Collective resonances in silver clusters: Role of d electrons and the polarization-free surface layer

Silver microclusters display giant dipole collective resonances with an unusual size dependence of the resonance frequency. This behavior is due to the fact that the clusters contain polarizable atomic-core d electrons in addition to a delocalized electron cloud. We describe a self-consistent field analysis of the response properties of such a system. It is shown that the efficiency of core screening is controlled by the overlap between d-electron and delocalized-electron wave functions. This provides microscopic justification for the two-region model in which core dielectric screening is missing in a layer near the particle surface. The dipole-excitation energies of such particles are calculated, and it is demonstrated that the model can account for the unusual blueshift of Ag_N⁺ resonances. The discussion of the polarization-free layer is also applicable to metallic surfaces.