title: High-frequency longitudinal and transverse dynamics in water.
abstract: High-resolution, inelastic x-ray scattering measurements of the dynamic structure factor S (Q,omega) 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 ( 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 S (Q,omega) as soon as the purely viscous regime is left.
authors.affiliation: Dipartimento di Fisica and INFM, Universitá di Roma La Sapienza, I-00185, Roma, Italy.
authors.forename: E
authors.initials: E
authors.lastname: Pontecorvo
authors.forename: M
authors.initials: M
authors.lastname: Krisch
authors.forename: A
authors.initials: A
authors.lastname: Cunsolo
authors.forename: G
authors.initials: G
authors.lastname: Monaco
authors.forename: A
authors.initials: A
authors.lastname: Mermet
authors.forename: R
authors.initials: R
authors.lastname: Verbeni
authors.forename: F
authors.initials: F
authors.lastname: Sette
authors.forename: G
authors.initials: G
authors.lastname: Ruocco