FEMTOSECOND TIME-RESOLVED SPECTROSCOPIC STUDIES OF THE DYNAMICS OF THE RELAXATION OF EXCITONS IN THE LOWEST ADIABATIC POTENTIAL-ENERGY SURFACE IN NACL

We have measured time-resolved optical absorption spectra following the excitation of the self-trapped excitons (STEs) in NaCl from the lowest triplet state to the next higher electron-excited state. In addition to the rapid reduction of the band due to the electron transition of the STE (STE band), a new absorption band peaked between the STE band and the F band is found to be temporarily generated. The heights of the new band and that of the STE band show anticorrelated oscillations with a period of 1 ps and moreover the new band disappears and the STE band recovers within 7 ps after the excitation. It is shown that the new band is due to the transition from the adiabatic potential energy surface on which the lowest STE is located and the oscillation is interpreted to be a motion of a wave packet on the adiabatic potential energy surface. We find also that the F band grows in the relaxation process.