The hydrogen bond network of water supports propagating optical phonon-like modes

TL;DR

This study reveals that liquid water supports propagating optical phonon-like modes within its hydrogen bond network, showing similarities to ice and challenging previous notions of water's vibrational dynamics.

Contribution

The paper demonstrates, through molecular dynamics simulations, the existence of dispersive optical phonon-like modes in liquid water, highlighting long-range coherence in its vibrational dynamics.

Findings

Optical phonon-like modes are present in water's vibrational spectrum.

These modes propagate over distances up to two nanometers.

Long wavelength modes show longitudinal-transverse splitting, indicating coherent dipole interactions.

Abstract

The local structure of liquid water as a function of temperature is a source of intense research. This structure is intimately linked to the dynamics of water molecules, which can be measured using Raman and infrared spectroscopies. The assignment of spectral peaks depends on whether they are collective modes or single molecule motions. Vibrational modes in liquids are usually considered to be associated to the motions of single molecules or small clusters. Using molecular dynamics simulations we find dispersive optical phonon-like modes in the librational and OH stretching bands. We argue that on subpicosecond time scales these modes propagate through water's hydrogen bond network over distances of up to two nanometers. In the long wavelength limit these optical modes exhibit longitudinal-transverse splitting, indicating the presence of coherent long range dipole-dipole interactions, as in ice. Our results indicate the dynamics of liquid water have more similarities to ice than previously thought.