Isosteres have been measured for submonolayerfilms of helium-four adsorbed on a sintered coppersponge precoated with each of the other noble gases,These measurements were carried out at temperaturesbetween 3°K and 4°K, and at coverages between 0.5and 1.0 monolayer; the pressures encountered werein the range from 10-7 Torr to 10-1 Torr. On theneon-coated substrate, the lower binding energyand higher lateral mobility made it feasible to takeuseful data at coverages as low as 0.025 monolayer.From the isosteres, information is derived aboutthe partial molar entropy, entropy and heat capacityof the film.
It appears that the submonolayer film is awell-defined entity whose qualitative propertiesremain unchanged as the substrate precoating isvaried, These include a transition into a highlyordered state as the coverage is increased, a totalheat capacity which is not monotonic with coverage,and significant lateral mobility. The transitioninto the ordered state is initiated at higher coverageas the temperature is lowered, and this anomaly isdiscussed in terms of a model in which two dimensionalcrystalline patches are broken up by thermal fluctuations.
