High-resolution neutron scattering measurement of the dynamic structure factor of heavy water

The low momentum collective dynamics of heavy water has been investigated b y means of neutron three-axis spectroscopy. Working at the resolution limit of this instrument, an energy resolution of 2.6 meV was achieved for a con stant analyzer energy of 120 meV. This good resolution allowed us to establ ish definitely that the collective dynamics of water is dominated by the pr esence and interaction of opticlike and acousticlike branches, coupling at wave-vector transfers of 0.3 to 0.35 Angstrom(-1). The transition from slow to fast sound has been attributed to the interaction between opticlike mod es and a solidlike fast acoustic mode.