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 nm(-1)) and temperature (260-570 K), using pressure (0-2 kbar) to keep the density at rho approximate to 1 g/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.