Surface-plasmon spectrum of Ag(001) measured by high-resolution angle-resolved electron-energy-loss spectroscopy

An angle-resolved high-resolution electron-energy-loss-spectroscopy study is reported of the surface-plasmon spectrum on Ag(001) along different azimuths. A single loss energy is observed which disperses linearly with exchanged momentum ${\mathit{q}}_{\mathrm{?}}$ from 3.69\ifmmode\pm\else\textpm\fi{}0.01 eV with a slope of 1.5\ifmmode\pm\else\textpm\fi{}0.1 eV \AA{}. The data were recorded for different impact energies and scattering angles. Within experimental accuracy, no dependence of plasmon frequency and dispersion on the crystal azimuth was observed. The peak width measures 95 meV at small wave vectors and broadens linearly with ${\mathit{q}}_{\mathrm{?}}$. Beyond 0.10 A${\mathrm{\r{}}}^{\mathrm{\ensuremath{-}}1}$ the width increases drastically because of the opening of efficient damping channels connected with interband transitions involving the d bands. Plasmon energy on Ag(100) agrees with the value reported for Ag(111) by Suto et al. and by Contini and Layet, while the form of the dispersion curves differs remarkably, being linear instead of quadratic as in the case of (111) and (110) surfaces.