  We report data of the O2- ion mobility in neon gas over broad density and
temperature ranges along with its theoretical description in terms of the
thermodynamic, free volume model that has successfully been adopted for the
interpretation of electron and ion mobility in superfluid and normal helium.
The free volume model, which is aimed at computing the free volume accessible
for the ion motion, along with the Millikan- Cunningham slip factor correction,
is able to describe the ion mobility in the crossover region connecting the
dilute gas regime described by the classical Kinetic Theory to the high density
region ruled by the laws of hydrodynamic transport.
