Title: Detection of collective optic excitations in molten NaI
Author(s):

	S. Hosokawa (Department of Physics, Kumamoto University, Kumamoto 860-8555, Japan; Department of Chemistry, Physical Chemistry, Philipps University of Marburg, 35032 Marburg, Germany) ,
	M. Inui (Graduate School of Integrated Arts and Sciences, Hiroshima University, Higashi-Hiroshima 739-8521, Japan) ,
	T. Bryk (Institute for Condensed Matter Physics of the National Academy of Sciences of Ukraine, 79011 Lviv, Ukraine) ,
	I. Mryglod (Institute for Condensed Matter Physics of the National Academy of Sciences of Ukraine, 79011 Lviv, Ukraine) ,
	W.-C. Pilgrim (Department of Chemistry, Physical Chemistry, Philipps University of Marburg, 35032 Marburg, Germany) ,
	Y. Kajihara (Graduate School of Integrated Arts and Sciences, Hiroshima University, Higashi-Hiroshima 739-8521, Japan) ,
	K. Matsuda (Department of Physics, Kyoto University, Kyoto 606-8502, Japan) ,
	Y. Ohmasa (Neutron Science Laboratory, Institute for Solid State Physics, The University of Tokyo, Kashiwa 277-8581, Japan) ,
	S. Tsutsui (Japan Synchrotron Radiation Research Institute (JASRI), Sayo 679-5198, Japan)

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.

Key words: inelastic x-ray scattering, molten salt, optic excitations, ab initio simulations
PACS: 61.10.Eq, 61.20.Lc, 78.30.Cp

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