Direct probing of low-energy intra d-band transitions in gas-phase cobalt clusters.

The interplay between constituent localized and itinerant electrons of metal clusters defines their physical and chemical properties. In turn, the electronic and geometrical structures are strongly entwined and exhibit strong size-dependent variations. Current understanding of low-energy excited states of metal clusters relies on stand-alone theoretical investigations and few comparisons with measured properties, since direct identification of low-lying states is lacking hitherto. Here, we report on the measurement of low-lying electronic transitions in cationic cobalt clusters using infrared photofragmentation spectroscopy. Broad and size-dependent absorption features were observed within 0.056 – 0.446 eV, well above the energies of the sharp absorption bands caused by cluster vibrations. Complementary time-dependent density functional theory calculations reproduce the main observed absorption features, providing direct evidence that they correspond to transitions between electronic states of mainly d-character, arising from the open d-shells of the Co atoms and the high spin multiplicity of the clusters. Large classes of metal clusters are expected to have dipole-allowed electronic transitions in the near-infrared, but precise experimental characterization of such low-lying states is lacking. Here, the authors probe cationic cobalt clusters of 4 to 15 atoms using IR photofragmentation spectroscopy with krypton as a messenger atom to show that low-lying electronic excited states are responsible for the size-dependent radiative decay of these highly excited clusters. [ABSTRACT FROM AUTHOR]