The far-infrared spectra of neutral and cationic niobium clusters: Nb5 0/+ to Nb9 0+.

Far-infrared absorption spectra of small neutral and cationic niobium clusters containing five to nine Nb atoms have been obtained by multiple photon dissociation spectroscopy of their argon complexes. The experimental far-IR spectra are recorded in the 85-600 cm(-1) region and cover the range of the structure-specific vibrational fundamentals, i.e., the finger-print range, for these metal clusters. The experiments are accompanied by quantum chemical calculations employing the density-functional theory. A comparison of the experimental and calculated far-IR spectra allows to identify the cluster structures. Although the experimental spectra for clusters containing five, six, eight, and nine Nb atoms are very different for cationic and neutral clusters, the comparison with theory reveals that, nevertheless, the overall geometries for cations and neutrals are very similar, except for Nb(6) (0+).