Clarification of the uniqueness of supercritical water from the view point of energy transfer dynamics

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We have developed an optical Kerr gate system for the time-resolved fluorescence detection under high-temperature and high-pressure fluids such as supercritical water (SCW) and supercritical methanol (SCMe). Using a third-harmonic output from a Ti : Sapphire laser as pump pulse and a fundamental output as a gate pulse, we attained the time-resolution of 1.2 ps by using a glass with high refractive index as a Kerr medium. We applied the system for the measurement of the fluorescence dynamics of perylene in SCMe, and found that the initial variation of the spectrum shape after the photo-excitation. The change was interpreted as the energy dissipation process by comparing the excess energy dependence of the fluorescence lineshape in vapor. The vibrational energy dissipation (VER) rate thus determined varied little by changing the solvent density from the critical density to twice of it, while the temperature effect on VER rate was large. The mechanism of VER was also investigated by other non-linear laser spectroscopy.We have also measured Raman spectra of p-nitroaniline in SCW and SCMe. Based on Raman spectra of chemical compounds with similar molecular structures measured in various solvents, we found that the vibrational frequency of the NO_2 stretching mode was strongly affected by the bulk property of solvent such as the dielectric constant, while the vibrational frequency of the NH_2 stretching mode was strongly affected by the hydrogen-bonding from the solvent molecule. The similarity of the density dependence of the VER rate of perylene to the vibrational spectral shift of NH_2 mode of p-ntroaniline may suggests the VER through the hydrogen-bonding is effective in SCMe.In the future, we are planning to measure the VER rate in SCW which could not be tried within the period of this program, and investigate the mechanism of VER more intimately with the vibrational spectroscopy.