$Ni_{2}$ REVISITED: REASSIGNMENT OF THE GROUND ELECTRONIC STATE IN AGREEMENT WITH LIGAND FIELD AND ab initio RESULTS

Resonant two-photon ionization spectroscopy was used to study $Ni_{2}$ produced by laser vaporization of a nickel target in the throat of a supersonic nozzle using argon as a carrier gas. Spectral regions previously investigated urging helium as a carrier gas were reinvestigated, and the improved cooling achieved in this study was found to suppress transitions arising from an $\\omega = 4$ state that had been thought to be the ground state. Seven new vibronic progressions were assigned, with spectroscopic constants determined for the ground and excited states. The predissociation threshold in $Ni_{2}$ was reinvestigated, and a revised value for the binding energy is given as $D^{0}_{0}(Ni_{2}) = 2.042 \\pm 0.0002$ eV. The ionization energy of $Ni_{2}$ was fund to be $7.430 \\pm 0.025$ eV, and from this value and the revised binding energy of the neutral, the binding energy of the cation was calculated to be $D^{0}_{0}(Ni_{2}) = 2.245 \\pm 0.025$ eV. Twenty bands were rotationally, all originating from a lower state of $\\omega = 0^{0}_{0} or 0^{-}_{u}$ (which are indistinguishable in our experiments). We argue that is the true ground state, in agreement with ligand field and ab initio theoretical studies. A weighted average of the $B^{\\prime\\prime}$ values obtained from the rotational analyses yielded a ground state bond length of $2.1542 \\pm 0.0003 {\\AA}$ for $^{58}Ni_{2}$.