Experimental Determination of the Structural Relaxation in Liquid Water

The transition from normal to fast regimes of the longitudinal sound velocity has been studied by inelastic x-ray scattering in liquid water as a function of momentum transfer ( $1–12{\mathrm{nm}}^{-1\mathrm{}}$) and temperature (260–570 K), using pressure (0–2 kbar) to keep the density at $\rho \approx 1\mathrm{g}/{\mathrm{cm}}^3$. As in many glass-forming liquids also in water this transition is induced by the structural ( $\alpha$) relaxation. The relaxation time $\tau$, however, when close to the melting point, is 2 orders of magnitude shorter than in glass formers. The activation energy, deduced from the Arrhenius behavior of $\tau$, suggests that such relaxation is associated to the rearrangement of the local structure induced by the hydrogen bond.