Adsorption and specific heat studies of⁴He in 0.2-μm Nuclepore filters

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 to T² with a characteristic two-dimensional Debye temperature of Θ_D=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.