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T. P. MARCY AND D. G. LEOPOLD, Department of Chemistry, University of Minnesota, Minneapolis, MN 55455..

We have observed a surprisingly simple, vibrationally resolved spectrum for the bare niobium octamer cluster by photoelectron spectroscopy of mass selected Nb₈^- anions. The clusters were prepared in a cw cathode discharge source within a liquid nitrogen cooled flow tube (0.5 Torr), and the spectra were obtained at 488 nm (2.540 eV) with an electron kinetic energy resolution of about 5 meV (40 cm^-1). The measured electron affinity of 1.513(7) eV is consistent with that obtained in the earlier anion PES study . In addition, the present spectrum exhibits a single clearly resolved vibrational progression in the transition between the ground electronic states of the anionic and neutral clusters. The frequency of this active mode is 185(15) cm^-1 in the neutral molecule and 160(20) cm^-1 in the anion, and the short progression indicates a small normal mode displacement of about 0.5 amu^1/2Å. These results are discussed in the context of recently reported density functional calculations \footnoteH. Grönbeck, A. Rosén and W. Andreoni, Phys. Rev. A \textbf58, 4630 (1998); R. Fournier, T. Pang and C. Chen, Phys. Rev. A, \textbf57, 3683 (1998). on Nb8 and Nb8^-.