HIGH FREQUENCY COLLECTIVE EXCITATIONS IN LIQUID AND SOLID WATER BY INELASTIC X-RAY SCATTERING. G. Ruocco¹, U. Bergmann², M. Krisch², C. Masciovecchio², V. Mazzacurati¹, F. Sette², G. Signorelli¹, R. Verbeni². ¹Universita' di L'Aquila and INFM, I-67100, L'Aquila, Italy; ²ESRF, B.P. 220 F-38043 Grenoble Cedex, France

Since the development of intense and collimated synchrotron sources, inelastic x-ray scattering with meV energy resolution has attracted an increasing interest for its potential applications in the study of the high frequency collective dynamics in condensed matter. This is traditionally the domain of neutron spectroscopies. Neutron techniques, however, due to kinematc limitations, are difficult to use on systems with a speed of sound larger than 1000 m/s and without translational invariance. These limitations are overcome using x-rays with 10-20 KeV energy and 1-3 meV energy resolution. The performance of the ring at the European Synchrotron Radiation Source in Grenoble, with its unprecedented low emittance, and our development of new and performing perfect crystal optics, allowed us to construct an undulator based beamline for inelastic x-ray scattering, and to obtain 1.4 meV energy resolution and high flux. We will present an outline of the instrument and of the technique. We will then concentrate on a study of the dynamic structure factor, S(Q,W), of water in the Q=1-10 nm-1 region. We show i) the existence in the whole investigated Q-region of longitudinal collective dynamics propagating, at Q > 4 nm-1, with a speed of sound of 3200 m/s, more than a factor of two larger than that in the hydrodinamic limit (fast sound phenomenon); and ii) the existence, in the Q=4-10 nm-1 region, of a second, weakly dispersing excitation with 4 meV peak energy. The fast sound phenomenom is discussed in view of existing theories, previous neutron data, and in connection to our study on solid water (ice Ih), were we point out that exists a mesoscopic wavelength region where the collective dynamics in the solid and in the liquid show striking similarities. The transition between the fast sound and the normal (hydrodynamical) behaviour of the density fluctuations takes place when the energy of the sound-like excitations equals that of the second mode, that it is show to be the reminescent of a transverse optical branch in ice Ih.