Thin He-4 films on alkali substrates: where do He-3 atoms bind?

The possible occurrence of bound states of He-3 atoms in the vicinity of a weakly attractive substrate coated with a thin superfluid He-4film is investigated by first principle computer simulations. No evidence is seen of such bound states, even in the case of the weakest substrate, i.e., Cs; a single He-3 atom always binds to the free He-4 surface, regardless of the thickness of the He-4 film. A comparison of He-4 density profiles computed in this work with those yielded by the Density Functional approach that led to the prediction of He-3 bound states near the substrate, shows that the latter may not have afforded a sufficiently accurate structural description of the adsorbed He-4 film.