Configurational switching and charge transfer for optically excited rare-gas adsorbates on simple-metal surfaces

Optical spectra in the range 5-17 eV for rare-gas atoms adsorbed on simple-metal surfaces are reported. Various features in the spectra are coverage dependent presumably owing to interactions among adsorbates. The results reveal an important new process of charge transfer induced by the optical excitation. The observed spectra fall into one of two clearly distinct categories. In one of these the rare-gas spectrum at low coverage persists in a form resembling that of the rare-gas solid, but with a noticeable broadening and shifting of prominent features. In the other, the threshold is blue shifted by several eV into a shoulder lacking analogs of the exciton lines. The demarcation between these forms occurs close to the condition $I^{*}=\phi$ at which the excited-state ionization energy $I^{*}$ equals the substrate work function $\phi$. Near this point the excited rare gas becomes unstable against charge transfer to the surface. Results are interpreted as indicating charge transfer induced by the optical process. The mechanism switching the system between configurations appears sharp to ∼0.1 eV in $I^{*}$ or $\phi$. Evidently the hopping matrix element connecting an excited electron on the rare gas to the bulk must be large (several eV). The observed line shapes for the cases of persistent spectra are difficult to understand in detail.