Detection of collective optic excitations in molten NaI

TL;DR

This study reports the detection of collective optic excitations in molten NaI using high-resolution inelastic x-ray scattering, confirming their presence at small momentum transfers and validating ab initio molecular dynamics simulations.

Contribution

First experimental observation of collective optic modes in molten NaI at small momentum transfers, supported by ab initio MD simulations.

Findings

Optic excitation modes observed at small Q in molten NaI.

Good agreement between experimental spectra and ab initio MD simulations.

Identification of collective optic modes with well-defined ionic phasing.

Abstract

High-resolution inelastic x-ray scattering measurements were carried out on molten NaI near the melting point at 680$^\circ$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, $Q\sim5$ nm$^{-1}$. The measured spectra are in good agreement, in both frequency and linewidth, with {\it 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.