Author Institution: Department of Chemistry, University of Minnesota, Minneapolis, MN 55455We report the 488 and 514 nm anion photoelectron spectra of Mo2ββ. Neutral Mo2β has been described in recent abinitio studies as having a bond order of six, predicted to be the highest of any homonuclear diatomic, exceeding even that of Cr2β(five). The photoelectron spectrum of Mo2ββconfirms the previously measured vibrational frequency of gas phase Mo2β and displays transitions to vibrational levels up to v=7 in its 1Ξ£g+β ground state. The electron affinity of Mo2β is measured to be 0.732 Β± 0.010 eV. The Mo2ββ ground state is assigned as a 2Ξ£u+β state, in which the extra electron occupies a formally antibonding Οuβ 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 Β± 0.02 \AA upon electron detachment. These results, and the similar vibrational frequencies measured for Mo2β and Mo2ββ, suggest that the anion HOMO is essentially nonbonding. Weak photodetachment transitions to excited states of Mo2β 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 Mo2β 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 Mo2ββ anion