Abstract
We have made adsorption studies and specific heat measurements for helium adsorbed on Nuclepore filters with a pore size of 0.2 µm. We have been able to identify the first and second layer completion on this substrate by a Langmuir and BET analysis of the data. For coverages near the second layer completion the data obey a Frenkel-Halsey-Hill isotherm, and we have obtained the van der Waals constant α=(2.4±0.4)×10−37 erg-cm3. We find this result in excellent agreement with a theoretical estimate of 2.17 × 10−37 erg-cm3. The interpretation of the isotherm data is confirmed by measurements of the specific heat. Data near a monolayer completion is found proportional toT2 with a characteristic two-dimensional Debye temperature of ΘD=23±1 K. Measurements of specific heat with helium samples equivalent to about 17 layers and higher show two λ transitions shifted in temperature, as is characteristic of helium confined to two different small dimensions. These data are in agreement with the thermodynamic instability for capillary condensation in a cylindrical geometry as calculated by Saam and Cole.
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