6S-ELECTRONS IN STABILIZED AU55-CLUSTERS

It is concluded from experiments, both on the optical excitation of plasmon-polaritons and on the complex low frequency dielectric function, that the 6s-electrons of ligand-stabilized Au55-clusters are delocalized, but the transition to the metallic behaviour of Au is not yet terminated. The observed vanishing of the Mie resonance in Au55-clusters is explained by the strong size-dependence of the (5d-6sp)-interband transitions and by charge-transfer effects between clusters and ligands. The extreme temperature-dependence which was observed in the AC conductivity of Au55-cluster samples is interpreted as due to the influence of the Coulomb barrier, occurring when carriers tunnel between neighboring clusters.