The microwave spectrum of cobalt monoxide: Hyperfine interactions in the X 4Δ state

The rotational spectrum of CoO in the Δ4 ground state was observed using a source-modulated submillimeter-wave spectrometer. A direct current sputtering method using cobalt powder placed in the hollow cathode electrode was used to generate the CoO radical in an atmosphere of oxygen and helium. In total 84 spectral lines were precisely measured in the frequency region of 255–465 GHz for eight rotational transitions of the two lowest spin substates. Each rotational transition consists of eight components due to the Co59 (I=7/2) hyperfine interaction. A least squares analysis of the measured frequencies gave a new set of molecular parameters: rotational constant, hyperfine coupling constants, a, bF, c, and eQq, centrifugal distortion corrections to rotation and spin–orbit interaction, and the less well known higher-order spin–orbit distortion term to the Fermi contact interaction, bS. A comparison of the determined molecular hyperfine coupling constants was made with those of atomic/ionic Co in order to assess the inherent characteristics of the molecular orbitals around the Co nucleus.