Unoccupied states of individual silver clusters and chains on Ag(111)

Size-selected silver clusters on Ag(111) were fabricated with the tip of a scanning tunneling microscope. Unoccupied electron resonances give rise to image contrast and spectral features which shift toward the Fermi level with increasing cluster size. Linear assemblies exhibit higher resonance energies than equally sized compact assemblies. Density functional calculations reproduce the observed energies within 0.6 eV and enable an assignment of the resonances to hybridized atomic 5s and 5p(z) orbitals with silver substrate states.