Spectroscopy of NbO: Characterization of the doublet manifold

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 levels upsilon = 0-4 of a low-lying a(2) Delta state of configuration sigma(2) delta and four additional doublet states (c(2) Pi, d(2) Delta(i), e(2) Phi, and f(2) Pi(i)) have been recorded at high resolution. Furthermore, a (2) Sigma state, assigned as b(2) Sigma(-), has been observed through the c(2) Pi (upsilon = 0) --> b(2) Sigma(-) (upsilon = 0, 1) transitions in the 1.6 mu m region. Rotational analyses of the doublet bands have been performed. Most of the excited doublet states lie only slightly above or below the well known B-4 Pi state. The density of states is fairly high immediately above B-4 Pi, 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 the A(4) Pi --> X-4 Sigma(-) (0, 0) transition. Nuclear hyperfine effects are barely detectable in orbitally degenerate states of the doublet manifold at Doppler-limited resolution, while the b(2) Sigma(-) state of configuration sigma delta(2) shows 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 the d(2) Delta(i) state. 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: (4) Phi at 8545 cm(-1), 2 Gamma at 8430 cm(-1) and (2) Sigma(+) at 9648 cm(-1). (C) 1997 Academic Press.