Thermodynamic Model for the Oxygen Ion Mobility in Dense Argon Gas

We report experimental data of the mobility of O$_2^-$ ions in argon gas as a function of the density in the temperature range between $180\,\mbox{K}\le T\le 300\,\mbox{K}$. At the intermediate, though fairly large, densities of the experiment both the kinetic theory and the hydrodynamic theory fail at describing the experimental data. By contrast, the free volume model, originally developed to describe electron and ion mobility in superfluid helium, gives a satisfactory agreement with the data as previously obtained for the O$_2^-$ ion mobility in both dense neon and helium gases.