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, nea r 6600 Angstrom, has been analyzed from spectra taken at sub-Doppler r esolution. The transition is notable for the great width of its Nb nuc lear hyperfine structure, which is caused principally by the unpaired 5s sigma electron in the ground state interacting with the large magne tic moment of the(41)(93) Nb nucleus (1=9/2). A fit to the ground-stat e 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 parameter s. Prominent among these are the third-order spin-orbit distortions of the spin-rotation interaction and the Fermi contact interaction, whic h are large and well. determined, reflecting different degrees of spin -orbit contamination of the the (4) Sigma(1/2) and (4) Sigma(3/2) comp onents 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 as ymmetric, because of strong perturbations by a (II)-I-2 state which ha s been identified in this work, from among the various weak bands in t he NbO spectrum near 7000 Angstrom; the result is that many high order centrifugal distortion terms are needed in an effective Hamiltonian m odel for the rotation. Second, the hyperfine structure is perturbed, n ot 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 c m(-1) appears to be one of these. The distant states generate large ap parent 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 F ermi contact matrix elements and are responsible for the unexpected ne gative 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.