Monolayer Adsorption of Helium on Argon

Abstract

Experimental isotherms and heats of adsorption of helium on an argon surface are reported for temperatures from 10° to 20°K. The potential energy of interaction of a helium atom with an argon surface is calculated by summing the He–Ar pair interaction over the solid lattice. The experimental data are analyzed in terms of a quantum mechanical modification of the theoretical treatment presented previously [W. A. Steele and M. Ross, J. Chem. Phys. 33, 464 (1960) (I); 35, 850 (1961) (II)]. It is shown that the adsorption properties calculated from the a priori potential function are in quantitative agreement with the experiments, in the region where a quantitative comparison can be made, and that the remainder of the data is in agreement with semiquantitative theoretical estimates. A detailed description of the probable adsorption process for the He–Ar system is deduced from the comparison between experiment and theory. It is concluded that the first half of the helium atoms comprising a monolayer on this surface are highly localized at equivalent positions over the argon lattice unit cells, and are nearly all in their quantum mechanical ground state at temperatures up to 20°K. The second half of the atoms in the monolayer is adsorbed over a different position relative to the solid lattice, and is more weakly bound than the first. Although the quantitative comparison between the theory and the experiment is somewhat limited because the quantum version of the theory is not as complete as the classical, it is concluded that the agreement between experiment and theory indicates that this approach to the problem of physical adsorption may prove useful in the analysis of data taken on other suitable systems.