HEAT-CAPACITY STUDY OF HE-4 DESORBING FROM H-2-PLATED GRAPHITE

We report measurements of the surface binding energy of He-4 absorbed bed on 1.45, 2.15, or 3.15 layers of hydrogen plating graphite. The heat capacity of partial monolayer coverages of helium was measured using adiabatic heat pulse calorimetry from 0.2 to 9K. Desorption of low surface density 4He films was indicated above approximate to 2K by a broad peak in the heat capacity. A model was developed to calculate the specific heat of an ideal gas in two dimensions as it desorbs into an ideal gas in three dimensions. This model was used to obtain the binding energies from our experimental data. The binding energies per atom are -39, -25.8, and -22 +/- 0.5 K, respectively, for the three hydrogen platings. The 2-d compressibility of the partial solid second layer of H-2 in the 1.45 layer H-2 coverage was calculated from an increase in its melting temperature with increase of He-4 2-d pressure.