Cation size dependence of phosphorescence lifetime of Eu²⁺ doped alkali halide crystals

Phosphorescence lifetimes of europium doped alkali halide crystals have been measured and compared with each other. The lifetime values resulted to be more influenced by the cation size following an increasing order from Li to Rb. The effect of covalency decrease from Li to Rb and the corresponding mixing of 4f⁷ and 4f⁶5d states of europium ions suggests an influence of the low lying ⁶P_7/2 level of the 4f ⁷ electronic configuration on the relaxation from the 4f ⁶5d excited state to the ⁸S_7/2 ground state.