Optical absorption spectra of rare-gas atoms adsorbed on metal surfaces exhibit a bimodal behavior, which, according to Cunningham, Greenlaw and Flynn, can be correlated with the difference I′ - φ (where I' is the ionization energy of the excited (gas phase) state of the rare gas atom and φ the work function of the metal) controlling the energetics of charge transfer from the excited atom to the metal. In this paper we propose a model which allows to treat this charge transfer and some accompanying many-body effects in detail. Strong Coulomb attraction between the core hole and the excited electron on the adatom is taken into account as well as the interaction with surface plasmons. An improved charge transfer criterion is obtained which, besides the important parameter I ∗ - φ, involves additional parameters such as the adsorbate-metal coupling strength and the plasmon frequency.