Multipole plasmon modes at a metal surface

The existence of multipole surface-plasmon modes at simple-metal surfaces is demonstrated both experimentally and theoretically. Inelastic reflection electron-scattering experiments on smooth films of K and Na show loss peaks originating from the ordinary surface plasmon as well as from a higher-energy multipole mode. Microscopic density-functional calculations of the electron-loss function reproduce the energy and momentum dispersion of both of these surface modes. For Al, the multipole mode is shown to be too weak to be observable in electron-loss spectroscopy.