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

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Fluorescence lifetimes of Eu²⁺ doped in CaF₂ are measured at various temperatures between 4K and 450K. The lifetime increases with between 100K and 300K, in contrast to the usual lifetime-temperature dependence. At higher or lower temperatures the lifetime decreases with increasing temperature. The phenomenon is explained by simulation of the energy levels and lifetimes of low-lying 4f⁶5d states involved in the fluorescence.