Mobility of O-2(-) ions in near critical Ar gas

We have measured the drift mobility mu of O-2(-) in Ar gas in the temperature range (151.5 < T < 157.0) K near T-c (150.86 K) and in the density range (2 < N < 130) X 10(26) atoms/m(3) around N-c (80.8 x 10(26) atoms/m(3)). We have observed a strong reduction of the zero-field density-normalized mobility mu(0)N as T --> T-c but for N < N-c. We suggest that this reduction is due to the effect of the long-range ion-atom polarization interaction and of the divergence of the gas compressibility. A satisfactory agreement with the data can be obtained by suitably modifying the Stokes's hydrodynamic expression for mu(0).