Origin of the 4.1-eV luminescence in pure CsI scintillator

The spectra and decay times of the 4.3- and 3.7-eV luminescence bands associated with the self-trapped excitons (STE's) in CsI have been observed in a wide temperature range (4.5--300 K) by using two-photon spectroscopy. The results reveal that the 4.1-eV luminescence in pure CsI, which is well known as the room-temperature scintillation, appears mainly from the 4.3-eV STE state: the 4.3-eV STE luminescence is dominant below 6 K and is quenched above 6 K, but appears again at 4.1 eV at room temperature by thermal activation of STE's from the 3.7-eV STE state to the 4.3-eV STE state.