Emission of Tm2+ in alkaline-earth fluoride crystals

The luminescence spectra of 4f-4f and 5 d-4f transitions and absorption spectra of 4f-4f transitions of Tm2+ ions were studied in crystals of alkaline earth fluorides CaF2, SrF2, BaF2. The 5 d-4f emission of Tm2+ ions was detected in the 680–800 nm region with a decay time in the micro-millisecond time interval. The 4f-4f emission is thermally quenched at temperatures of 250, 300, and 350 K in BaF2, SrF2, and CaF2, which correlates with the distance from the excited level to the bottom of the conduction band. The splitting of the atomic F7/22, F5/22 levels of Tm2+ by the cubic field decreases linearly with increasing interion distance. The splitting tends to zero at a distance between fluorines of about 4 Å. The positions of the Tm2+ bands in alkaline earth fluorides are well described by ab initio quantum chemical calculations.