Interaction of Acoustic and Quasi-Elastic Modes in Liquid Water on Nanometer Length Scales

TL;DR

This study reveals how acoustic and quasi-elastic modes interact in liquid water at nanometer scales, clarifying spectral features and supporting hydrodynamic models through high-resolution inelastic x-ray scattering.

Contribution

It demonstrates the significance of acoustic-quasi-elastic interaction in liquid water spectra, providing a hydrodynamic explanation without extra modes, and clarifies previous contradictory interpretations.

Findings

Interaction explains spectral overlap in IXS data.

Sound velocity and mode intensities plateau at high Q.

Results align with viscoelastic Landau-Placzek relation.

Abstract

We investigate the presence of an acoustic-quasi-elastic interaction contribution in the IXS spectra of liquid water at 301K using inelastic x-ray scattering with sub-meV energy resolution at momentum transfers 0.77<=Q<=4.20 /nm. The contribution appears due to the overlap the acoustic mode with the tail the quasi-elastic mode and is fully consistent with hydrodynamic theory. Incorporating this interaction allows us to describe the dynamic structure factor, S(Q,w), without introducing an extra mode, and may help explain earlier contradictory interpretations. The sound velocity, and relative intensity of the quasi-elastic and acoustic mode, plateau for Q>2/nm at values consistent with a viscoelastic generalization of the Landau-Placzek relation.