Transition from normal to fast sound in liquid water

Inelastic x-ray scattering data from water at 5 degrees C show a variation of the velocity of sound from 2000 to 3200 m/s in the momentum transfer range 1-4 nm(-1). The transition occurs when, at approximate to 4 meV, the energy of the sound excitations equals that of a second weakly dispersing mode. This mode is reminiscent of a phonon branch in ice Ih crystals, which is shown here to be of optical transverse character. The present work accounts for most of the highly debated difference between hydrodynamic (approximate to 1500 m/s) and high-frequency (approximate to 3200 m/s) velocities of sound in water.