Energy shift of He groundstate close to an aluminum surface

He atoms and ions with keV energies are scattered under a grazing angle of incidence from a flat and clean Al(1 1 1) surface. From the analysis of angular distributions for scattered projectiles we reveal that the angular shifts between distributions for incident neutral atoms and positive ions show a reversal in sign as function of distance of closest approach to the surface plane. This observation allows us to deduce the energy shift of the He groundstate as function of distance from the surface. Our work provides important information towards a fundamental microscopic understanding of atomic interactions close to metal surfaces.