ELECTRONIC SPECTROSCOPY OF THE NIOBIUM DIMER MOLECULE - EXPERIMENTAL AND THEORETICAL RESULTS

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 supersoni c expansion. The molecular beam containing niobium dimer molecules was interrogated in the range 400-900 nm using a pulsed dye laser to exci te fluorescence. Numerous OMEGA = 0 <-- OMEGA = 0 and OMEGA = 1 <-- OM EGA = 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 OMEGA = 0 <-- OMEGA = 0 systems originating from a common lower state of 0g+ symmetry, and three OMEGA = 1 <-- OMEGA = 1 systems originatin g from a common lower state of 1g symmetry. The two lower states were assigned as the OMEGA = 0 and OMEGA = 1 spin-orbit components of the X 3SIGMA(g)- ground state, which is derived from the electron configurat ion 1pi(u)(4)1sigma(g)(2)2sigma(g)(2)1delta(g)2. The two spin-orbit co mponents are split by several hundred cm-1 due to a strong, second-ord er isoconfigurational spin-orbit interaction with the low-lying 1SIGMA (g)+ state. Evidence for significant 4d orbital participation in the N b2 bond is furnished by the short bondlength [r(e) = 2.077 81(18) angs trom] and large vibrational frequency [omega(e) = 424.8917(12) cm-1] d etermined for the X3SIGMA(g)-(0g+) state (2sigma error bounds). The el ectronic structure of niobium dimer was investigated using density fun ctional theory. For the electronic ground state, the predicted spectro scopic properties were in good agreement with experiment. Calculations on excited states reveal congested manifolds of triplet and singlet e lectronic states in the range 0-3 eV, reflecting the multitude of poss ible electronic promotions among the 4d- and 5s-based molecular orbita ls. The difficulties of correlating the experimentally observed electr onic transitions with specific valence electronic promotions are addre ssed. Comparisons are drawn between Nb2 and the isoelectronic molecule V2.