Electron-energy-loss channels and plasmon confinement in supported silver particles

The present paper is aimed at distinguishing the surface and particle contributions to the electron-energy-loss reflection spectrum from supported metallic particles in the thin layer limit k_d*1, where k_ is the electron parallel momentum transfer and d* the dielectric effective layer thickness. In the same way as with light excitation, the key constituents of the response of supported clusters are the cluster polarizabilities which create the long-range dipole oscillating field resulting in energy loss upon electron scattering. Contributions to the plasmon peak from either the substrate (Ag/Si, via the model) or a mixture of parallel and perpendicular excitations linked to the cluster shape (Ag/MgO, model and experiments) are identified. In the case Ag/MgO, a prototype for the study of plasmon excitation because of the negligible substrate contribution, a plasmon dispersion is observed even within a k_ range in which plasmon confinement is expected. This is interpreted as the signature of the island polarizabilities oscillatorlike behavior.