ULTRAVIOLET PHOTOELECTRON-SPECTROSCOPY AND PHOTOCONDUCTIVITY OF CAF2

Vacuum ultraviolet photoelectron spectroscopy and a systematic study o f 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 charg ing for monitoring photoconductivity. Attempts to reduce charging incl uded crystal heating, use of negatively charged grids at the rear of t he sample, and flooding with low-energy electrons. At crystal temperat ures between 220 and 350 degrees C both the negative grid and the floo d-gun technique produce charge-free surfaces. The enhanced photoconduc tivity during UV irradiation is explained by thermally activated diffu sion of V-k centers. The observed temperature dependence yields an act ivation energy of 0.30+/-0.06 eV.