Zhao Hang, Tan Yong, Zhang Liangliang, Zhang Rui, Shalaby Mostafa, Zhang Cunlin, Zhao Yuejin, Zhang Xi-Cheng
Beijing Key Laboratory for Precision Optoelectronic Measurement Instrument and Technology, School of Optics and Photonics, Beijing Institute of Technology, Beijing, 100081 China.
Beijing Advanced Innovation Center for Imaging Technology and Key Laboratory of Terahertz Optoelectronics (MoE), Department of Physics, Capital Normal University, Beijing, 100048 China.
Light Sci Appl. 2020 Aug 4;9:136. doi: 10.1038/s41377-020-00370-z. eCollection 2020.
The fundamental properties of water molecules, such as their molecular polarizability, have not yet been clarified. The hydrogen bond network is generally considered to play an important role in the thermodynamic properties of water. The terahertz (THz) Kerr effect technique, as a novel tool, is expected to be useful in exploring the low-frequency molecular dynamics of liquid water. Here, we use an intense and ultrabroadband THz pulse (peak electric field strength of 14.9 MV/cm, centre frequency of 3.9 THz, and bandwidth of 1-10 THz) to resonantly excite intermolecular modes of liquid water. Bipolar THz field-induced transient birefringence signals are observed in a free-flowing water film. We propose a hydrogen bond harmonic oscillator model associated with the dielectric susceptibility and combine it with the Lorentz dynamic equation to investigate the intermolecular structure and dynamics of liquid water. We mainly decompose the bipolar signals into a positive signal caused by hydrogen bond stretching vibration and a negative signal caused by hydrogen bond bending vibration, indicating that the polarizability perturbation of water presents competing contributions under bending and stretching conditions. A Kerr coefficient equation related to the intermolecular modes of water is established. The ultrafast intermolecular hydrogen bond dynamics of water revealed by an ultrabroadband THz pump pulse can provide further insights into the transient structure of liquid water corresponding to the pertinent modes.
水分子的基本性质，如分子极化率，尚未得到阐明。氢键网络通常被认为在水的热力学性质中起着重要作用。太赫兹（THz）克尔效应技术作为一种新型工具，有望用于探索液态水的低频分子动力学。在此，我们使用一个强而超宽带的太赫兹脉冲（峰值电场强度为14.9 MV/cm，中心频率为3.9 THz，带宽为1 - 10 THz）来共振激发液态水的分子间模式。在自由流动的水膜中观察到双极太赫兹场诱导的瞬态双折射信号。我们提出一个与介电常数相关的氢键谐振子模型，并将其与洛伦兹动力学方程相结合，以研究液态水的分子间结构和动力学。我们主要将双极信号分解为由氢键拉伸振动引起的正信号和由氢键弯曲振动引起的负信号，这表明水的极化率扰动在弯曲和拉伸条件下呈现出相互竞争的贡献。建立了一个与水的分子间模式相关的克尔系数方程。超宽带太赫兹泵浦脉冲揭示的水的超快分子间氢键动力学可以为对应相关模式的液态水瞬态结构提供进一步的见解。
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