High-resolution, inelastic x-ray scattering measurements of the dynamic structure factor S(Q,?) 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 nondispersive) 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 a complete transition from the adiabatic sound velocity (c0) (viscous limit) to the infinite-frequency sound velocity (c?) (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 S(Q,?) as soon as the purely viscous regime is left.
High frequency longitudinal and transverse dynamics in water.
G Ruocco
2005
Abstract
High-resolution, inelastic x-ray scattering measurements of the dynamic structure factor S(Q,?) 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 nondispersive) 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 a complete transition from the adiabatic sound velocity (c0) (viscous limit) to the infinite-frequency sound velocity (c?) (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 S(Q,?) as soon as the purely viscous regime is left.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
