Triple point of the first monomolecular layer of krypton adsorbed on the cleavage face of graphite. Influence of the structure of this surface on the structure of the first adsorbed layer of rare gases

Abstract

From adsorption isotherms of krypton on graphite measured between 79.24 and 88.46 K in the monolayer domain a two-dimensional triple point of 84.8 ± 0.5 K is determined for this system. Thermodynamic analysis of these isotherms and of the isotherms of xenon on graphite measured by Thomy and Duval suggests that the surface structure plays a prominent role in the formation of dense monolayers on the (0001) face of graphite. To explain the striking difference of behaviour of the two rare gases, two factors have to be taken into account; the height of potential barriers to translation and dimensional incompatibility between the lattices of the bulk adsorbates and of the (0001) face of graphite, the first “solid” layer appearing being in registry with the substrate. A single isotherm of argon on graphite measured at 68.78 K suggests that a more exhaustive study of the Ar + graphite system could be useful in establishing the relative importance of both factors.