Influence of the core polarization on the dielectric properties of polyvalent metals

We have measured the dispersion of plasmon excitations in polycrystalline Sn, Pb, Cd and Zn using electron energy-loss spectroscopy (EELS) in transmission. These metals show a remarkable deviation of their plasmon behaviour from standard theoretical expectations which can be attributed to the presence of shallow core levels in the energetic neighborhood of the plasmon energy. The position of the plasmon resonance in the long wavelength limit is in line with predictions from model assumptions taking into account the core level polarization effects. We show that not only the plasmon energy in the optical limit but also the plasmon dispersion is altered by the interaction of the charge carrier plasmon and the core level excitations. Received: 15 June 1998 / Revised and Accepted: 9 July 1998