Terahertz insights into liquid water physics from an experimental perspective

Terahertz (THz) spectroscopy provides unique access to low-frequency molecular motions, enabling detailed investigation of hydrogen-bond networks, collective solvent dynamics, and ultrafast solvation processes-key phenomena in liquid behavior that remain challenging to resolve with conventional techniques. Traditional far-infrared and THz time-domain spectroscopy have been well demonstrated in identifying the spectral fingerprint of solvents and ion pairs. Recent advances in nonlinear, time-resolved THz spectroscopy have enabled the direct tracking of solvated electron dynamics, interfacial energy transfer, and transient anisotropic responses in liquids. This article reviews the fundamental principles and cutting-edge advances in linear and nonlinear terahertz (THz) spectroscopy as applied to liquid-phase systems, covering the interactions of water with ions, biomolecules, electrons, plasmons, and photons. In particular, the experimental consideration and the data analysis are summarized to give a clue to extract the useful information. The remarkable capabilities of THz spectroscopy are poised to advance our understanding and control of liquid systems, offering broad interdisciplinary appeal across chemistry, biology, and physics.