Surface plasmon dispersion on sputtered and nanostructured Ag(001)

The surface plasmon dispersion of ion bombarded Ag(001) is investigated by angle resolved high resolution electron energy loss spectroscopy. We find it to be parabolic and dominated by the quadratic term, contrary to the case of flat Ag(001) where the linear term is dominant. The case is reminiscent of surface plasmon dispersion on the oxygen covered, missing row reconstructed Ag(001) surface, for which case the change of the dispersion from nearly linear to quadratic was attributed to the removal of a surface plasmon decay channel associated to the surface interband transition at (X) over bar. We show moreover, that for the sputtered surface, slope and curvature of the dispersion depend on surface morphology at the nanoscale. When sputtering is performed at low crystal temperature and no ordered superstructure forms, the quadratic term coincides with the isotropic value dictated by bulk properties, reported for flat Ag(110) and Ag(111) and for reconstructed Ag(001). When on the contrary sputtering is performed at room temperature and the checkerboard superstructure develops, the quadratic term becomes twice as large.