Band gap shiftings in Co-doped Nb_n (n = 3-15) clusters: Influence of Co 3d electrons on the electronic structure

The photoelectron spectra of bimetallic, mass-selected Nb _nCo^- clusters (n = 3-15) measured at 4.66eV photon energy are reported. The electronic structure changes of the binary clusters are discussed in comparison with the electronic structures of pure Nb _n^- clusters as a function of size evolution. Spectroscopic parameters like electron affinities (EAs) and vertical detachment energies (VDEs) are determined. The addition of one Co atom to the Nb _n cluster core significantly perturbs the electronic structures of pure Nb_n clusters at small cluster sizes. Electronic state merging can be explained by hybridization of Co 3d and 4s as well as Nb_n 4d-derived valence orbitals. Tentative geometric structure postulations are carried out revealing the influence of the incoming Co atom toward the intact Nb_n cluster core. In strong contrast to pure Nb_n clusters, a highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO-LUMO) gap closing of Nb_nCo at n = 6 indicates a bulk-like evolution of electronic structures induced by charge transfer of Co 3d electrons.