Electronic spectroscopy of the niobium dimer molecule: Experimental and theoretical results

Abstract

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.