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

We prepared niobium carbide clusters, Nb_nC_m, 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 (E_i’s) of Nb_nC_m (n = 3−10, m = 0−7). The E_i’s of Nb₄C₄ and Nb₅C₃ are found to be the lowest in the clusters studied. When the experimental E_i’s are compared with the E_i’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 E_i’s of carbon-rich clusters, Nb_nC_m (n ≤ m), tend to be higher (>5 eV) than the E_i’s of the niobium-rich clusters. The high E_i’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.