Adsorption of Neon on a Xenon Surface

Abstract

Measurements of isotherms and heats of adsorption are reported for neon at submonolayer coverages on a xenon surface which had been prepared by preadsorbing one layer of xenon on a graphitized carbon black substrate. Also, heat capacities of a nominal half layer of neon were measured from 6° to 21°K. These data were used to calculate the absolute entropy of a half‐layer of adsorbed neon, and also to locate the critical temperature for two‐dimensional condensation in this system. It was found that the neon zero‐point entropy was zero within experimental error, and that the critical temperature was 15.3° ± 0.3°K. Theoretical estimates of the energy and partition function of an isolated neon atom on the surface were compared with experiment, as were calculated values of the mutual interaction energy of a pair of adsorbed neon atoms. It was concluded that the neon atoms behave as if they were adsorbed on the points of a lattice congruent with a regular close‐packed xenon lattice having sites centered over triangular groups of xenon atoms in the surface. It was shown that the experimental value of the critical temperature is commensurate with this picture, but is definitely anomalous compared to predictions based on other models for the adsorbed film.