Propagating Librations in Ice XI: Model Analysis and Coherent Inelastic Neutron Scattering Experiment

We report on propagating librational ( rotational) modes intrinsic to ice XI. Contrary to the theoretical picture presumed so far, namely, that all librational modes are localized in each water molecule, our calculations identify large dispersions for some of the modes, which basically originate from intermolecular dipole-dipole interactions. On the basis of this finding, the known incoherent inelastic neutron scattering spectrum and infrared absorption spectrum are newly analyzed to give the interpretation that the spectra are mixtures of contributions of localized and propagating modes. Furthermore, we perform an experiment on coherent inelastic neutron scattering using the MAPS spectrometer in order to search omega-Q space more directly than in a previous work and find that the observation also strongly suggests the existence of large dispersions.