Photoemission from pure and electron irradiated CaF 2

Abstract

We describe changes of the electronic structure of CaF 2 (1 1 1) surfaces caused by electron irradiation, with an emphasis on radiation stimulated oxidation. CaF 2 crystals cleaved in air and in ultra high vacuum (UHV) were investigated with ultraviolet photoelectron spectroscopy using 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 cleaved in UHV. When irradiating the air cleaved crystals with low energy electrons, we observed the formation of a new peak in the band gap region that is attributed to oxygen. From the electron dosage dependence of this new peak, we conclude that it is mainly formed during irradiation by oxygen already present at the crystal surface. For crystals cleaved in UHV, no new features are observed following irradiation, although a metallic film on the crystal surface is visible with the bare eye. However, dosage with oxygen also yields the band gap peak. The oxidized surface is modeled by ab-initio calculations, based on the Hartree–Fock method. Results are compatible with experimental observations.