Configurations of excitons in BaF2-LaF3 crystals

Exciton luminescence was investigated in La-doped BaF2 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 BaF2-LaF3 was similar to that of the exciton emission band of pure BaF2 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 BaF2-3 wt% LaF3 and has a maximum at 8.7 eV at 290 K. The decay time for the 4.26 eV emission band of BaF2-3% LaF3 considerably increased with decreasing temperature like the exciton decay time for pure BaF2, where the decay time of unperturbed excitons grows to 121 As at 17 K. In contrast to this, the decay time of the 3.82 eV band increases from 4.1 mus at 290 K to 6.2 mus 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.