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，例如分子极化率，尚未阐明。通常认为氢键网络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颖的工具，预计将有助于探索液态水的低频分子动力学。在这里，我们使用强超宽带太赫兹脉冲（峰值电场强度为14.9 MV/cm，中心频率为3.9太赫兹，带宽为1-10太赫兹）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引起的瞬态双折射信号。我们提出了与介电磁化率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。我们主要将双极性信号分解为氢键伸缩振动引起的正信号和氢键弯曲振动引起的负信号，表明水的极化率扰动在弯曲和拉伸条件下呈现出竞争贡献。建立了与水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进一步了解与相关模式相对应的液态水的瞬态结构。