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

Abstract

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 from 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.