Determination of the geometric structure of neutral niobium carbide clusters via infrared spectroscopy.

We report experimental vibrational spectra of small neutral niobium carbide clusters in the 350-850 cm-1 spectral range. Clusters were first irradiated by IR light and subsequently probed using UV light with photon energies just below the ionization threshold. Upon resonance with an IR vibrational mode, the number of cluster ions increases, allowing to record a vibrational spectrum. In complementary density functional theory calculations, we have simulated the IR spectra for several low-energy isomers. We were able to assign the spectra experimentally obtained for each cluster size to a specific geometric structure based on the match with the computed spectra. The number of the cluster sizes investigated here allows to follow the evolution of the geometric structure of the niobium and carbon components of clusters separately. For Nb6Cm (m = 4, 5, 6), we observe the emergence of the cubic crystal structures similar to the bulk.