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

We have investigated the internal conversion, intramolecular energy redistr ibution, and vibrational cooling of Coumarin 481 in cyclohexane after the p hotoexcitation by a third harmonic of the Ti:Sapphire laser (267 nm). Follo wing the photoexcitation to a higher electronic state, we have observed the fluorescence up-conversion signals at several wavelengths. The experimenta l results show that the fluorescence signals rise with time constants of 22 0-280 fs at all wavelengths, and there is no drastic change of the spectral shape within a few picoseconds. This suggests that the observed dynamics a re mainly due to the internal conversion from the S-n to S-l state and the intramolecular energy redistribution takes place much faster than the forme r process. We have also performed a model calculation of the fluorescence s pectrum by assuming three Franck-Condon active modes to obtain information on the relaxation processes. A spectral change due to vibrational cooling i s observed on a time scale of 10 ps, which is simulated quite well in terms of the thermal diffusion equation. (C) 1999 Elsevier Science B.V. All righ ts reserved.