Transition fromNormaltoFastSound in Liquid Water

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