The Thermodynamics of the ⁴HE Submonolayer Film Adsorbed on Grafoil

The heat capacity and vapor pressure of thin films of helium adsorbed on graphite have been measured simultaneously. Sufficient data were taken for a complete model~independent thermodynamic analysis from 4.5 K to 15 K and from 0.01 to 1.2 monolayer. Heat capacity down to 1s4 K showed reproducibility with other laboratories. The data allow definitive tests of many models suggested for these films. Large heat capacity peaks at the melting transition are shown to be the result of interactions with the second layer and the bulk gas phase. Large deviations from ideal gas behavior at low densities are explained quantitatively by interactions with inhomogeneities in the substrate. The data may therefore be corrected to determine experimentally the behavior of strictly two-dimensional helium on a completely homogeneous surfaces.