Brillouin neutron scattering in heavy water: Evidence for two-mode collective dynamics

Abstract

A high resolution (1.5 meV) inelastic neutron scattering experiment was carried out, aiming at an accurate investigation of the high frequency and low momentum dynamic response in heavy water. The experimental data confirm the existence of a dispersionless mode, besides the ordinary longitudinal collective dynamics. A simplified model, based on the interaction of two vibrational branches, is proposed to interpret the observed features of the dynamic spectra. The validity extent of this scheme is proved by applying it to room temperature neutron and x-ray data, to temperature and pressure dependent x-ray data, and to room temperature neutron data of vibrational density of states. The overall successfull results provided by this model, in conjunction with the combined analysis of the x-ray and neutron data on collective dynamics, enable a deeper insight into the complex mechanisms of the water dynamics and provide a simple phenomenological explanation for the transition from ordinary to fast sound.

High-frequency longitudinal and transverse dynamics in water

“…[9,10] The data reported in Fig. 12, together with the outcome listed in items (1) and (2) before, are the main results of this paper.…”

“…This second excitation has a (almost) Q-independent energy, and becomes visible only at Q larger than ≈4 nm −1 . This second feature, observed in the S(Q, ω) of ambient condition liquid water by INS [6,8,9,10] and IXS [12,13,16], has been attributed to transverse-like dynamics on the basis of a MD study [18] where the comparison between longitudinal and transverse current spectra was performed. The existence of the second feature and its transverse origin can be explained within the same viscoelastic framework which has been successfully used to describe the longitudinal dynamics.…”

“…A different interpretation for the origin of the two excitations appearing in the S(Q, ω) of water has recently been proposed by Petrillo et al [9] and by Sacchetti et al [10], who analyzed the INS and IXS data using a solid-like framework: the mode-mode interaction and the symmetry avoided crossing. In this context, the second dispersion-less feature is assigned to a local inter-molecular vibration, resembling one of the optic modes in ice [19].…”

“…The key quantity in most of these studies is the so-called dynamical structure factor S(Q, ω) ( Q and ω = E denote the momentum and energy transfer, respectively), which is the time and space Fourier transform of the atomic density-density pair correlation function [5]. Starting from the pioneering work by Bosi et al [6] and by Texeira et al [7], several neutron experiments [8,9,10] and x-ray studies [11,12,13,14,15,16,17] have been devoted to the study of the THz-frequency dynamics in liquid water.…”

Pressure effects on collective density fluctuations in water and protein solutions

“…A 10% wt/wt solution of lysozyme powder in D 2 O buffer has been prepared. Protein was purchased from SIGMA Chemical Co in fully hydrated form and was employed without further purification It has been dissolved in pure D 2 O solvent to exchange labile hydrogen atoms, followed by lyophilization.…”