Free Volume Model Analysis of the O₂− Ion Mobility in Dense Ar Gas

We report the results of the experiment aimed at measuring the mobility of
$\text{O}_{{2}}^{-}$
ions in dense argon gas in the temperature range
${180}\,{{\mathrm {K}}}\leq {T} \leq {300}$
K. We show that an adequate theoretical description of the data is obtained by using the thermodynamic free volume (FV) model, originally developed to describe the electron bubble mobility in superfluid helium and successfully exploited for describing the
$\text{O}_{{2}}^{-}$
mobility in near critical neon gas. The model goal is to thermodynamically predict the free space available for ion motion. By implementing the FV model with the Millikan–Cunningham (MC) slip correction factor, we can describe the ion mobility in the crossover region bridging the dilute gas kinetic regime to the high-density hydrodynamic regime of ion transport. These results confirm the validity of the model and the universality of some of its features.