Collective electronic excitations in clusters in the vicinity of metal surfaces

We study the collective excitations of metal clusters approaching a metal surface. Using a simple model for the frequency-dependent dielectric constant ε(ω) and the multiple scattering method, we numerically investigate the shift in the plasmon resonance due to the coupling of the collective modes of the sphere with those of its mirror image. Results of the model calculation are verified by means of ab initio theory. As a prototype system we study Na₉ ⁺ cluster on the Cu(100) surface. The representation of the solid surface by a cluster of several, typically 54 substrate atoms, is used in combination with a high level configuration interaction (CI) calculation. PACS 31.15.Dv; 36.40.Gk; 73.20.-f