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High frequency longitudinal and transverse dynamics in water

E. Pontecorvo·M. Krisch·A. Cunsolo·G. Monaco·A. Mermet·R. Verbeni·F. Sette·G. Ruocco
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Abstract

High-resolution, inelastic x-ray scattering measurements of the dynamic structure factor S(Q,ω) of liquid water have been performed for wave vectors Q between 4 and 30 nm^-1 in distinctly different thermodynamic conditions (T= 263 - 420 K ; at, or close to, ambient pressure and at P = 2 kbar). In agreement with previous inelastic x-ray and neutron studies, the presence of two inelastic contributions (one dispersing with Q and the other almost non-dispersive) is confirmed. The study of their temperature- and Q-dependence provides strong support for a dynamics of liquid water controlled by the structural relaxation process. A viscoelastic analysis of the Q-dispersing mode, associated with the longitudinal dynamics, reveals that the sound velocity undergoes the complete transition from the adiabatic sound velocity (c_0) (viscous limit) to the infinite frequency sound velocity (c_\infinity) (elastic limit). On decreasing Q, as the transition regime is approached from the elastic side, we observe a decrease of the intensity of the second, weakly dispersing feature, which completely disappears when the viscous regime is reached. These findings unambiguously identify the second excitation to be a signature of the transverse dynamics with a longitudinal symmetry component, which becomes visible in the S(Q,ω) as soon as the purely viscous regime is left.

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E. Pontecorvo, M. Krisch, A. Cunsolo, G. Monaco, A. Mermet, R. Verbeni, F. Sette, G. Ruocco. 2005-01-10. High frequency longitudinal and transverse dynamics in water. https://doi.org/10.1103/physreve.71.011501

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