Detection of collective optic excitations in molten NaI

High-resolution inelastic x-ray scattering measurements were carried out on molten NaI near the melting point at 680 • 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 frequency and 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.