Energy-loss experiments with 50-keV electrons on thin polycrystalline films of Al and In yielded data for the surface plasmon dispersion up to $k_{\parallel }=0.47\mathrm{} \mathrm{and} 0.42$ ${\mathrm{\AA }}^{-1\mathrm{}}$, respectively. The wave-vector dependence of the surface plasmon frequency of Al shows a significant increase for $k_{\parallel }>0.3\mathrm{}$ ${\mathrm{\AA }}^{-1\mathrm{}}$, while in the case of In there is only a weak increase for $k_{\parallel }>0.2\mathrm{}$ ${\mathrm{\AA }}^{-1\mathrm{}}$.

• Received 21 July 1976

DOI:https://doi.org/10.1103/PhysRevLett.37.1355