Density fluctuation of fluids and dynamics of photo-dissociation reactions

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In order to reveal the dynamics of the molecular pair created by the photo-excitation in supercritical fluids with the large density fluctuation, we have studied two reaction systems ; the charge transfer complex which creates the charge-separated excited state upon photo-excitation, and disulfide compounds which creates the radical pair upon photo-excitation. We have chosen the charge transfer complex of hexamethylbenzene and tetracyanoethylene as the former system, and measured the relaxation process from the excited state in several fluids (ethane, carbon dioxide, trifluoromethane, and nitrous oxide) by the pico-second transient absorption method. According to the results, the electron transfer rate from the excited state increases with increasing the solvent polarity and the solvent density. The relative change of the electron transfer rate in various conditions has been well reproduced by the theoretical calculation using the reaction free energy and the solvent reorganization energy estimated from the absorption spectrum. The electron transfer rate showed a significant isotope effect, which could not be reproduced by the theoretical calculation.We have applied the transient grating method to the measurement of the photolysis quantum yield, and have succeeded in determining the photolysis quantum yield of diphenyl disulfide in supercritical fluids. The density dependence of the photolysis quantum yield and the non-geminate recombination rate could not be explained by the model used for the recombination process of iodine. Our results suggests the possibility of the primary geminate recombination just after the photo-excitation. We have also succeeded in measuring the absorption spectrum in pico-second order time resolution using the high pressure flow system constructed in this study. Now we are analyzing the rise process of the absorption of the radical just after the creation, in several fluids.