Existence and nature of a helium monolayer film

A variational calculation is presented of the energy of monolayer He films adsorbed on a surface. We find that the ground state of He4 is always a liquid, in agreement with the result for a strictly two-dimensional calculation, but in disagreement with results obtained recently with perturbation theory for the case of extremely weak adsorption potentials. The calculated energies provide a bound on the regime of parameter space corresponding to a bare surface ground state. For the most part, the latter is consistent with both the conclusions obtained recently with a density functional theory and with experiments showing nonwetting for He4 on Cs and Rb. For He3, calculations are presented under the assumption that the quasi-two-dimensional ground state is a gas. The resulting threshold potential for the monolayer to form is somewhat less attractive than in the He4 case.