SELF-TRAPPED EXCITON LUMINESCENCE IN DILATED NAI CRYSTALS - RELAXATION PROCESS OF EXCITONS IN ALKALI-HALIDES

The effect of dilatational stress on the relaxation of excitons in NaI is investigated by means of photoluminescence and decay time measurements. When the lattice spacing is expanded, a new luminescence band, which is ascribed to the self-trapped exciton in off-center configuration, appears around 3.05 eV in addition to the pi-band at 4.15 eV. The decay profile of the pi-band, consisting of 1.2-ns and 110-ns decay components, is not affected by the lattice expansion. The 3.05 eV band exhibits two decay components of 6.0 ns and 1.5-mu-s. These lifetimes are significantly longer than the corresponding values of the pi-band, indicating that the electron-hole overlap is smaller for the 3.05 eV-luminescent state. The relaxation processes of a free exciton into the self-trapped state in alkali halides are discussed with the help of the present and previous results.