Seminar of the Institute for Condensed Matter Physics

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. We will then concentrate on a study of the dynamic structure factor, S(Q,w), of water in the Q=1 - 10 inverse nm region. We show i) the existence in the whole investigated Q-region of longitudinal collective dynamics propagating, at Q > 4 inverse nm, 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 inverse nm region, of a second, weakly dispersing excitation with about 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.