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.
Ion transport through a nanoporous C2N membrane: the effect of electric field and layer number
