Anharmonic Coupling between Intermolecular Motions of Water Revealed by Terahertz Kerr Effect

TL;DR

This study uses intense terahertz pulses to directly excite and measure low-frequency intermolecular motions in water, revealing anharmonic coupling between rotational and translational degrees of freedom, which sheds light on water's local structure.

Contribution

It demonstrates a novel experimental approach to directly probe and quantify anharmonic coupling of intermolecular motions in water using terahertz Kerr effect.

Findings

Resonant excitation of water's reorientational motions observed.

Transient optical birefringence linked to specific molecular configurations.

Estimated strength of anharmonic coupling between rotational and translational motions.

Abstract

Formation of local molecular structures in liquid water is believed to have marked effect on the bulk properties of water, however, resolving such structural motives in an experiment is challenging. This challenge might be handled if the relevant low-frequency structural motion of the liquid is directly driven with an intense electromagnetic pulse. Here, we resonantly excite diffusive reorientational motions in water with intense terahertz pulses and measure the resulting transient optical birefringence. The observed response is shown to arise from a particular configuration, namely the restricted trans-lational motion of water molecules whose motions are predominantly orthogonal to the dipole moment of the excited neighboring water molecules. Accordingly, we estimate the strength of the anharmonic coupling between the rotational and the restricted translational degrees of freedom of water.