Theoretical Answers to Some Experimental Questions about Gaseous Ion Transport

Abstract

It is shown that the diffusion coefficients parallel to the electric field in an ion swarm experiment are greater than the perpendicular diffusion coefficients at low values of E/N, the ratio of the electric field strength to the gas number density, but that at high E/N the two are approximately equal or the inequality is reversed. For electron swarms the inequalities are the reverse of those for ion swarms, unless there is a significant Ramsauer minimum in the momentum-transfer cross section. A second point is that for many ion–atom systems the zero-field mobility is not close to its polarisation limit even when the gas temperature is 4.35 K. It is also shown that the unusual mobilities observed for Ar⁺ ions in He gas at 4.35 K are not due to quantum-mechanical (resonant scattering) effects as postulated by the experimenters. Finally, it is shown that it is possible to compute gaseous ion transport coefficients for molecular ions from a knowledge of the fundamental interaction potentials rather than from assumed models for the cross sections.