Electronic and geometric structures of Co2Cn− and V2Cn−: Initial growth mechanisms of late and early 3d transition-metal carbide clusters

Photoelectron spectra of Co2Cn− (n=2, 3) and V2Cn− (n=2–4) were measured in the energy range below 3 eV. Analyses of these spectra by the density-functional theory deduced their electronic states and geometric structures. The growth mechanisms of the 3d transition-metal carbide clusters were discussed on the basis of the structural models obtained. The geometric structures of Co2Cn− exhibit a tendency that carbon atoms aggregate to form a Cn substructure. In contrast, V2Cn− consists of VC2 building blocks, which prelude the formation of a vanadium-carbide network. These features illustrate the differences in the carbide-formation processes of the late and the early 3d transition metals, that is, only the latter forms large metal-carbide networks such as metallocarbohedrens and metal carbide compounds.