Decay time measurements of intrinsic luminescence in alkali halides using single-bunched light pulses from UVSOR

The combination of single-bunched light pulses (duration: 0.5 ns, interval: 177.6 ns) from UVSOR (Okazaki, Japan) with a method of time-correlated single-photon counting was applied to study decay behaviors of intrinsic luminescence in nine typical alkali halide crystals (Na, K, Rb) × (Cl, Br, I). Under excitation into the band-to-band transition, two emission bands, so far called σ emission (fluorescence with a short lifetime) and π emission (phosphorescence with a much longer lifetime), were observed in general at LHeT. The decay behaviors observed were essentially the same as those reported in previous studies where pulsed particles were used as an excitation source. A novel situation has, however, been recognized in the π emission in both NaBr and NaI: A fluorescent component was found clearly to coexist with the long-lived phosphorescent component. This suggests that the initial state of the π emission in NaBr and NaI is a singlet-triplet pair slightly split by the exchange interaction.