Novelli Fabio, Hoberg Claudius, Adams Ellen M, Klopf J Michael, Havenith Martina
Department of Physical Chemistry II, Ruhr University Bochum, 44780 Bochum, Germany.
Institute of Radiation Physics, Helmholtz-Zentrum Dresden-Rossendorf, Dresden, Germany.
Phys Chem Chem Phys. 2022 Jan 4;24(2):653-665. doi: 10.1039/d1cp03207k.
The dynamical complexity of the hydrogen-bonded water network can be investigated with intense Terahertz (THz) spectroscopy, which can drive the liquid into the nonlinear response regime and probe anharmonicity effects. Here we report single-color and polarization-dependent pump-probe experiments at 12.3 THz on liquid water, exciting the librational mode. By comparing results obtained on a static sample and a free-flowing water jet, we are able to disentangle the distinct contributions by thermal, acoustic, and nonlinear optical effects. We show that the transient transmission by the static water layer on a time scale of hundreds of microseconds can be described by thermal (slow) and acoustic (temperature-dependent) effects. In addition, during pump probe overlap we observe an anisotropic nonlinear optical response. This nonlinear signal is more prominent in the liquid jet than in the static cell, where temperature and density perturbations are more pronounced. Our measurements confirm that the THz excitation resonates with the rotationally-damped motion of water molecules, resulting in enhanced transient anisotropy. This model can be used to explain the non-linear response of water in the frequency range between about 1 and 20 THz.
可以用强太赫兹（THz）光谱研究氢键水网络的动力学复杂性，该光谱可使液体进入非线性响应区域并探测非谐效应。在此，我们报告了在12.3太赫兹下对液态水进行的单色和偏振相关泵浦 - 探测实验，激发了水分子的转动模式。通过比较在静态样品和自由流动水射流上获得的结果，我们能够区分热效应、声学效应和非线性光学效应的不同贡献。我们表明，在数百微秒时间尺度上，静态水层的瞬态透射可以用热（慢）效应和声学（温度相关）效应来描述。此外，在泵浦 - 探测重叠期间，我们观察到各向异性的非线性光学响应。这种非线性信号在液体射流中比在静态样品池中更明显，在静态样品池中温度和密度扰动更显著。我们的测量证实，太赫兹激发与水分子的旋转阻尼运动发生共振，导致瞬态各向异性增强。该模型可用于解释水在约1至20太赫兹频率范围内的非线性响应。
