The Far-Infrared Spectra of Neutral and Cationic Niobium Clusters: Nb50/+ to Nb90/+

Far-infrared absorption spectra of small neutral and cationic niobium clusters containing 5 to 9 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 density-functional theory. A comparison of the experimental and calculated far-IR spectra spectra allows to identify the cluster structures. Although the experimental spectra for clusters containing 5, 6, 8, and 9 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 Nb60/