Study on the photo-thermalization process of photo-excited molecules in supercritical fluid by the non-linear spectroscopy

本工作的目的是阐明溶液中光热化过程的分子机制。利用超临界流体，我们进行了两种不同的实验研究；一种是探测溶质分子的振动能，另一种是探测溶剂的平动能。对于前一种方法，我们通过测量随时间变化的荧光光谱，确定了S_2 azulene在各种超临界流体中的振动能量弛豫（VER）速率。在S_2状态下，VER速率与溶剂密度的关系与基态下非常相似。这表明溶剂密度对VER速率的影响可以通过气相分离二元碰撞模型的扩展来解释。分子动力学模拟的结果也支持了这一结论，即在分子距离上，溶剂-溶剂相关性不影响溶质-溶剂相关性，在分子距离上，排斥性相互作用占主导地位。另一方面，我们开发了一套将瞬态光栅方法应用于超临界流体的系统。在这种方法中，我们通过分析交叉光束脉冲光激发后光热过程引起的声传播信号来确定对溶剂的能量耗散率。在系统持续时间（5ns）的限制下，我们将系统设置成大的交叉波束角度（150°）来测量声信号。我们成功地测量了声速最慢的溶剂临界密度附近的信号。现在我们正在测量三氟甲烷中的电荷转移（CT）配合物。在测量超临界流体之前，我们研究了类似的CT配合物在极性液体中，如乙腈，并证明了光热化时间常数在10皮秒左右。

The aim of this work is to elucidate the molecular mechanism of photo-thermalization process in solution. Using the supercritical fluid, we have made two different experimental studies ; one is to probe the vibrational energy of the solute molecule and the other is to probe the translational energy of the solvent.As for the former method, we have determined the vibrational energy relaxation (VER) rate of the S_2 azulene in various supercritical fluids by measuring the time-dependent fluorescence spectrum. The solvent density dependence of the VER rate in the S_2 state is quite similar to that in the ground state. This indicates that the solvent density effect on the VER rate can be interpreted by the extension of the isolated binary collision model in the gaseous phase. This conclusion is also supported by the result of the molecular dynamics simulation that the solvent-solvent correlation does not affect the solute-solvent correlation in the molecular distance where the repulsive interaction is dominant.On the other hand, we developed a system to apply the transient grating method to the supercritical fluids. In this method, we determine the energy dissipation rate to the solvent by analyzing the sound propagation signal caused by the photo-thermal processes after the photo-excitation by the crossing beam pulses. We set up the system with a large angle (150°) of the crossing beam to measure the sound signal under the limitation of time duration (5ns) of our system. We have succeeded in measuring the signal near the solvent critical density where the sound velocity is the slowest. Now we are measuring the charge transfer (CT) complex in trifluoromethane. Before the measurement of the supercritical fluids, we have studied the similar CT complexes in polar liquids such as acetonitrile, and demonstrated that the photo-thermalization time constant is around 10 pico-second.