VO and NbO in their X 4 E ground states

Microwave spectra of VO and NbO arising from the lowest rotational transition in the ground state have been observed and analysed by Suenram, Fraser, Lovas and 

Gillies [237]. They used the technique of laser ablation to form nozzle beams of the molecules, injected into a Fourier transform microwave spectrometer. In both cases the groimd electronic state is an example of which we met in chapter 9, the excited a state of the CH radical. However in that example the coupling was close to case (b), whereas VO and NbO provide examples of a case (a) state. The = 1 /2 and 3/2 states are split by second-order spin rbit coupling the splitting is 

The case (a) effective rotational Hamiltonian for NbO in its ground state may be written 

In addition, the Nb magnetic hyperline coupling was large enough to be readily observed, even in the electronic spectrum. It was fitted to the normal Frosch and Foley Hamiltonian for a E state. 

Similar aspects arise for VO, except that vanadium belongs to the 37 transition element group. The electronic structure of VO is, however, similar to that of NbO and a large isotropic hyperfine interaction arises from a 4 spin density of around 30%. 

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