Abstract
Calcium difluoride bulk crystals cleaved along (111) planes in air and in ultra-high vacuum have been irradiated with 2.5 keV electrons and 1486 eV photons. Radiation induced changes in the surface electronic structure were monitored as a function of irradiation dosage with ultraviolet photoelectron spectroscopy (UPS) and metastable impact electron spectroscopy (MIES). The radiation induced decomposition of the surface is directly evidenced in MIES spectra where upon increased dosage a continuous band appears at the Fermi energy while UPS performed with 21.2 eV photons is found not to be sensitive for the detection of metallized areas at the surface. For crystals cleaved in air after irradiation an additional structure 3 eV above the fluorine 2p valence band edge is found in UPS and MIES spectra that is attributed to oxidized metal patches at the surface. This assignment is confirmed by spectroscopy on Ca films on silicon surfaces dosed with oxygen.
Published Version
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