Probing collective electronic excitations in as-deposited and modified Ag thin films grown on Cu(111)

The nature and the dispersion of the electronic collective excitations in ultrathin Ag layers deposited onto the Cu(111) surface were investigated by angle-resolved high-resolution electron-energy-loss spectroscopy. For two Ag layers we found a nearly flat behavior of the surface-plasmon energy (absence of dispersion) as a function of the parallel momentum transfer. For higher coverages the surface plasmon is confined in Ag grains. The confinement of the surface plasmon was removed upon annealing. On the contrary, on the sputtered Ag film the quadratic term dominates.