An ellipsometric technique was used to measure a set of adsorption isotherms for ethylene on a single cleaved surface of graphite in the vicinity of the triple point (T3-104 K) of ethylene. At least seven or eight discrete layers are observed at temperatures slightly above T3, contrary to a previous suggestion of a prewetting transition after two monolayers. Evidence is presented that this layered liquid film becomes unstable with respect to bulk liquid at about ten layers, so that wetting is incomplete even above T3.

Layer transitions in CF4 adsorbed on graphite are studied by vapor-pressure isotherm measurements in the range one to eight layers and 52 K & T & 104 K, using an ellipsometric technique to measure the coverage on a single surface. A maximum of two layers adsorb below 71 K. Above 71 K we observe for the first time the condensation of a double-layer (third and fourth), which splits into separate layers above a triple point at 75 K. The fifth layer appears at 80 K, the sixth at 87 K, and at least eight discrete layers are seen over several kelvin above the bulk melting point at 89.5 K. The slopes of the layer chemical potentials versus temperature yield estimates of the locations of phase transitions and of entropy differences.In recent years a rich variety of two-dimensional phases and phase transitions has been found in monolayer physisorbed systems. ' There is evidence of a similar richness in multilayer systems.Attention has focused particularly on wetting, that is, the stability of uniform thick films with respect to bulk condensed phases, 5 on melting of wetting films and its relation to bulk melting, and on layering in wetting films and the relation of the layer critical points to roughening of the bulk phase surface. At intermediate thicknesses the topology of the phase diagrams may involve layer appearance transitions at finite temperatures, prewetting, melting of individual layers or of multilayer phases, ' layer critical points, " and layer coalescence or splitting transitions. ' The last has not been observed previously experimentally.If a particular layer is stable over an appreciable temperature range, there may be additional phase transitions involving changes in structure or molecular orientation. 'The system CF4 on graphite exhibits many of the phenomena just mentioned. X-ray' and neutron' diffraction measurements have been reported, but so far only for the first layer. Zhang, Kim, and Chan (ZKC) have made an extensive heat-capacity study covering the monolayer, ' ' bilayer, ' and multilayer ' regimes. Volumetric isotherm measurements have been reported by Dolle, Matecki, and Thorny, ' including two isotherms below the bulk melting temperature at 89.5 K. Gay, Bienfait, and Suzanne' have studied CF4 on graphite below 50 K by reflection highenergy electron diffraction and interpret their results as showing a wetting transition at 37 K. They also report stepwise attenuation of substrate low-energy electron diffraction intensity, which indicates condensation of a third and possibly fourth layer in isotherms between 51 and 58 K.In this paper we report extensive adsorption measurements for CF4 on a single surface of highly oriented pyrolytic graphite.The CF4 coverage on a 0.1x0.2-mm area of graphite is measured by a phase-modulation ellipsometric technique. ' The ellipsometer output I& at the modulation frequency is directly proportional to coverage, in the range of interest. For very thick uniform films, Ii and the output at twice the modulation frequency I2 should vary periodically with film thickness...

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The physisorption system ethane on graphite was investigated by an ellipsometric technique.We report layer condensation critical temperatures for layers 2-9. T,(2) is 120.8~0.3 K, somewhat lower than previously reported. T, (n) decreases with increasing layer number n until for n 8 and 9 the extrapolated critical temperatures are below the bulk melting point (90.3 K), where these layers have been preempted by bulk solid. The coefficient of the compressibility above T, increases strongly with n. This phenomenon is not predicted in a theory based on the two-dimensional Ising model, and probably reflects the importance of a second interaction parameter related to the substrate holding potential.

A torsional oscillator cell is described, by means of which simultaneous precision measurements of (prl) and of the molar volume can be made in liquid 4He-4He mixtures over the temperature range between 0.5 and 3 K. Here p is the mass density, rl the shear viscosity and in the superfluid phase they become the contributions p, and qn of the normal component. The results of ;l for 4He near the superfluid transition are compared with the predictions by Schloms, Pankert and Dohm, and by Ferrell. Measurements of (P~l) are reported for mixtures with 0.64 < X < O. 74, where X is the 3He mole fraction. Those for X =0.67 and 0.70 are compared with data by Lai and Kitchens. The viscosity experiments show no evidence of a weak singularity at the tricritical point.