Triplet luminescence of cadmium centres in alkaline-earth fluoride crystals

The emission and excitation spectra as well as decay of emissions of alkaline-earth fluoride crystals doped with CdF2 were investigated in the 2-24 eV range at temperatures in the range 8-300 K. Emission bands at 4.2 and 3.5 eV, respectively, were found under excitation into the Cd absorption region in CaF2-Cd and SrF2-Cd crystals at low temperatures. Both emission bands have slow luminescence decay times of microsecond timescale. No Cd-related emission was found in BaF2-Cd crystals. The calculations of the geometrical configurations of excited triplet Cd 2, centres and the Cd-related electron transitions were carried out by using the ab initio Hartree-Fock method. The results of experiments and calculations lead us to the conclusion that the observed Cd2+ emission bands are due to the triplet-singlet transitions from the Cd s-states to the nearest fluorine ions. The calculated energies are in good agreement with experimentally observed ones.