Anomalous vibrational dependence of the rotational and hyperfine parameters in the B-4 Pi-X-4 Sigma(-) 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 hyperfi ne constants for the levels v = 0-3 of the B(4)Pi state and v = 1 of the X( 4)Sigma (-) 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 m anifold from Launila et al. (J. Mol. Spectrosc. 186, 131-143 (1997)), the n ew data give a very complete picture of the vibrational energy level patter n in this region. Strong irregularities in the vibrational dependences of t he B(4)Pi rotational and hyperfine constants can be interpreted in terms of spin-orbit interaction between the B(4)Pi state and the f(2)Pi, c(2)phi, a nd d(2)Delta states. The interaction is strong enough that all three double t states can be seen in absorption from the X(4)Sigma (-) ground state, add ing to the complexity of the spectrum. The hitherto unknown sigma delta sig ma* (4)Delta state is estimated to lie near 17 500 cm(-1), from the change of sign in the spin-rotation parameter gamma of the B(4)Pi state between v = 2 and 3. (C) 2001 Academic Press.