Measurement of the chemical potential of thin helium films
Shirron, Peter John
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https://hdl.handle.net/2142/22107
Description
Title
Measurement of the chemical potential of thin helium films
Author(s)
Shirron, Peter John
Issue Date
1989
Doctoral Committee Chair(s)
Mochel, J.M.
Department of Study
Physics
Discipline
Physics
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Physics, Condensed Matter
Language
eng
Abstract
High-precision measurements of third sound velocities in atomically thin $\sp4$He films on Ar, Ne, CO$\sb2$, and H$\sb2$ substrates in the T = OK limit reveal a periodic structure not corresponding to whole layers. We analyze the data in two extreme limits. First, treating the helium film as an incompressible, continuous fluid, the data indicate that the chemical potential has a contribution proportional to $\ell\sp{-6}$, where $\ell$ is a layer coverage, which is modulated at intervals of.62 $\pm$.04 layers, and which weakens the van der Waals potential. Second, if we assume the potential is exactly the van der Waals potential, the data indicate the existence in the film of a damped smectic density wave with a periodicity of.54 $\pm$.04 layers, independent of substrate. The correct description of He films must lie between these extremes. We find the inert layer coverages to be 1 full layer on Ar, Ne, and CO$\sb2$, and.36 layers on H$\sb2$.
Coverages scales were determined using helium adsorption isotherms. A self-consistent analysis method was developed based on the modified Brunauer-Emmet-Teller method for low coverages and the Fenkel-Halsey-Hill vapor pressure relation at high coverages. The analysis establishes the layer capacities for helium films and helium-substrate chemical potential constants, $\Gamma$.
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