Monte Carlo model of ion mobility and diffusion for low and high electric fields.

A Monte Carlo method is described to model the mobility and diffusion of ions drifting in response to an electric field in a neutral gas. The model uses a collision frequency that is dependent upon the ion velocity and neutral gas thermal velocity. When implemented with a constant collision cross section for momentum transfer, the model gives a mobility that is constant for small electric fields (those giving a subsonic drift velocity) and that for larger fields falls inversely with the square root of the electric field. For argon ions drifting in argon, the model gives a close agreement with experimental data for the mobility for a wide range of electric fields when implemented with an energy-dependent cross section. For modeling of transverse diffusion, agreement with data is improved if the collisions are a combination of idealized charge-exchange collisions and hard-sphere collisions.