Equilibrium Properties of Submonolayer He⁴ Films

Citation

Wallace, John Longstreet (1971) Equilibrium Properties of Submonolayer He⁴ Films. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/A99W-KR64. https://resolver.caltech.edu/CaltechTHESIS:06262017-102857880

Abstract

Isosteres have been measured for submonolayer films of helium-four adsorbed on a sintered copper sponge precoated with each of the other noble gases, These measurements were carried out at temperatures between 3°K and 4°K, and at coverages between 0.5 and 1.0 monolayer; the pressures encountered were in the range from 10^-7 Torr to 10^-1 Torr. On the neon-coated substrate, the lower binding energy and higher lateral mobility made it feasible to take useful data at coverages as low as 0.025 monolayer. From the isosteres, information is derived about the partial molar entropy, entropy and heat capacity of the film.

It appears that the submonolayer film is a well-defined entity whose qualitative properties remain unchanged as the substrate precoating is varied, These include a transition into a highly ordered state as the coverage is increased, a total heat capacity which is not monotonic with coverage, and significant lateral mobility. The transition into the ordered state is initiated at higher coverage as the temperature is lowered, and this anomaly is discussed in terms of a model in which two dimensional crystalline patches are broken up by thermal fluctuations.

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