COLLECTIVE ELECTRONIC EXCITATIONS IN METAL-COATED C-60

A two-shell jellium-on-jellium model has been used to study the collec tive electronic excitations of C60 coated by an increasing number N of Na atoms. We predict a transition from the pi-electron polarization o f the fullerene to the metallic sodium polarization as a function of i ncreasing N. For low coverages, the Na layer produces only a broadenin g and fragmentation of the pi plasmon of C60. However, if the coverage is large enough, a Na surface plasmon appears. This occurs only after the electron density of the cluster clearly shows two distinct region s, the outer one having an ''average'' density similar to that in pure Na metal or Na clusters. The induced density at the collective-mode f requency shows structure corresponding to two interfaces, C60-Na and N a vacuum, the first peak becoming less pronounced as the coverage incr eases due to a transfer of oscillator strength to the metallic counter part. The static polarizability per electron also shows this trend and tends slowly to the Na value from below after an initial sharp rise i n the region N is similar to 13-21.