Experimental determination of the structural relaxation in liquid water

The transition from normal to fast regimes of the longitudinal sound veloci ty has been studied by inelastic x-ray scattering in liquid water as a func tion 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 c lose to the melting point, is 2 orders of magnitude shorter than in glass f ormers. The activation energy, deduced from the Arrhenius behavior of tau, suggests that such relaxation is associated to the rearrangement of the loc al structure induced by the hydrogen bond.