15min:

BEAU J. BARKER, SUNIL BAIDAR, SEAN M. CASEY AND DOREEN G. LEOPOLD, Department of Chemistry, University of Minnesota, Minneapolis, MN 55455.

We report the 488 and 514 nm anion photoelectron spectra of Mo₂^-. Neutral Mo₂ has been described in recent ab initio studies as having a bond order of six, predicted to be the highest of any homonuclear diatomic, exceeding even that of Cr₂(five). The photoelectron spectrum of Mo₂^-confirms the previously measured vibrational frequency of gas phase Mo₂ and displays transitions to vibrational levels up to v=7 in its ¹ _g⁺ ground state. The electron affinity of Mo₂ is measured to be 0.732 \pm 0.010 eV. The Mo₂^- ground state is assigned as a ² _u⁺ state, in which the extra electron occupies a formally antibonding _u orbital of primarily 5s atomic parentage. A Franck-Condon analysis of the vibrational band intensities indicates a change in the equilibrium bond length of only 0.03 \pm 0.02 Å upon electron detachment. These results, and the similar vibrational frequencies measured for Mo₂ and Mo₂^-, suggest that the anion HOMO is essentially nonbonding. Weak photodetachment transitions to excited states of Mo₂ lying within 1.2 eV of its ground state are also observed. DFT calculations using the BPW91/SDD method show good agreement with experiment for the electron affinity of Mo₂ and the bond lengths in the anion and neutral molecule ground states. It is hoped that these spectroscopic results will motivate and assist high level theoretical studies of the Mo₂^- anion.