Solvent-Solute Interaction in Liquid and Solid Phases of Ethanol Probed by Photon Echo Spectroscopy

染料分子在溶液中的光学退相已被广泛地用各种实验方法研究，结果表明，退相发生在飞秒的时间尺度上，具有非马尔科夫性质。由于光子回声光谱是研究溶剂化动力学的强大工具，几个小组报告了利用飞秒激光脉冲在室温下测量的回声响应。然而，关于溶剂化动力学与温度的关系的报道相当有限。温度依赖性的研究是非常重要的，因为它提供了关于溶剂化动力学的详细信息。在本研究中，用非相干光子回波光谱研究了不同温度下β-胡萝卜素分子在乙醇中的溶剂化动力学。实验结果基于飞秒消相的动力学模型和慢能量起伏的随机模型进行了分析。利用由荧光光谱确定的动力学模型的参数值，我们得到了乙醇固相中皮秒时间尺度的慢能量涨落的幅度和关联时间。结果表明，回波信号在飞秒时间尺度上的快速响应本质上来自于该系统中较强的电子-声子耦合，而缓慢的能量涨落减小了回波信号的复相过程，从而使响应速度大大加快。非相干光子回波探测的是皮秒量级的慢能量起伏，比两个激发脉冲之间的延迟时间要长。

The optical dephasing of dye molecules in solution has been extensively investigated by various experimental methods and it is shown that the dephasing occurs on the time scale of femtoseconds with a non-Markovian nature. Since photon echo spectroscopy is a powerful tool for investigating solvation dynamics, several groups report echo responses measured at room temperature utilizing femtoseconds laser pulses. However reports on the temperature dependence of solvation dynamics are quite limited. The investigation of temperature dependence is important because it gives detailed information about the solvation dynamics.In this research, the solvation dynamics of beta-carotene molecules in ethanol has been investigated by incoherent photon echo spectroscopy at various temperatures from 300 K to 50 K.Experimental results are analyzed based on a dynamical model for a dephasing in femtoseconds and a stochastic model for a slow energy fluctuation. Using parameter values for the dynamical model determined from fluorescence spectra, we obtain amplitude and correlation time of the slow energy fluctuation in picoseconds time scale in solid phase of ethanol. It is shown that the fast response of echo signals in femtoseconds time scale essentially comes from the relatively strong electron-phonon coupling in this system and that the slow energy fluctuation decreases the rephasing process resulting in much faster response. The incoherent photon echo probes the slow energy fluctuation in picosecond order, Which is longer than the delay time between two excitation pulses.