Microwave spectrum of the NiO radical in the X3Σ− state

The microwave spectrum of NiO in the 3Σ− electronic ground state has been observed using a source-modulated millimeter and submillimeter wave spectrometer. NiO was generated by dc-sputtering vaporization of powdered NiO. Rotational transitions of 58NiO and 60NiO in the vibronic ground state were measured in the 110–460 GHz region and those of 58NiO in the v = 1 state in the 240–370 GHz region. Precise molecular constants were obtained by a least-squares analysis of the measured frequencies. The re bond length is obtained to be 1.6271291(31) Å with three standard errors in parentheses.