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

Abstract

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\ifmmode\pm\else\textpm\fi{}0.5 K and 100.8\ifmmode\pm\else\textpm\fi{}1.0 K, and the critical temperatures are estimated to be 86.8\ifmmode\pm\else\textpm\fi{}1.0 K and 119\ifmmode\pm\else\textpm\fi{}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 $P\ensuremath{-}T$ range studied. A qualitative estimate places the Ar 2D critical temperature at about 65 K.