Ultraviolet photoelectron spectroscopy and photoconductivity of CaF2

Vacuum ultraviolet photoelectron spectroscopy and a systematic study of charging during irradiation with 21.2-eV photons was carried out on cleaved (111) surfaces of CaF2 crystals. The goal was twofold: (1) to find ways of suppressing charging effects and (2) to use surface charging for monitoring photoconductivity. Attempts to reduce charging included crystal heating, use of negatively charged grids at the rear of the sample, and flooding with low-energy electrons. At crystal temperatures between 220 and 350 degrees C both the negative grid and the flood-gun technique produce charge-free surfaces. The enhanced photoconductivity during UV irradiation is explained by thermally activated diffusion of V-k centers. The observed temperature dependence yields an activation energy of 0.30+/-0.06 eV.