Resonant two photon ionization (R2PI) spectroscopy was used to obtain detailed spectroscopic information on the neutral and cation ground states of the jet-cooled molecules V2, VNb, and Nb2. By recording photoionization efficiency (PIE) spectra, their adiabatic ionization potentials were determined to be 51 269(5) cm−1 (V2), 51 554(10) cm−1 (VNb), and 51 359(10) cm−1 (Nb2). In VNb, we used different ionization routes to determine that the splitting between the Ω=0 and Ω=1 spin–orbit components of the X 3Σ− ground state was 230(3) cm−1. In the case of V2 and VNb, two thresholds were observed in the PIE spectra recorded via Ω=1 intermediate states. We were thus able to assign the ground states of V+2 and VNb+ as having 4Σ− symmetry, with second-order spin–orbit splittings of 20(3) and 82(3) cm−1, respectively. A simple model was applied to calculate the locations of the 1Σ+ and 2Σ+ states which are responsible for the second-order spin–orbit splitting of the neutral and cation ground states, respectively. One-color R2PI spectroscopy was employed to determine the bond dissociation energy of VNb, the result D00=30 562(10) cm−1 being obtained. The implications of our measurements regarding the relative bond strengths of the neutral and cationic dimers are discussed.
Rotationally resolved electronic spectra of the niobium dimer molecule are reported for the first time. The molecules were produced by laser vaporization of a niobium target rod and cooled in a helium supersonic expansion. The molecular beam containing niobium dimer molecules was interrogated in the range 400–900 nm using a pulsed dye laser to excite fluorescence. Numerous Ω=0←Ω=0 and Ω=1←Ω=1 vibronic transitions were discovered in the region 630–720 nm and investigated at 200 MHz resolution using the cw output of a single mode ring dye laser. The principal features were classified into five Ω=0←Ω=0 systems originating from a common lower state of 0+g symmetry, and three Ω=1←Ω=1 systems originating from a common lower state of 1g symmetry. The two lower states were assigned as the Ω=0 and Ω=1 spin–orbit components of the X 3Σ−g ground state, which is derived from the electron configuration 1π4u1σ2g2σ2g1δ2g. The two spin–orbit components are split by several hundred cm−1 due to a strong, second-order isoconfigurational spin–orbit interaction with the low-lying 1Σ+g state. Evidence for significant 4d orbital participation in the Nb2 bond is furnished by the short bondlength [re=2.077 81(18) Å] and large vibrational frequency [ωe=424.8917(12) cm−1] determined for the X 3Σ−g(0+g) state (2σ error bounds). The electronic structure of niobium dimer was investigated using density functional theory. For the electronic ground state, the predicted spectroscopic properties were in good agreement with experiment. Calculations on excited states reveal congested manifolds of triplet and singlet electronic states in the range 0–3 eV, reflecting the multitude of possible electronic promotions among the 4d- and 5s-based molecular orbitals. The difficulties of correlating the experimentally observed electronic transitions with specific valence electronic promotions are addressed. Comparisons are drawn between Nb2 and the isoelectronic molecule V2.
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