Helium prewetting and nonwetting on weak-binding substrates

New wetting and nonwetting phenomena for He4 adsorbed on weak-binding substrates such as the alkali metals are predicted. With a nonlocal density-functional model, we calculate the density profile of He4 films adsorbed on surfaces characterized by an attractive van der Waals potential tail of strength C3 and a well depth D. As a function of C3 and D, He4 films exhibit both nonwetting and wetting, the latter being accompanied by a prewetting transition. Specifically, we find that He4 will not wet all the alkali metals; only on Li and Na is wetting behavior, with prewetting, expected.