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Ionization Energies of Niobium Carbide Clusters NbnCm (n = 3−10, m = 0−7)

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journal contribution
posted on 19.03.2009, 00:00 by Naoya Fukushima, Ken Miyajima, Fumitaka Mafuné
We prepared niobium carbide clusters, NbnCm, in the gas phase by a double laser ablation technique. A photoionization efficiency was measured as a function of the wavelength of an ionization laser to determine ionization energies (Ei’s) of NbnCm (n = 3−10, m = 0−7). The Ei’s of Nb4C4 and Nb5C3 are found to be the lowest in the clusters studied. When the experimental Ei’s are compared with the Ei’s estimated by the density functional calculation previously reported (Harris, H.; Dance, I. J. Phys. Chem. A 2001, 105, 3340−3358), we determined Nb5C3 to have a similar 2 × 2 × 2 cubic structure as Nb4C4. We also found that the Ei’s of carbon-rich clusters, NbnCm (nm), tend to be higher (>5 eV) than the Ei’s of the niobium-rich clusters. The high Ei’s are due to the structure of the carbon-rich clusters: carbon−carbon bonding is preferred when the number of carbon atoms exceeds the number of metal atoms. The structure was also discussed in relation to the reactivity of cluster ions with a hydrogen molecule.