TIME-RESOLVED SPECTROSCOPIC STUDY ON THE TYPE-I SELF-TRAPPED EXCITONS IN ALKALI-HALIDE CRYSTALS .1. EMISSION-SPECTRA AND DECAY BEHAVIOR

Decay behaviors of Self-Trapped Exciton (STE) luminescence categorized into type I in seven alkali halides, i.e., the sigma bands in NaCl, KBr, RbBr, KI and RbI, and the pi bands in NaBr and NaI, are studied by using synchrotron radiation pulses from UV-SOR under single-bunch operation. Counting photons over four orders of magnitude reveals that every sigma band involves a phosphorescent component with a lifetime longer than 100 ns, in addition to the main fluorescent component. This evidences that the sigma band originates from the lowest orbital state of the STE. Simultaneous detection through two different time windows discloses a split in the peak energy of the fluorescent and phosphorescent components. From this result, a slight difference in the location of the minima is suggested in the adiabatic energy surfaces for a nearly degenerate singlet-triplet STE pair with the on-center configuration.