We have studied, for the first time, the energy and the linewidth dispersion of a plasmon in a dense two-dimensional electron system in a metallic surface-state band on a silicon surface. As expected from the considerably high effective density and long Fermi wavelength of the system, the plasmon energy dispersion exhibited an excellent agreement with the nearly free-electron theory. However, in a small wave number region below the Landau edge, we have observed an anomalous linewidth dispersion which nearly free-electron theories do not predict.

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Collective surface electronic excitations on Ag͑111͒ and Ag͑110͒ have been studied by energy-lossspectroscopy LEED. The data allow separation of the contribution to the loss intensity due to monopole and to multipole surface plasmons, thus demonstrating the existence of the latter mode also on noble-metal surfaces. Multipole plasmons are therefore a general property of the optical response of metal surfaces whenever the charge-density profile at the interface is not too abrupt. ͓S0163-1829͑96͒51544-4͔ EVIDENCE FOR THE PRESENCE OF THE MULTIPOLE . . .