Adsorption of Kr, Xe, and Ar on highly uniform MgO smoke

The first atomic layer of Kr, Xe, and Ar adsorbed on MgO smoke is explored by using adsorption isotherms. A procedure has been developed by which we produce MgO smoke powders of higher homogeneity than previously reported. On these powders, apparent two-dimensional (2D) solid-vapor, fluid-vapor, and solid-fluid coexistence regions are observed for Kr and Xe. Triple-point temperatures are estimated to be 66.6±0.5 K and 100.8±1.0 K, and the critical temperatures are estimated to be 86.8±1.0 K and 119±2.0 K, respectively, for these two elements. A comparison between Kr and Xe 2D-solid densities at the fluid-solid transition suggests that the 2D-solid phases are incommensurate with the substrate. The Ar isotherms are qualitatively different from those of Kr and Xe, showing no features corresponding to 2D phase transitions in the $P-T$ range studied. A qualitative estimate places the Ar 2D critical temperature at about 65 K.