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

Abstract We investigated the effect of the surface condition on luminescence processes in a surface layer of single CaF2 crystals under the excitation by vacuum-ultraviolet (VUV) light (hν≤13.5 eV). It has been shown that the intensity of the excitonic emission at 4.4 eV in polished crystals essentially decreases with respect to that in freshly cleaved crystals. The decrease at the interband excitation is larger than that at the direct 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 strict correlation with the excitonic absorption band at 11.2 eV. It is almost absent in polished crystals. The ageing of the cleaved surface of CaF2 crystals causes decrease in the intensities of the 4.4 and 2.55 eV emissions with different rates. The etching of a polished surface restores both emission bands. The correlation has been established between the evolution of these 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 cleavage, polishing or etching of the sample.