PHOTOEMISSION FROM PURE AND ELECTRON-IRRADIATED CAF2

We describe changes of the electronic structure of CaF2 (1 1 1) surfac es caused by electron irradiation. with an emphasis on radiation stimu lated oxidation. CaF2 crystals cleaved in air and in ultra high vacuum (UHV) were investigated with ultraviolet photoelectron spectroscopy u sing HeI light (21.2 eV). Surfaces produced by cleavage in air show a large density of states in the band gap region, in contrast to those c leaved in UHV. When irradiating the air cleaved crystals with low ener gy electrons, we observed the formation of a new peak in the band gap region that is attributed to oxygen. From the electron dosage dependen ce of this new peak, we conclude that it is mainly formed during irrad iation by oxygen already present at the crystal surface. For crystals cleaved in UHV, no new features are observed following irradiation. al though a metallic film on the crystal surface is visible with the bare eye. However, dosage with oxygen also yields the band gap peak. The o xidized surface is modeled by ab-initio calculations, based on the Har tree-Fock method. Results are compatible with experimental observation s. (C) 1998 Elsevier Science B.V. All rights reserved.