No abstract

A calorimetric and vapor-pressure study of the behavior of 'He adsorbed on Grafoil III the temperature range 4 to 15 K is reported. These results have been combined with preexisting low-temperature heat-capacity data to form a complete thermodynamic description of the film, The Grafoil substrate evidently consists almost entlrely of basal-plane graphite, with only a small fraction of energetically distinct adsorption sites. Techniques of semiempirical modelling are introduced which make rt possible to eliminate the effects both of these inhomogeneitres and of the formation of second and higher layers, so that a detailed picture may be formed of the behavior of the 'He first layer on an ideal graphite substrate. The binding energy of a 'He atom on the graphite substrate is reported to be 143 i 2 K. with a first excited state at 89 3 K. A lattice-gas ordering transition occurs near 3 K at

Two separate lines of investigation have recently converged to produce a highly detailed picture of the behavior of helium atoms physisorbed on graphite basal plane surfaces. Atomic beam scattering experiments on single crystals have yielded accurate values for the binding energies of several· states for both 4 He and 'He, as well as matrix elements of the largest Fourier component of the periodic part of the interaction potential. From these data, a complete three-dimensional description of the potential has been constructed, and the energy band structure of a helium atom moving in this potential calculated. At the same time, accurate thermodynamic measurements were made on submonolayer helium films adsorbed on Grafoil .. The binding energy and low-coverage specific heat deduced from these measurements are in excellent agreement with those calculated from the band structures.

We present both an improved model and new experimental data concerning the problem of melting in multilayer adsorbed films. The model treats in a mutually consistent manner all interfaces in a stratified film. This results in the prediction of substrate freezing, a phenomenon therrnodynamically analogous to surface melting. We also compare the free energies of stratified films to those of homogeneous films. This leads to an orderly classification of multilayer phase diagrams in the vicinity of the bulk triple point. The results of the model are compared with the experimentally known systems. Of these, only methane/graphite exhibits melting from homogeneous solid to homogeneous liquid in multilayer films. The systems Ne/graphite and Ar/graphite, studied by Zhu and Dash, exhibit surface melting and substrate freezing instead. We observe experimentally, by means of pulsed nuclear magnetic resonance, that melting in methane adsorbed on graphite extends below the film thickness at which the latent heat of melting is known to vanish. The multilayer melting curve in this system is a first-order prewetting transition, extending from triple-point dewetting at bulk coexistence down to a critical point where the latent heat vanishes at about four layers, and apparently extending to thinner films as a higher-order, two-dimensional phase transition. It would therefore seem that methane/graphite is an ideal system in which to study the evolution of melting from two dimensions to three dimensions.

We present heat-capacity data detailing the evolution of the first six layers of argon adsorbed on graphite foam. The second and third layers have liquid-solid-gas triple points similar to the first layer. These layers exhibit a phase diagram consisting of two-dimensional solid, liquid, and gas phases on top of a solid film. Above the temperatures of the individual-layer triple points, the melting transition for each layer appears to be first order, and the first two layers show evidence of registry transitions prior to melting. For films of a total thickness of about four layers and up, the melting of each of the first three layers occurs at temperatures above the bulk triple point, as reported by Zhu and Dash [Phys. Rev. B 38, 11673 (1988)]. Our results confirm those of an ellipsometry study [H. S. Youn and G. B. Hess, Phys. Rev. Lett. 64, 918 (1990)) that found layering transitions above what were believed to be the layering critical-point temperatures. We observe heat-capacity peaks identified with these transitions and with melting transitions that join them with the low-temperature layering transitions. A phase diagram based on these data may represent the signature of a preroughening transition and a disordered flat phase in the bulk-crystal interface.