Ultrafast Spectroscopic Study of Photochemical Dynamics in Solution and Microscopic Environments

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We studied picosecond and femtosecond dynamics of fundamental molecules in sondensed phase using time-resolved vibrational, electronic spectroscopy both in the time and frequency domain. The major outcome of this research project is summarized as follows.1. We constructed a non-colinear OPA system that generates sub-10 fs optical pulses. Using the ultrashort optical pulse, we carried out pump-probe experiments for S_1 trans-stilbene, and observed beating signals due to the vibrational coherence (V_<25> mode) in the S_1 state.2. We investigated the vibronic relaxation process of polyatomic molecules in solution, taking tetrace ne as an example. We succeeded in measuring both S_2 and S_1 time-resolved fluorescence, and clarified the electronic relaxation, IVR, vibrational cooling and rotational diffusion process which successively occur after S_2←S_1 photoexcitation.3. We studied photoisomerization of azobenzene by picosecond Raman and femtosecond fluorescence spectroscopy. The obtained data directly showed that the S_2 state is almost exclusively relaxed to the 'planar' S_1 state. It means that the 'long-time believed' rotational isomerization pathway in the S_2 state does not exist, and that isomerization after S_2 excitation takes place in S_1 state after electronic relaxation.4. We investigated ultrafast intramolecular proton transfer of 1,8-dihydroxyanthraquinone, and found that tautomer-type fluorescence appears within 50 fs, which mean that proton translocation takes place within 50 fs in the excited state of this molecule.