NbO molecule: ESR and optical spectra in inert matrices at 4 °K; establishment of its ground electronic state as 4Σ

NbO molecules, produced by the vaporization of a mixture of Nb metal and solid Nb₂O₅ or by passing O₂ gas over hot Nb metal, were trapped in solid Ne or Ar at 4°K and investigated by ESR and optical spectroscopy. Analysis of the ESR spectrum established that the molecule has a ⁴Σ ground state (as derived from a σδ² configuration) exhibiting large hyperfine (hf) splittings from interaction with the ⁹³Nb $\mathrm{(I=}\frac{9}{2})$ nucleus. Essentially all lines were accounted for as effectively ΔM_I=0, ±1, and ±2 transitions (or as multiple trapping sites). Hyperfine and g tensor elements have the following values in a neon matrix: g_∥=2.0023(10), $g_{\perp }\text{=1.9577(10)}$ , |A_∥(⁹³Nb)|=1587(1) MHz, and ${\mathrm{|A}}_{\perp }{(}^{93}\mathrm{Nb}\text{)|=1647(1)\hspace{0.17em}}\mathrm{MHz}$ . The magnetic parameter b ≅ 0.19 cm⁻¹ found in the gas phase optical spectrum is here determined to be b =0.165(1) cm⁻¹. These magnetic parameters are in accord with the expected electronic properties of NbO (assuming the A values are positive) and are compared with those of VO. Both Nb¹⁶O and Nb¹⁸O have been investigated by optical spectroscopy, and the spectra correlate well with gas phase data. The ⁴Σ⁻←X ⁴Σ⁻ blue system appears at 461.4 nm in a Ne matrix and contains several vibrational perturbations. At least five systems of bands appear between 530 and 740 nm. It is conjectured that the progression with (0,0) at 655.3 nm is a ⁴Σ←X ⁴Σ transition and that the remaining bands arise from a ⁴Π←X ⁴Σ system where the upper state is partially Hund's case (c) coupling. In the infrared the vibrational transition of NbO appears as a triplet at 965.5, 973.0, and 977.1 cm⁻¹ in neon; a similar triplet was recently observed in argon by Green, Korfmacher, and Gruen.