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.

Investigation of the use of density functionals in second‐ and third‐row transition metal dimer calculations

“…For the niobium dimer Nb 2 , experiment determines r e = 2.078 Å and ω e = 424.8917 ± 0.0012 cm ). 29,31,32 The ab initio CASSCF method significantly underestimates the dissociation energy (D e = 2.24 eV) despite the accurate bond length (r e = 2.10 Å) and harmonic frequency (ω e = 448 cm −1 ) for this ground state. 33 The fairly small dissociation energy of B-null may, therefore, indicate that dynamical correlation is required to increase the dissociation energy.…”

Isomer selective infrared spectroscopy of neutral metal clusters

“…28 As a check, for Nb 2 this approach yields a calculated vibrational frequency of 445 cm −1 , close to the experimental value of 425 cm −1 . 31 We tested a large number of different geometric structures for Nb 9 , including some that have been suggested previously in the literature. 29, 30 Moreover, we tested a number of geometric distortions of these structures, and calculated the energy and the IR absorption spectra for different spin states.…”

The far-infrared spectra of neutral and cationic niobium clusters: Nb5∕+ to Nb9∕+

“…For comparison, an increased force constant is also found for the neutral niobium dimer. 57 This stronger bonding can be related to a more efficient overlap of the larger 4d orbitals of niobium than the 3d orbitals of vanadium.…”

“…Using the matrix isolation resonance Raman spectroscopy the dimer stretching frequency was found to be e = 421 cm −1 , 55,56 in agreement with the gas-phase value of 424.9 cm −1 from the rotationally resolved electronic spectrum. 57 For the trimer, vibrational fundamentals of 235 and 227 cm −1 have been derived from the observed band progressions in the Raman spectrum and they are assigned to the totally symmetric stretch and the doubly degenerate bend of a nearly D 3h symmetric structure. 58 Theory identifies a triangular 2 B 1 state with an obtuse angle of 63.4°as the ground state for this species.…”