LONGITUDINAL ELECTRIC-RESPONSE AND LOSS-FUNCTION OF METALLIC MICROSPHERES AND VOIDS

The response of a spherical metallic particle and a void in a metal on an arbitrary external electrical (gradient) field is given analytical ly within two models: a macroscopic dielectric description in terms of dielectric functions and a linearized hydrodynamic theory which takes into account boundary effects as well as spatial dispersion. From the response functions, the eigenfrequencies of the collective modes and the electron-energy-loss function are obtained. For small radii we fou nd collective modes extending beyond the classical l = infinity limit of surface-plasmons up to the volume-plasmon. Concerning electron-ener gy-losses, our results demonstrate that the ''blue-shift'' of the coll ective modes which were observed in potassium clusters or calculated f or voids in metals are almost of classical origin and stem from spatia l dispersion.