Specific heat of adsorbed films of3He,4He,and isotopic helium mixtures near monolayer coverages

Specific heat results are presented for near-monolayer films of³He,⁴He, and isotopic He mixtures adsorbed on Vycor porous glass in the temperature range between 1 and 4°K. In the case of the pure He monolayers and submonolayers, the specific heats depend only onT ² , like a two-dimensional Debye elastic medium. A film of slightly greater than monolayer coverage needs an additional temperature-independent term. Monolayer coverages of mixtures are difficult to define, but all coverages used, some of which are submonolayer, need bothT ² andT-independent terms. The constant term may arise from atoms in the second layer forming a classical noninteracting system, which in the mixture case would indicate selective adsorption of⁴He in preference to³He at the substrate wall, in agreement with previous adsorption experiments on the same substrate.Work supported in part by the U.S. Atomic Energy Commission.