Femtosecond cascade-excitation spectroscopy for nonradiative deexcitation and lattice relaxation of the self-trapped exciton in NaCl

Optical-absorption change following photoexcitation of the lowest triplet self-trapped exciton (STE) in NaCl has been measured in the femtosecond time regime. It is found that a short-lived intermediate state is generated within 1 ps and decays accompanied by restoration of the lowest STE. An oscillation in the population of the lowest STE and the intermediate state with a period of 1.1 ps was observed. Formation of Frenkel pairs has been found to occur not at the initial stage of relaxation but as a result of the relaxation leading to the lowest STE.