Multiple isomers in the photoelectron spectra of small mono-niobium carbide clusters

We calculate the photoelectron spectrum of small mono-niobium carbide clusters (NbC_n) using density functional theory for clusters with n = 2-7 and the symmetry adapted cluster configuration interaction method for the smallest clusters (n = 2-4). Theoretical spectra of a single structure cannot explain all peaks present in the spectrum measured by Zhai et al. [J. Chem. Phys. 115, 5170 (2001)]. However, we can match all peaks in the experimental spectra if we assume that the beam contains a combination of cyclic and linear structures. This finding is even more surprising given the fact that some of the excited metastable geometries have energies as large as 0.5 eV above the ground state. Our result is confirmed by both theoretical approaches. We suggest further experiments, using additional beam cooling, to corroborate this observation.