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

我们开发了一种光学克尔门系统，用于高温高压流体（如超临界水（SCW）和超临界甲醇（SCMe））下时间分辨荧光检测。利用Ti: Sapphire激光器的三次谐波输出作为泵浦脉冲，基波输出作为门脉冲，我们利用高折射率玻璃作为Kerr介质，获得了1.2 ps的时间分辨率。我们将该系统应用于SCMe中苝的荧光动力学测量，发现光激发后光谱形状的初始变化。通过比较蒸汽中荧光线形状的多余能量依赖性，将这种变化解释为能量耗散过程。当溶剂密度为临界密度的两倍时，测定的振动能量耗散率变化不大，而温度对其影响较大。用其他非线性激光光谱学方法研究了VER的作用机理。我们还测量了SCW和SCMe中对硝基苯胺的拉曼光谱。基于不同溶剂中具有相似分子结构的化合物的拉曼光谱，我们发现NO_2拉伸模式的振动频率受溶剂的体积性质（如介电常数）的强烈影响，而NH_2拉伸模式的振动频率受溶剂分子氢键的强烈影响。对苯胺NH_2模式的振动谱移与苝的VER速率的密度依赖性相似，表明通过氢键的VER在SCMe中是有效的。未来，我们计划在本项目期间无法测试的SCW中测量VER速率，并利用振动光谱更深入地研究VER的机理。

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.