Low-temperature O2- Mobility In High-density Neon Gas

We report measurements of the drift mobility of O2- ions in supercritical neon gas at high density at a temperature T = 45.03 K, 0.6 K above the critical temperature. Our measurements span a very large density range, up to values comparable with those of a liquid [(15-240) x 10(20) cm-3), and cover the transition region from the kinetic to the hydrodynamic regime. We show that the classical hydrodynamic Stokes formula for the drag on a sphere moving through a fluid describes accurately enough the ionic mobility as a function of the gas density, if it is modified to take into account the spatial dependence of the gas density and viscosity around the ion due to electrostriction.