Dependence of luminescence processes and transmission in vacuum-ultraviolet region on surface condition in CaF2 single crystals

We investigated the effect of the surface condition on luminescence process es in a surface layer of single CaF2 crystals under the excitation by vacuu m-ultraviolet (VUV) light (hv less than or equal to 13.5 eV). It has been s hown that the intensity of the excitonic emission at 4.4 eV in polished cry stals essentially decreases with respect to that in freshly cleaved crystal s. The decrease at the interband excitation is larger than that at the dire ct optical creation of excitons. A new luminescence band peaking at 2.55 eV has been discovered and ascribed to the oxygen-related centres situated in a surface layer of CaF2. The main excitation band of this band is in stric t correlation with the excitonic absorption band at 11.2 eV. It is almost a bsent in polished crystals. The ageing of the cleaved surface of CaF2, crys tals causes decrease in the intensities of the 4.4 and 2.55 eV emissions wi th different rates. The etching of a polished surface restores both emissio n bands. The correlation has been established between the evolution of thes e luminescence phenomena and the changes in VUV transmission spectra at 7.5 -10.2 eV near the fundamental absorption edge of CaF2, induced by the cleav age, polishing or etching of the sample. (C) 2000 Elsevier Science B.V. All rights reserved.