Ni2 revisited: Reassignment of the ground electronic state

Resonant two-photon ionization spectroscopy was used to study jet-cooled Ni2 produced by pulsed laser ablation of a nickel target in the throat of a supersonic nozzle using argon as the carrier gas. Spectral regions previously investigated using helium as the carrier gas were reinvestigated, and the improved cooling achieved was found to suppress transitions arising from an Ω=4 state that had been thought to be the ground state. Seven new vibronic progressions were assigned, with spectroscopic constants determined for the excited states. The predissociation threshold in Ni2 was reinvestigated, and a revised value for the binding energy is given as D○0(Ni2)=2.042±0.002 eV. The ionization energy of Ni2 was found to be 7.430±0.025 eV, and from this result and the revised bond dissociation energy of the neutral, the binding energy of the cation was calculated to be D○0(Ni+2)=2.245±0.025 eV. Similarly, D○0(Ni−2)=1.812±0.014 eV is obtained using D○0(Ni2) and the electron affinities of Ni and Ni2. Twenty bands were rotationally resolved, all originating from a lower state of Ω″=0+g or 0−u which we argue is the true ground state, in agreement with ligand field and ab initio theoretical studies. The rotational analysis also yielded a ground state bond length of 2.1545±0.0004 Å for 58Ni2.