PERTURBATIONS IN THE ROTATIONAL AND HYPERFINE STRUCTURES OF THE $B^{4}\Pi - X^{4} \Sigma^{-}$ AND $C^{4} \Sigma^{-} -X^{4}\Sigma^{-}$ SYSTEMS OF VO

$^{1}$D. Richards and R.F. Barrow, Nature, 219, 1244 (1968).Author Institution:Analyses of the rotational and hyperfine structure have been carried out for various bands of the $B^{4}\Pi - X^{4} \Sigma^{-}$ and $C^{4} \Sigma^{-} -X^{4}\Sigma^{-}$- systems of VO, which lie respectively near 8000 {\AA} and 5700 {\AA}. All the bands of both systems are perturbed, with some quite sizeable avoided crossings occurring in the levels of the $B^{4}\Pi$ state. The available evidence indicates that the principal perturbing state is another $^{4}\Sigma^{-}$ state. Hyperfine structure due to the vanadium nucleus (I=7/2) is resolved in branches involving the $F_{1}$ and $F_{4}$ electron spin components of the ground state. The determination of accurate rotational constants for the ground state is greatly complicated by internal $hyperfine^{1}$ perturbations caused by $\Delta=0, \Delta=0, \Delta=\pm 1$ matrix elements of the hyperfine Hamiltonian, centred round N=15; similar internal perturbations also occur in the $C^{4}\Sigma^{-}$ state near N = 5. These are currently being studied by high-resolution laser-induced fluorescence methods