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

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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.