We present the heat capacity of partial second-layer He films adsorbed on Grafoil'below 2 K. The second-layer liquefaction temperature occurs near 0.75 K which is substantially less than the 1.3 K observed for monolayer liquefaction. This rescaling in T is thought to result from a vertical spread in the wave functions and hence a reduced Van der Waals interaction between adsorbed helium atoms. A consistent picture emerges when the total heat-capacity signals are modified by compressible-monolayer and film-heterogeneity considerations. The resulting liquefaction phase 0 diagram clearly indicates a critical point near 0.025jA and some altered higher-coverage behavior.

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Thermal resistance and vapor pressure isotherms were taken near superfluid onset for ultrathin helium films adsorbed on a ZYX graphite wafer between 1-2 K and 3-7 atomic layers. Our data are consistent with previous graphite onsets and are compatible with a current model of film droplet formation. Overlap of thermal resistance curves at 1.19 and 139 K is believed to be associated with discrete layering effects of 2D superfluid film properties.

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