A NEGATIVE ION PHOTOELECTRON SPECTROSCOPIC AND COMPUTATIONAL STUDY OF Mo$_{2}$ AND Mo$_{2}^{-}$

Abstract

Author Institution: Department of Chemistry, University of Minnesota, Minneapolis, MN 55455We report the 488 and 514 nm anion photoelectron spectra of Mo$_{2}^{-}$. Neutral Mo$_{2}$ 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 Cr$_{2}$(five). The photoelectron spectrum of Mo$_{2}^{-}$confirms the previously measured vibrational frequency of gas phase Mo$_{2}$ and displays transitions to vibrational levels up to v=7 in its $^{1}\Sigma_{g}^{+}$ ground state. The electron affinity of Mo$_{2}$ is measured to be 0.732 $\pm$ 0.010 eV. The Mo$_{2}^{-}$ ground state is assigned as a $^{2}\Sigma_{u}^{+}$ state, in which the extra electron occupies a formally antibonding $\sigma_{u}$ orbital of primarily 5$s$ 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 \AA $$ upon electron detachment. These results, and the similar vibrational frequencies measured for Mo$_{2}$ and Mo$_{2}^{-}$, suggest that the anion HOMO is essentially nonbonding. Weak photodetachment transitions to excited states of Mo$_{2}$ 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 Mo$_{2}$ 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 Mo$_{2}^{-}$ anion