Plasmon dispersion in metallic lithium-ammonia solutions

Inelastic X-ray scattering spectra of lithium (Li)-ammonia solutions with concentrations of 9, 13, and 17 mol.% metal have been measured with 0.05- and 0.38-eV resolution for momentum transfer between 0.18 and 0.56 ?-1. The peak growing with Liconcentration was found around 2 eV, and was assigned to the plasmon of delocalized Li-2s electrons. The plasmon shows a positive dispersion, indicating that exchange-correlation effects in actual nearly free electrons are considerably smaller than expected.