Strong Anisotropy in Liquid Water upon Librational Excitation using Terahertz Laser Fields

TL;DR

This study demonstrates that terahertz laser fields can induce strong transient anisotropy in liquid water by resonantly exciting librational modes, revealing a significantly enhanced nonlinear response compared to optical frequencies.

Contribution

The paper introduces a novel experimental approach to excite and measure water's librational modes with terahertz pulses, showing a three orders of magnitude increase in nonlinear response.

Findings

Strong transient anisotropy observed in liquid water at 12.3 THz.

Third order nonlinear response exceeds optical range values by three orders of magnitude.

Salt addition reduces the perturbability of water molecules by THz pulses.

Abstract

Tracking the excitation of water molecules in the homogeneous liquid is challenging due to the ultrafast dissipation of rotational excitation energy through the hydrogen-bonded network. Here we demonstrate strong transient anisotropy of liquid water through librational excitation using single-color pump-probe experiments at 12.3 THz. We deduce a third order response of chi^3 exceeding previously reported values in the optical range by three orders of magnitude. Using a theory that replaces the nonlinear response with a material response property amenable to molecular dynamics simulation, we show that the rotationally damped motion of water molecules in the librational band is resonantly driven at this frequency, which could explain the enhancement of the anisotropy in the liquid by the external Terahertz field. By addition of salt (MgSO4), the hydration water is instead dominated by the local electric field of the ions, resulting in reduction of water molecules that can be dynamically perturbed by THz pulses.