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Adsorption and Specific-Heat Studies of Monolayer and Submonolayer Films of He3 and He4

McCormick, W. D. and Goodstein, D. L. and Dash, J. G. (1968) Adsorption and Specific-Heat Studies of Monolayer and Submonolayer Films of He3 and He4. Physical Review, 168 (1). pp. 249-260. ISSN 0031-899X. doi:10.1103/PhysRev.168.249.

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A study has been made of the adsorption of He3 and He4 at 4°K on a substrate consisting of a monolayer of argon adsorbed on a sintered copper sponge. The isotherms display distinct steps indicating the completion of first and second adsorbed layers. Comparisons among the adsorption isotherms of helium and of Ar and N2 at 77.4°K yield a self-consistent set of molecular areas. Measurements have been made of the specific heat of five submonolayer coverages of He3 and He4 on Ar-plated Cu sponge. The heat capacities of nearly complete monolayers vary as T2 from 0.3 to 4°K, yielding two-dimensional Debye temperatures Theta (He4)=28±1°K, and Theta (He3)=31±1°K. At lower coverages the molar heat capacities increase and develop contributions linear in T below 1°K. At an intermediate coverage, the heat capacity of He4 exhibits a broad and pronounced maximum near 3°K. Possible mechanisms for the linear terms and the maximum are discussed briefly. Evidence for considerable mobility of He atoms along the surface is adduced from the temperature and coverage dependence of the heat capacity. The T2 behavior for the complete monolayers yields an upper limit of ~10^-11 sec for the lifetime of a He atom in any individual adsorption site, consistent with a theoretical estimate.

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Additional Information:©1968 The American Physical Society Received 19 October 1967 Research supported by the National Science Foundation under Grant No. GP 5693. The authors gratefully acknowledge many valuable discussions during the course of this work with Professor G. D. Halsey and Professor R. E. Peierls. We are also grateful to G. A. Stewart for his technical assistance.
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Deposited On:24 Dec 2005
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