Tests of approximate formulae for the calculation of ion mobility and diffusion in gas mixtures

Abstract

Approximate formulae from momentum-transfer theory for the mobility and diffusion coefficients of ions in gas mixtures are tested. Most of the test ‘data’ are simulations obtained from moment solutions of the Boltzmann equation. The mobility formula works well, but requires some simple iteration; a linearized version proposed to avoid the iteration can fail badly in some cases. The formulae for ion diffusion, here tested for the first time, are less successful because of approximations made to find the ion temperatures perpendicular and parallel to the electric field, but nevertheless give a useful qualitative accuracy. The results properly apply to systems undergoing only elastic collisions, but some limited results on the influence of inelastic collisions are given.