Critical behavior of O-2(-) ions in argon gas

We measured the drift mobility of O-2(-) ions in argon gas close to the cri tical point for (1.005 < T/T-c < 1.04) above T-c approximate to 150.7 K in the density range (0.025 < N/N-c < 1.733) around the bulk critical density N-c = 8.08 atoms.nm(-3) The density-normalized zero-field mobility mu(0)N o f the ions shows a deep minimum as a function of the gas density N as T --> T-c(+). This anomalous reduction of mu(0)N occurs at a density N-m approxi mate to 0.76N(c). We believe that this behavior is due to the strong electr ostriction exerted by the ion on the highly compressible gas. By introducin g suitable contributions to the: effective ion radius R due to the large ga s compressibility and taking into account short-range local density and vis cosity augmentation due to electrostriction, the hydrodynamic Stokes formul a mu 0 = e/6 pi eta R, where eta is the gas viscosity, is in good agreement with the experimental data.