate the transport kinetics of charged particles in a non equilibrium background of atoms or molecules. The method is an implementation of the null collision Monte Carlo method of Skullerud, extended in order to include the translational distribution of neutrals, also in non uniform cases. As such, it delivers an exact solution of the linear Boltzmann equation. Two equivalent formulations of the method are proposed: an inductive one based on physical considerations, and a more formal one in which the method is deduced from the appropriate transport equation. As a test case, the method is applied to the thermal diffusion of H3+ ions diluted in H2 under an isobaric thermal gradient and with a transversal electric field.