We report of the first gas-phase electronic spectrum of V2. The dimer was generated in an expansion cooled molecular beam by laser vaporization of vanadium metal. Using the technique of resonant two photon ionization spectroscopy a strong band system with origin near 7000 Å was observed. Rotationally resolved spectra of this band system conclusively demonstrate that the transition is of the type 3Πu(a)←3Σ−g(a), and we assign the 3Σ−g state as the ground electronic state of V2. The short ground state bond length re″=1.77 Å and high vibrational frequency ωe″=535 cm−1 are indicative of extensive 3d-orbital participation in the bonding of this molecule. The unusual case (a) coupling and large second-order spin-orbit splitting observed in the 3Σ−g state implies that the ground state electronic configuration contains a half-filled π(3d) or δ (3d) orbital. The most plausible valence configuration consistent with the data is σg(3d)2πu(3d)+σg(4s)2δg(3d)2. In the excited state there is strong evidence of predissociation occuring for levels higher than 14 872 cm−1 above the ground state, thus providing an upper bound of 1.85 eV for the dissociation energy of V2.

We report the first gas-phase electronic spectrum of nickel dimer. Ni2 is produced by laser vaporization of metallic nickel in the throat of a supersonic nozzle. Using resonant two photon ionization, bands previously observed in inert matrices and attributed to Ni2 are conspicuous in their absence. Further to the red, an abrupt onset of complicated spectral structure indicates rapid predissociation above 16 680 cm−1. We argue that this represents the true dissociation limit, and places D0=2.068±0.01 eV. A congested pattern of spectral features from 6000 to 9000 Å confirms theoretical predictions of a large number of low-lying electronic states in nickel dimer. Rotationally resolved bands near 8500 Å are indicative of a ΔΩ=+1 transition, with Ω″=4, Ω′=5. Rotational analysis of these bands yields a bond length of 2.200±0.007 Å for the ground state of Ni2, which must be of either 1Γg or 3Γu electronic symmetry species. Both the long bond length of 2.20 Å and the high value of Ω″ are in agreement with theoretical predictions, and confirm that no substantial 3d participation contributes to the chemical bonding of Ni2.

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