Knowledge Bank

Negative ion photoelectron spectroscopy provides a means of obtaining vibrational data for atoms and small molecules ""chemisorbed"" on size-selected metal clusters. In the present study, $Nb3O-$, $Nb4O-$ and $Nb4CO-$ were prepared in a flowing afterglow ion-molecule reactor equipped with a metal cathode cluster source. The 488 nm photoelectron spectrum of the mass-selected $Nb3O-$ anions shows a vertical transition to the ground state of neutral $Nb3O-$, with weak progressions in the $Nb3-O$ stretching (710$\pm$$20cm-1$ in $Nb3O$) and $Nb3$ bending (320$\pm$$15cm-1$ in both $Nb3O$ and $Nb3O-$) vibrational modes. These results indicate that the $Nb3O-$ anion, like $Nb3O$ and $Nb3O-1$, has a planar $C3$, structure with the O atom bridging two Nb atoms. The $Nb4O-$ spectrum shows resolved transitions to the ground state of $Nb4O$ and to an excited electronic state lying 3050$\pm$$20cm-1$ higher in energy. In analogy with $Nb3O$ results, the 670$\pm$$20cm-1$ frequencies observed in the ground and excited states, respectively, to a bending mode of the metal cluster. The electron affinities of $Nb3O$ and $Nb4O$ are 1.402 and 1.178 ($\pm$0.006) cV, respectively. Preliminary, ongoing studies of mass selected $Nb4CO-$ anions prepared under a variety of source conditions thus far suggest the presence of two isomers, one with a greatly weakened but intact CO bond as indicated by a very low CO stretching frequency of about $1300cm-1$, and the other with the dissociated C and O atoms bound separately to the niobium cluster.