Spectroscopy of NbO: Characterization of the Doublet Manifold

Abstract

Doublet bands of NbO in the near infrared region have been observed in emission with FTS techniques using an electrodeless 2450 MHz discharge as a source. Transitions involving the vibrational levelsv= 0–4 of a low-lyinga2Δ state of configuration σ2δ and four additional doublet states (c2Π,d2Δi,e2Φ, andf2Πi) have been recorded at high resolution. Furthermore, a2Σ state, assigned asb2Σ−, has been observed through thec2Π (v= 0) →b2Σ−(v= 0, 1) transitions in the 1.6 μm region. Rotational analyses of the doublet bands have been performed. Most of the excited doublet states lie only slightly above or below the well-knownB4Π state. The density of states is fairly high immediately aboveB4Π, and characterizations of some of the states in this region involve difficulties. A currently unanalyzed band, centered at 11 820 cm−1, has been preliminarily attributed to theA4Π → X4Σ−(0, 0) transition. Nuclear hyperfine effects are barely detectable in orbitally degenerate states of the doublet manifold at Doppler-limited resolution, while theb2Σ−state of configuration σδ2shows partly resolved magnetic hyperfine structure (b= −0.08191 cm−1). Large-scale all-electron CI calculations have been performed on doublet and quartet manifolds of NbO up to 22 000 cm−1. The calculations have been performed in two steps. In the first step, the spin–orbit effects within electronic states were not considered. An excellent overall agreement with experimental energies was obtained, except for thed2Δistate. In the second step, all spin–orbit interactions were included. Also these calculations resulted in a good agreement with the experimental observations. The calculations also predict three hitherto unobserved low-lying states:4Φ at 8545 cm−1,2Γ at 8430 cm−1and2Σ+at 9648 cm−1.