Crossover between Monopole and Multipole Plasmon of Cs Monolayers on Si(111) Individually Resolved in Energy and Momentum

The evolution of the plasmon spectrum of the Si(111) (7x7)-Cs surface has been studied by energy loss spectroscopy individually resolved in energy and momentum during the transition from substrate to Cs overlayer metallization. The multipole plasmon is identified by an extremely narrow angular distribution of the inelastic electron scattering, unaccounted for by standard dipole scattering theory. A crossover between multipole and monopole surface plasmon is observed at finite surface wave vectors qparallel, depending on Cs coverage, and reveals a high sensitivity of the short-wavelength multipole components on surface morphology