The unusual temperature dependence of the Eu2+ fluorescence lifetime in CaF2 crystals

Fluorescence lifetimes of Eu2+ doped in CaF2 are measured at various temperatures between 4 and 450 K. The lifetime increases between 100 and 320K, in contrast to the usual lifetime-temperature dependence. At higher or lower temperatures, the lifetime decreases with increasing temperature. This phenomenon is explained by simulation of the energy levels and lifetimes of low-lying 4f65d states involved in the fluorescence.