Critical Behavior of 2− Ions in Argon Gas

Abstract

We measured the drift mobility of 2− ions in argon gas close to the critical point for (1.005 < T/Tc < 1.04) above Tc ≈ 150.7 K in the density range (0.025 <N/Nc< 1.733) around the bulk critical density Nc = 8.08 atoms . nm−3. The density-normalized zero-field mobility μ0N of the ions shows a deep minimum as a function of the gas density N as T → Tc+ This anomalous reduction of μ0N occurs at a density Nm ≈ 0.76Nc. We believe that this behavior is due to the strong electrostriction exerted by the ion on the highly compressible gas. By introducing suitable contributions to the effective ion radius R due to the large gas compressibility and taking into account short-range local density and viscosity augmentation due to electrostriction, the hydrodynamic Stokes formula μ0 = e/6πηR, where η is the gas viscosity, is in good agreement with the experimental data.