Evidence for the presence of the multipole plasmon mode on Ag surfaces

Collective surface electronic excitations on Ag(111) and Ag(110) have been studied by energy-loss-spectroscopy 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. {} 1996 The American Physical Society.