Helium prewetting and nonwetting on weak-binding substrates

New wetting and nonwetting phenomena for ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ adsorbed on weak-binding substrates such as the alkali metals are predicted. With a nonlocal density-functional model, we calculate the density profile of ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ films adsorbed on surfaces characterized by an attractive van der Waals potential tail of strength ${\mathit{C}}_3$ and a well depth D. As a function of ${\mathit{C}}_3$ and D, ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ films exhibit both nonwetting and wetting, the latter being accompanied by a prewetting transition. Specifically, we find that ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ will not wet all the alkali metals; only on Li and Na is wetting behavior, with prewetting, expected.