Netection of collective optic excitations in olten NaI.

High-resolution inelastic x-ray scattering measurements were carried outon molten NaInearthe melting point at 680c'C at SPring-8. Small and damped indications of longitudinal optic excitation modes were observed on the tails of the longitudinal acoustic modes at small momentum transfers, Q ~ 5 nm-1. The measured spectra are in good agreement, in both frequencyand linewidth, with ab initio molecular dynamics (MD) simulations but not classical MD simulations. The observation of these modes at small Q and a good agreement with the simulation permits clear identification of these as collective optic modes with well defined phasing between different ionic motions. [ABSTRACT FROM AUTHOR]