Ultrafast relaxation processes from a higher excited electronic state of a dye molecule in solution: a femtosecond time-resolved fluorescence study

摘要

We have investigated the internal conversion, intramolecular energy redistribution, and vibrational cooling of Coumarin 481 in cyclohexane after the photoexcitation by a third harmonic of the Ti:Sapphire laser (267 nm). Following the photoexcitation to a higher electronic state, we have observed the fluorescence up-conversion signals at several wavelengths. The experimental results show that the fluorescence signals rise with time constants of 220-280 fs at all wavelengths, and there is no drastic change of the spectral shape within a few picoseconds. This suggests that the observed dynamics are mainly due to the internal conversion form the S_n to S_1 state and the intramolecular energy redistribution takes place much faster than the former process. We have also performed a model calculation of the fluorescence spectrum by assuming three Franck-Condon active modes to obtain information on the relaxation processes. A spectral change due to vibrational cooling is observed on a time scale of 10 ps, which is simulated quite well in terms of the thermal diffusion equation.