Angle-resolved reflection inelastic electron scattering has been used to measure the dispersion of the electron-gas collective modes at the surface of Li(110) and Mg(0001). The relatively strong crystal potential in Li shifts the surface plasmon energy from the value predicted by free electron theory, but the dispersion for small q‖ is still negative. For Mg the band structure has little effect and the energy and dispersion of the surface plasmon and multipole mode are normal.
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Sprunger et al. (1992) studied this question.