NEGATIVE ION PHOTOELECTRON SPECTROSCOPY OF $Nb^{-}_{3}, Nb_{3}H^{-}$, AND $Nb^{-}_{4}$

$^{1}$ H. Wang, R. Craig, H. Haouari, Y. Liu, J.R. Lombardi, and D.M. Lindsay, J. Chem. Phys. 105, 5355 (1996).Author Institution: Department of Chemistry, University of MinnesotaWe report the 488 nm negative ion photoelectron spectra of three small niobium cluster molecules. The negative ions were prepared in a flowing afterglow ion-molecule reactor by a niobium cathode discharge source in a liquid nitrogen cooled flow tube. $H_{2}$ or $D_{2}$ diluted in argon was added downstream to prepare $Nb_{3}H^{-}$ or $Nb_{3}D^{-}$. The ions were separated by a sector magnet mass analyzer and photodetached by the 488 nm line of an argon ion laser. $Nb_{3}, Nb_{3}H$, and $Nb_{4}$ display similar electron affinities of 1.032, 1.070, and $1.009 \pm 0.006$ eV. Two vibrational modes each are observed for neutral $Nb_{3}$ and $Nb_{4}$. The fundamental frequencies of $Nb_{3}$ are observed to be 295 and $220 \pm 15 cm^{-1}$ which can be compared to the matrix observations of Lindsay and $coworkers^{1}$ of $334.9 \pm 2.8$ and $227.4 \pm 2.9 cm^{-1}$. The latter mode exhibits a frequency of $215 \pm 15 cm^{-1}$ in the anion. The two observed $Nb_{4}$ frequencies are 205 and $140 \pm 15 cm^{-1}$, with a $120 \pm 20 cm^{-1}$ value for the latter mode in the anion. $Nb_{3}H$ has only one mode active with a frequency of $220 \pm 15 cm^{-1}$ in the neutral cluster and $210 \pm 15 cm^{-1}$ in the anion