Absence of anomalous dispersion features in the inelastic neutron scattering spectra of water at both sides of the melting transition

Abstract

Inelastic neutron scattering spectra of water a few degrees below and above melting have been measured. The excitations seen in the hot solid are analyzed in terms of the wave vector dependence of the average frequencies characterizing the envelopes of the mostly acoustic and mostly optical manifolds, which are confined to energy transfers below 40 meV. The spectra corresponding to the liquid are analyzed by means of a model which includes stochastic, zero-frequency motions (long range translational and rotational diffusion) using the values for the relevant transport coefficients measured under high-resolution conditions. The excitation frequencies in the solid and liquid phases follow a behavior common to other molecular liquids examined so far, approaching hydrodynamic sound at low momentum transfers.