VIBRATIONALLY RESOLVED NEGATIVE ION PHOTOELECTRON SPECTROSCOPIC STUDIES OF NIOBIUM CLUSTERS
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Date
1996
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Ohio State University
Abstract
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, $Nb_{3}O^{-}$, $Nb_{4}O^{-}$ and $Nb_{4}CO^{-}$ 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 $Nb_{3}O^{-}$ anions shows a vertical transition to the ground state of neutral $Nb_{3}O^{-}$, with weak progressions in the $Nb_{3}-O$ stretching (710$\pm$$20 cm^{-1}$ in $Nb_{3}O$) and $Nb_{3}$ bending (320$\pm$$15 cm^{-1}$ in both $Nb_{3}O$ and $Nb_{3}O^{-}$) vibrational modes. These results indicate that the $Nb_{3}O^{-}$ anion, like $Nb_{3}O$ and $Nb_{3}O^{-1}$, has a planar $C_{3}$, structure with the O atom bridging two Nb atoms. The $Nb_{4}O^{-}$ spectrum shows resolved transitions to the ground state of $Nb_{4}O$ and to an excited electronic state lying 3050$\pm$$20 cm^{-1}$ higher in energy. In analogy with $Nb_{3}O$ results, the 670$\pm$$20 cm^{-1}$ frequencies observed in the ground and excited states, respectively, to a bending mode of the metal cluster. The electron affinities of $Nb_{3}O$ and $Nb_{4}O$ are 1.402 and 1.178 ($\pm$0.006) cV, respectively. Preliminary, ongoing studies of mass selected $Nb_{4}CO^{-}$ 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 $1300 cm^{-1}$, and the other with the dissociated C and O atoms bound separately to the niobium cluster.
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$^{1}$ D.-S. Yang, M.Z. Zgierski, D.M. Rayner, P.A. Hackett, A. Martinez, D.R. Salahub, P.-N. Roy and T. Carrington, J. Chem. Phys. 103, 5335 (1995).
Author Institution: Department of Chemistry, University of Minnesota
Author Institution: Department of Chemistry, University of Minnesota