Viscoelastic model for the transition from normal to fast sound in water

The cross-over from hydrodynamic to fast sound in water at 5 C has recently been mapped experimentally by means of inelastic X-ray scattering. We show that a viscoelastic model gives a good account of both the longitudinal-current autocorrelation Function and the dynamic structure factor in the region of the cross-over. Evidence is also given for a weakly dispersing excitation observed in these experiments being a liquid state remnant of an oxygen-related optic mode in ice.