Adsorption of3He and4He on copper and on argon-coated copper below 20 K

Measurements have been made of adsorption isotherms of³He and of⁴He on copper and on a monolayer of argon deposited on copper in the temperature range 6.18–18.55 K and in the pressure range 0.25 to 75 Torr. From these many isotherms, calculations have been made of the isosteric heat of adsorptionQ_st/R. In the limit of zero coverage on the argon monolayerQ_st/R=76±2 K for³He and 76±2 K for⁴He. For adsorption on the bare copper,Q_st/R is difficult to extrapolate to zero coverage, but it probably lies (for both³He and⁴He) between 135 and 165 K. At theoretical monolayer helium coverage,Q_st/R=44±2 K for³He on the argon monolayer and 47±2 K for⁴He. At theoretical monolayer helium coverage on the bare copper,Q_st/R=61±4 K for³He and 77±5 K for⁴He. The results are compared with theoretical evaluations for helium adsorbed on an argon monolayer and with some previous experimental data, and the agreement is found to be fair. All the data are summarized in tables. Finally, a review is given of evaluations, including those from this work, of the monolayer capacity of³He and⁴He on the substrates studied.Work supported by a contract with the Department of Defense (Themis Program) and with the Office of Naval Research and by a Grant from the National Science Foundation.