Electron Mobility Maximum in Near-Critical Argon Gas

Measurements of the drift mobility + of excess electrons in dense argon gas in proximity of the critical point of the liquid vapor transition are reported. The density and electric field dependence of + at two temperatures fairly close to the critical point, namely T=162.30 K (T Tcr1.08) and T=152.15 K (T Tcr 1.01) (Tc=150.7 K) in a density range (0.5 N 14) atoms } nm&3 (0.06 N Nc 1.73), encompassing the critical region of Ar (Nc=8.08 atoms } nm&3), are investigated. At the lower temperature a maximum of the zero-field density- normalized mobility +0N was observed at the same density as observed in the liquid. A density-modified kinetic model describes well all features of + in the gas phase, even at densities comparable to those of the liquid. It is argued that the electron scattering processes in the liquid phase can be described in terms of kinetic theory rather than in terms of the usual deformation potential model.