HYPERFINE-STRUCTURE IN HIGH-SPIN MULTIPLICITY ELECTRONIC STATES - ANALYSIS OF THE B(4)PI-X(4)SIGMA(-) TRANSITION OF GASEOUS NBO

The (0,0) band of the B (II)-I-4-X (4) Sigma(-) transition of NbO, near 6600 Angstrom, has been analyzed from spectra taken at sub-Doppler resolution. The transition is notable for the great width of its Nb nuclear hyperfine structure, which is caused principally by the unpaired 5s sigma electron in the ground state interacting with the large magnetic moment of the(41)(93) Nb nucleus (1=9/2). A fit to the ground-state combination differences, including four very precise microwave lines measured by Suenram et al. [J. Mol. Spectrosc. 148, 114 (1991)], has given a comprehensive set of rotational, spin, and hyperfine parameters. Prominent among these are the third-order spin-orbit distortions of the spin-rotation interaction and the Fermi contact interaction, which are large and well. determined, reflecting different degrees of spin-orbit contamination of the the (4) Sigma(1/2) and (4) Sigma(3/2) components of the ground state. The delta(2) pi B (II)-I-4 state was hard to fit, for a number of reasons. First, its spin-orbit structure is asymmetric, because of strong perturbations by a (II)-I-2 state which has been identified in this work, from among the various weak bands in the NbO spectrum near 7000 Angstrom; the result is that many high order centrifugal distortion terms are needed in an effective Hamiltonian model for the rotation. Second, the hyperfine structure is perturbed, not only by this (II)-I-2 state, but by distant Sigma and Delta states at higher energy. The delta(2) sigma* C (4) Sigma(-) State at 21 350 cm(-1) appears to be one of these. The distant states generate large apparent nuclear spin-rotation interactions, both within and between the Lambda components of the II state, as a result of cross terms between matrix elements of the operators -2BJ.L and aI.L. Similar cross terms arising from the operators AL.S and aI.L produce corrections to the Fermi contact matrix elements and are responsible for the unexpected negative sign of the magnetic hyperfine parameter d. The ’’off-diagonal’’ quadrupole parameter e(2)Qq(2) is very large, and causes some of the higher J line shapes of the B-X system to be noticeably asymmetric at Doppler limited resolution; its value is consistent with the electron configuration of the B (II)-I-4 state being delta(2) pi.