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. This study uses an intense and ultrabroadband THz pulse to resonantly excite intermolecular modes of liquid water, observing bipolar THz field-induced transient birefringence signals. A hydrogen bond harmonic oscillator model is proposed and combined with the Lorentz dynamic equation to investigate the intermolecular structure and dynamics. The bipolar signals are decomposed into a positive signal (hydrogen bond stretching vibration) and a negative signal (hydrogen bond bending vibration), indicating competing contributions under bending and stretching conditions. A Kerr coefficient equation related to the intermolecular modes is established, providing insights into the ultrafast intermolecular hydrogen bond dynamics.
