Heat Capacity of Helium Submonolayers on a Nonuniform Surface

Data of McCormick, Goodstein, and Dash, and Stewart and Dash, for heat capacities of He-3 and He-4 films on Ar-plated copper in the submonolayer region have been analyzed on the basis of a model of a long-range variation of adsorption potential. This variation allows the adatoms to form small islands on the surface. The total heat capacity of the film is the sum of two contributions, one a Fermi-like contribution due to surface heterogeneity, and the other a two. dimensional Debye-like contribution due to molecular vibrations. The data are roughly consistent with a Debye temperature of 32K for all the He-4 data and 38 K for the He-3 data. The role of surface heterogeneity in the heat capacity of films is briefly discussed.