Submillimeter-wave spectroscopy of VN (X3Δr) and VO (X4Σ−): A study of the hyperfine interactions

The pure rotational spectra of VN (X3Δr) and VO (X4Σ−) have been recorded in the frequency range 290-520 GHz using direct absorption spectroscopy. These radicals were synthesized in the gas-phase from the reaction of VCl4 with either N2 or H2O in an AC discharge. Seven rotational transitions were recorded for each molecule; in both sets of spectra, fine and hyperfine structures were resolved. The data sets for VN and VO were fit with Hund's case (a) and case (b) Hamiltonians, respectively, and rotational, fine structure, and hyperfine constants determined. For VN, however, an additional hyperfine parameter, Δa, was necessary for the analysis of the Ω = 2 sublevel to account for perturbations from a nearby 1Δ state, in addition to the usual Frosch and Foley constants. Determination of Δa suggests that the 1Δ state lies ∼3000 cm−1 above the ground state. In VO, the hyperfine structure in the F2 and F3 components was found to become heavily mixed due to an avoided crossing, predicted by previous optical studies to be near the N = 15 level. The hyperfine constants established for these two molecules are consistent with the proposed σ1δ1 and σ1δ2 electron configurations.