Configurations of excitons in BaF2–LaF3 crystals

Abstract

Exciton luminescence was investigated in La-doped BaF2 crystals. Twoluminescence 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 ofthe exciton emission band of pure BaF2 crystals. The excitation spectrum of the3.82 eV band shows a new band within the 8–9 eV region. The band becomesmost prominent for BaF2–3 wt% LaF3 and has a maximum at 8.7 eV at 290K.The decay time for the 4.26 eV emission band of BaF2–3% LaF3 considerablyincreased with decreasing temperature like the exciton decay time for pureBaF2, where the decay time of unperturbed excitons grows to 121 μ s at 17K. In contrast to this, the decay time of the 3.82 eV band increases from 4.1 μ s at 290 Kto 6.2 μs at 15 K. The significant reduction of the triplet–singlet decay time can beassociated with strong interaction of excited electrons with lanthanumions.It was concluded that the 4.26 eV emission band is associated with weaklyperturbed excitons, while the 3.82 eV band is associated with strongly perturbedexcitons. The possible spatial configurations of excitons are discussed.