Interaction Between Overlapping Quasi-Stationary States: He (2 1 S and 2 1 P ) Levels in Front of an Aluminium Surface

The interaction of He (2 1 S ) and He (2 1 P ) atoms with an aluminium surface is studied using the coupled-angular-mode method. The energy positions and widths of the perturbed atomic levels are determined as functions of the atom-surface distance Z . They display a very unusual behaviour: when Z decreases the two states “attract” each other when they start to overlap, they almost coalesce and then split with very different widths. This is quite different from the usual picture where interacting states repel each other. This behaviour is attributed to the existence of an imaginary coupling term between the two quasi-stationary states, which corresponds to indirect transitions via the metal surface continuum.