Abstract
Abstract Nominally pure LiF crystals from different sources show a very weak emission in the red spectral region with the maximum at 720 nm when excited with UV light. In pure crystals this photo-luminescence is measurable only under extreme excitation and registration conditions. Besides broad band and zero-phonon line spectra due to F-aggregate centres, LiF/CaWO4 crystals emit strong 720 nm fluorescence bands. Additionally to this feature, the LiF/CaWO4 solid system exhibits a highly structured photoluminescence on the short wavelength side of the 720 nm peak. This line spectrum is well resolved already at 77 K. The red emission is connected with the presence of Ca2+ ions in the LiF lattice. Transmission, excitation, and emission data, together with the results of annealing experiments strongly indicate that the measured low energy photoluminescence is due to optical transitions in Z2-centres.
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