Photoelectron spectroscopy of mono-niobium carbide clusters NbCn-(n=2-7): Evidence for a cyclic to linear structural transition

We investigated a series of mono-niobium carbide clusters, NbCn- (n = 2-7), using anion photoelectron spectroscopy. Vibrationally resolved photoelectr on spectra were observed for NbC2- and NbC3-, which were both shown to have cyclic C-2v structures. Two isomers were observed for NbC4- and NbC5-. The weak and low electron binding energy isomers were shown to be cyclic struc tures forming a series with NbC2- and NbC3-, and all have similar and low e lectron binding energies. The main isomers of NbC4- and NbC5-, which posses s much higher electron binding energies, were shown to be due to linear str uctures, which form a series with NbC6- and NbC7-. All the linear NbCn- clu sters were observed to have high electron binding energies and exhibit an e ven-odd alternation, similar to that observed for pure linear carbon cluste rs in the same size range. A cyclic to linear structural transition was thu s observed for the NbCn- clusters from NbC3- to NbC4-, with the cyclic stru ctures favored for the smaller clusters and the linear isomers favored for the larger clusters. (C) 2001 American Institute of Physics.