Interplay between single-particle and plasmonic excitations in the electronic response of thin Ag films

High-resolution electron energy loss spectroscopy is used to study the electronic properties of thin Ag layers on Ni(111). In addition to the ordinary surface plasmon at 3.8 eV, we observe a broad feature at 7–8 eV, whose nature is investigated as a function of scattering geometry and primary electron beam energy. Loss measurements unambiguously indicate that this mode has spectral components from both free-electron Ag plasmonic excitations (free-electron surface plasmons and multipole plasmons) and single-particle transitions. (paper)