Anomalous Vibrational Dependence of the Rotational and Hyperfine Parameters in the B4Π–X4Σ− Transition of NbO

The laser excitation spectrum of jet-cooled NbO in the region 16 000–18 000 cm−1 has been recorded at high resolution, giving rotational and hyperfine constants for the levels v=0−3 of the B4Π state and v=1 of the X4Σ− state; zero gaps have also been measured at low resolution for some weaker bands involving higher vibrational levels. Taken together with the laser data for the B–X (0,0) band from Adam et al. (J. Chem. Phys.94, 6240–6262 (1994)) and the Fourier transform emission data for the doublet manifold from Launila et al. (J. Mol. Spectrosc.186, 131–143 (1997)), the new data give a very complete picture of the vibrational energy level pattern in this region. Strong irregularities in the vibrational dependences of the B4Π rotational and hyperfine constants can be interpreted in terms of spin–orbit interaction between the B4Π state and the f2Π, e2Φ, and d2Δ states. The interaction is strong enough that all three doublet states can be seen in absorption from the X4Σ− ground state, adding to the complexity of the spectrum. The hitherto unknown σδσ* 4Δ state is estimated to lie near 17 500 cm−1, from the change of sign in the spin–rotation parameter γ of the B4Π state between v=2 and 3.