title: Is there any fast sound in water?
abstract: We measured the dynamic structure factor S(Q,omega) of liquid and undercooled water down to 253 K in the Q approximately 0.02-0.1 nm;{-1} momentum transfer region. We observe the neat departure of the apparent speed of sound from the adiabatic regime as a function of decreasing temperature. Our evaluation of the infinite-frequency limit of sound velocity, c_{infinity}, matches with the results obtained in the high momentum transfer limit by inelastic neutron and x-ray scattering. These results strongly support the viscoelastic interpretation of the dynamics of water. Hence, we propose to call c_{infinity} the high-frequency speed of sound and to abandon the term fast sound, which recalls a propagation mechanism through lighter atoms, like in gas mixtures.
authors.affiliation: Sincrotrone Trieste, s.s. 14, km 163.5, 34012 Basovizza, Trieste, Italy.
authors.forename: S C
authors.initials: SC
authors.lastname: Santucci
authors.forename: D
authors.initials: D
authors.lastname: Fioretto
authors.forename: L
authors.initials: L
authors.lastname: Comez
authors.forename: A
authors.initials: A
authors.lastname: Gessini
authors.forename: C
authors.initials: C
authors.lastname: Masciovecchio