Density fluctuation of fluids and dynamics of photo-dissociation reactions

为了揭示密度波动较大的超临界流体中光激发形成的分子对的动力学特性，我们研究了两种反应体系；在光激发下产生电荷分离激发态的电荷转移配合物，以及在光激发下产生自由基对的二硫化合物。我们选择了六甲基苯和四氰乙烯的电荷转移配合物作为前者体系，用皮秒瞬态吸收法测量了几种流体（乙烷、二氧化碳、三氟甲烷和氧化亚氮）从激发态开始的弛豫过程。结果表明，激发态的电子转移速率随溶剂极性和溶剂密度的增加而增加。利用吸收光谱估计的反应自由能和溶剂重组能，通过理论计算，很好地再现了不同条件下电子转移速率的相对变化。电子转移率表现出明显的同位素效应，这是理论计算无法再现的。我们将瞬态光栅法应用于光解量子产率的测量，并成功地测定了超临界流体中二苯二硫的光解量子产率。光解量子产率和非孪晶复合率的密度依赖性不能用碘的复合过程模型来解释。我们的研究结果表明，光激发后可能发生初级双极重组。我们还成功地利用本研究构建的高压流系统测量了皮秒级时间分辨率的吸收光谱。现在我们正在分析自由基形成后，在几种流体中吸收的上升过程。

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.