LASER SPECTROSCOPY OF THE LOW-LYING ELECTRONIC STATES OF NBN - ELECTRON-SPIN AND HYPERFINE EFFECTS IN THE STATES FROM THE CONFIGURATIONS SIGMA-DELTA AND DELTA-PI

Rotational and hyperfine analyses have been carried out for the (0,0) bands of the (CII)-I-3-X(3) Delta, e(1)II-X(3) Delta, and f(1) Phi-a(1) Delta transitions of gaseous NbN from laser excitation spectra taken at sub-Doppler resolution. The delta pi (CII)-I-3 and e(1)II states lie only 102 cm(-1) apart in zero order but the spin-orbit matrix element between them, which is the sum of the spin-orbit constants for the delta and pi electrons, is 698 cm(-1); as a result the (II1)-I-3 spin component lies below both the (II0)-I-3 and (II2)-I-3 components, and its hyperfine structure is highly irregular. This irregularity is an extreme example of how cross terms between the spin-orbit interaction and the Fermi contact hyperfine operator alter the apparent value of the hyperfine a constant, the coefficient of I.L in the magnetic hyperfine Hamiltonian. Molecular parameters for the (CII)-I-3 and e(1)II states have been obtained from a combined fit to the two of them. Including data fot the B-3 Phi state recorded earlier [Azuma et al., J. Chem. Phys. 91, 1 (1989)], detailed information is now available for all six of the electronic states from the electron configurations sigma delta and delta pi. It has been verified that the spin-orbit/Fermi contact cross terms cause roughly equal and opposite shifts in the hyperfine a constants for the singlet states and the Sigma=0 components of the triplet states. After allowing for this effect, it has been possible to interpret the hyperfine a constants in terms of one-electron parameters for the delta and pi electrons, in similar fashion to spin-orbit parameters. Wavelength resolved fluorescence, following selective laser excitation of the (CII)-I-3, e(1)II, and f(1) Phi states, has led to the discovery of three new electronic states, delta(2) c(1) Gamma,delta(2) A(3) Sigma-, and sigma(2) b(1) Sigma(+), besides giving the absolute position of a(1) Delta. Strong configuration interaction mixing is found to occur between the sigma(2) b(1) Sigma(+) and delta(2) d(1) Sigma(+) states. The low-lying electronic states of NbN are now well understood.