Adsorption and Specific-Heat Studies of Monolayer and Submonolayer Films ofHe3andHe4

A study has been made of the adsorption of ${\mathrm{He}}^3$ and ${\mathrm{He}}^4$ 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 ${\mathrm{N}}_2$ 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 ${\mathrm{He}}^3$ and ${\mathrm{He}}^4$ on Ar-plated Cu sponge. The heat capacities of nearly complete monolayers vary as $T^2$ from 0.3 to 4°K, yielding two-dimensional Debye temperatures $\Theta \left({\mathrm{He}}^4\right)=28\mathrm{}\pm 1°$K, and $\Theta \left({\mathrm{He}}^3\right)=31\mathrm{}\pm 1°$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 ${\mathrm{He}}^4$ 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 $T^2$ behavior for the complete monolayers yields an upper limit of ∼${10}^{-11\mathrm{}}$ sec for the lifetime of a He atom in any individual adsorption site, consistent with a theoretical estimate.