Abstract
Inelastic x-ray scattering experiments have been pushed to the meV energy resolution range at the European Synchrotron Radiation Facility in Grenoble. This opens the possibility to measure the high frequency dynamics in liquids and disordered systems. This technique has been used to study the collective dynamics in liquid water in the high frequency limit with the aim to investigate the long-standing issue of the fast sound. Propagating collective excitations have been found at momentum transfers between 1 and 14 nm−1, and the transition from a velocity of sound close to the hydrodynamic value to the fast sound value of ≈3200 m/s is observed between 1 and 4 nm−1. At Q larger than 4 nm−1 the excitations propagate with the fast sound.
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