Experimental Determination of the Structural Relaxation in Liquid ...

Experimental Determination of the Structural Relaxation in Liquid WaterThe investigation of large wavevector excitations in liquid water has shown the existence of apositive dispersion in the velocity of sound. This dispersion has been inferred in the pioneeringcomputational [1] and experimental [2] studies of the dynamic structure factor, S(Q,E), and hasbeen recently assessed by Inelastic X-ray Scattering (IXS) [3]. Using IXS, the transition of thelongitudinal sound velocity from the adiabatic value, c o~ 1500 m/s, to a value more than twicelarger, c ~ 3200 m/s, was studied at T = 5°C. This sound velocity dispersion is qualitativelysimilar to that observed in glass-forming liquids. There, the transition between the two dynamicregimes is determined by the coupling of the propagating density fluctuations with the dynamicsof the structural rearrangements of the particles in the liquid. The complex dynamics of such arearrangement can be described by a relaxation process with a characteristic time, . Thetransition takes place when the condition ~ 1 is fulfilled. In glass-forming liquids has a verysteep temperature dependence; its typical values are in the nanosecond range close to themelting point and dramatically increases near the calorimetric glass transition temperature T g.This relaxation process ( -process) has a cooperative nature and the density fluctuations areinfluenced differently in the two opposite frequency limits: the system has a solid like elasticbehaviour for >> 1, and a viscous one for

The analogy with the glass-formers phenomenology, implies that the fast relaxation processstudied in this work can be identified with an -process. The derived values of are consistentwith previous estimations: they roughly follow an Arrhenius behaviour with an activationenergy comparable to the hydrogen bond energy. This suggests that, in water, the -process isassociated with the rearrangement (making and breaking) of molecular structures kept togetherby the hydrogen bond.The IXS data were analysed both with an empirical model and with a visco-elastic model, andin each case it was possible to determine the detailed T and Q dependence of the relaxationtime, .References[1] Rahman and Stillinger, Phys. Rev., A 10, 368 (1974).[2] J. Teixeira, M.C. Bellisent-Funel, S.H. Chen, B. Doznez, Phys. Rev. Lett., 54, 2681 (1985).[3] F. Sette et al., Phys. Rev. Lett., 75, 850 (1995); Phys. Rev. Lett., L77, 83 (1996).Principal Publications and AuthorsA. Cunsolo (a), G. Ruocco (b), F. Sette (a), C. Masciovecchio (a), A. Mermet (a), G. Monaco(b), M. Sampoli (a), R. Verbeni (a), Phys. Rev. Lett., 82, 775 (1999).G. Monaco, A. Cunsolo, G. Ruocco, F. Sette, Phys. Rev., E 60, 5505 (1999).(a) ESRF(b) Università di L'Aquila and Istituto Nazionale di Fisica della Materia (Italy)(c) Università di Firenze and Istituto Nazionale di Fisica della Materia (Italy)http://www.esrf.eu/info/science/highlights/1999/es-ld/relaxn.html03/01/2007