Thickness dependence of surface plasmon damping and dispersion in ultrathin Ag films

The thickness dependence of the surface plasmon damping and dispersion of atomically flat and ultrathin silver films deposited on the Si(111)-(7×7) surface was investigated by a combined high-resolution electron-energy-loss spectroscopy (HREELS) and scanning tunneling microscopy (STM) system. We found stronger plasmon energy dispersion with the momentum parallel to the surface (q_) in thicker films, and a significant dependence of the damping edge on the film thickness. Both of them can be associated with the presence of quantum well states (QWS) in the direction perpendicular to the film surface, which influences the interband transitions between the lower and upper $5sp$ bands of silver.