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 ${\mathrm{Ag}}_{\mathit{N}}^{+}$ resonances. The discussion of the polarization-free layer is also applicable to metallic surfaces.

• Received 28 September 1994

DOI:https://doi.org/10.1103/PhysRevB.51.1844