Angular dependence of dipole scattering cross section: Surface-plasmon losses on Ag(100)

We report high-resolution angular-resolved electron-energy-loss measurements of the cross section of surface-plasmon excitation. The intensity shows a pronounced minimum as a function of exchanged momentum near ${\mathit{q}}_{\mathrm{\parallel }}$=0 whose position depends on impact energy and angle of incidence of the electron beam and is determined by the value of the phase shift occurring in the scattering process. The effect has been taken into account in the determination of energy and dispersion of the surface plasmon of Ag(100). A linear dispersion is found in the range of ${\mathit{q}}_{\mathrm{\parallel }}$ between 0 and 0.15 A${\mathrm{̊}}^{\mathrm{-}1}$ in contrast to the quadratic behavior found for Ag(111) and Ag(110).