Low-temperaureO2−mobility in high-density neon gas

We report measurements of the drift mobility of ${\mathrm{O}}_2^{\mathrm{-}}$ 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)×${10}^{20}$ ${\mathrm{cm}}^{\mathrm{-}3}$], 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.