Collective contribution to the frequency-dependent polarizability of an ion or metallic cluster immersed in a plasma

The frequency-dependent electric polarizability of an ion or metallic cluster immersed in a plasma is studied in the framework of Blochʹs classical hydrodynamic model. Both bound and free electrons are treated as a classical fluid moving in the potential created by the fixed ion and a uniform neutralizing background. The electrons respond collectively to a uniform oscillating applied electric field. The absorption edge at the bulk plasma frequency is studied in detail. It is shown that the photoabsorption cross section and the photoscattering cross section at the bulk plasma frequency are related in a simple manner for general electron density profile