Transition from Normal to Fast Sound in Liquid Water.

Abstract

Inelastic x-ray scattering data from water at 5 ‐ C show a variation of the velocity of sound from 2000 to 3200 mys in the momentum transfer range 1 4n m 2 1 . The transition occurs when, at 4 meV , the energy of the sound excitations equals that of a second weakly dispersing mode. This mode is reminiscent of a phonon branch in ice Ih crystals, which is shown here to be of optical transverse character. The present work accounts for most of the highly debated difference between hydrodynamic s 1500 mysd and high-frequency s 3200 mysd velocities of sound in water. [S0031-9007(96)00500-5] PACS numbers: 61.10.Eq, 63.50. + x, 78.70.Ck Water, in its liquid and solid forms, continues to fascinate for its complicated and quite unique properties. An interesting and still highly debated issue is the phenomenon of fast sound in the liquid. The velocity of acousticlike excitations, which is 1500 mys in the hydrodynamic limit up to momentum transfers Q 2 3 10 22 nm 21 , reaches the value of 3200 mys (fast sound) at Q above 4n m 2 1 . This large velocity increase was first predicted in a molecular dynamics (MD) calculation [1], which generated a great number of MD simulations aiming to understand the microscopic origin of this process [2,3]. The fast sound was first shown to exist using coherent inelastic neutron scattering (INS) [4]. Recently, it was confirmed in a wider Q region using inelastic x-ray scattering (IXS) [5], and, still using IXS, it was found that the velocity of sound is equivalent in liquid and solid (ice Ih) water in the whole investigated 4 14 nm 21 Q region [6]. These results confirm the existence of fast sound, show that at high frequencies the dynamical response of the liquid becomes solidlike, and imply that in the liquid the processes responsible for the large variation in the speed of sound must take place in the mesoscopic (Q-v) region with Q in the 0.05 4n m 2 1 range. This region, too high for light scattering, is also experimentally difficult to reach for both INS and IXS techniques.