Solid Layers in the Adsorbed Helium Film and Mobility of the Film at Low Coverages

Abstract

An examination is made of the evidence which led to the idea of the existence of solid layers in the adsorbed helium film at low temperatures. The experimental evidence from adsorption isotherm, specific heat, and flow measurements on the helium film is reviewed and the interpretation of this evidence as supporting the idea of a solid layer structure near the adsorbing surface is critically discussed. This discussion includes some evaluation of the applicability of the B.E.T. and other adsorption models used to describe helium adsorbed at very low temperatures. As a result of these considerations the author is of the opinion that the concept of a solid layer, or so, existing in an adsorbed helium film, is not nearly as well founded as is usually supposed. One particular inference from the solid layer picture; that the helium film at near monolayer coverages be considered immobilized is particularly questioned. The model of Dzyaloshinskii, Lifshitz, and Pitaevskii which considers no density variation with depth is discussed and a suggestion is also made for using some early calculations of Lennard-Jones and Devonshire to show that adsorbed helium at very low coverages should be quite mobile on the adsorbing surface.