We report measurements of photon-stimulated desorption of excited atoms from LiF and NaCl crystal surfaces. Based on these measurements, we propose a new model of photon-initiated, defect-mediated desorption involving exoergic surface reactions between alkali dimers and halogens. This model consists of two steps: (a) reactants are formed at the surface, directly or by the migration to the surface of holes and defects created in the near surface bulk, due to uv photon bombardment, and (b) excited alkali atoms are emitted following a reaction between the alkali and halogen surface species. jg p2.

The mechanism leading to photon-stimulated desorption (PSD) of excited atoms from the surfaces of alkali halides has been a subject of controversy since it was first discussed a decade ago. We have recently carried out a systematic experimental study of PSD from alkali halides under both valence-band and core-level excitation. Our data demonstrate that the desorption yields for excited alkali atoms track the excitonic optical response of these crystals even at photon energies below the bulk band gap. Secondary electron measurements, taken simultaneously, implicate the formation of excess metal in the time and dose dependence of PSD yields. These results suggest a unified picture of PSD in which the ion motion initiating desorption is pictured as the decay of a localized, highly deformed vibrational mode of the crystal. Plausible mechanisms for creation of the excited atomic state include relaxation of excited F-centers and resonant neutralization into the excited state on metallic microclusters.