Frequency-dependent extinction cross section of a spherical ion or metallic cluster immersed in a plasma

The frequency-dependent polarizability of an ion or metallic cluster immersed in a plasma is studied for the model of a uniform sphere with sharp boundary. The electron gas inside and outside the ion or cluster is treated in the framework of Bloch's hydrodynamic model. Due to the coupling to the bulk plasma, the sharp absorption lines of the isolated sphere are modified into an infinite sequence of broadened resonance peaks. The first peak corresponds to the Fröhlich mode of a metallic sphere. It accounts for the major part of the integrated extinction cross section. The shape of the giant resonance peak deviates strongly from a Lorentzian.