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Strong absorption in the vacuum ultraviolet region near the exciton edge was observed in Cd doped CaF 2, SrF2 and BaF2 crystals. The Cd ++ absorption band at 8.6 eV was observed in low doped BaF2 crystals. In SrF2 and CaF 2 the maximum of Cd ++ band cannot be resolved due to stronger overlapping with exciton bands. The calculations of geometry of Cd ++ and Cd + centres and Cd-related electron transitions were done using ab − initio Hartree-Fock and density functional methods. The experimental and calculated results lead us to the conclusion that strong Cd ++ absorption bands belong to allowed transitions from the nearest fluorides to the Cd ++ ion. Lattice relaxation around Cd + centres, created by x-irradiation, and optical transitions of these centres were also calculated. The calculated energies are in good agreement with experimental ones.

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Exciton luminescence was investigated in La-doped BaF 2 crystals. Two luminescence bands at 4.26 and 3.82 eV related to excitons have been observed. The excitation spectrum of the 4.26 eV band of BaF 2 -LaF 3 was similar to that of the exciton emission band of pure BaF 2 crystals. The excitation spectrum of the 3.82 eV band shows a new band within the 8-9 eV region. The band becomes most prominent for BaF 2 -3 wt% LaF 3 and has a maximum at 8.7 eV at 290 K.The decay time for the 4.26 eV emission band of BaF 2 -3% LaF 3 considerably increased with decreasing temperature like the exciton decay time for pure BaF 2 , where the decay time of unperturbed excitons grows to 121 µs at 17 K. In contrast to this, the decay time of the 3.82 eV band increases from 4.1 µs at 290 K to 6.2 µs at 15 K. The significant reduction of the triplet-singlet decay time can be associated with strong interaction of excited electrons with lanthanum ions.It was concluded that the 4.26 eV emission band is associated with weakly perturbed excitons, while the 3.82 eV band is associated with strongly perturbed excitons. The possible spatial configurations of excitons are discussed.

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