Microwave spectra of the AlO (X 2Σ+) radical in the v= 1 and 2 states

Abstract The microwave spectra of the aluminum monoxide radical (X 2 Σ + ) for v = 1 and 2 have been observed by using a source-modulated spectrometer combined with a free-space absorption cell. The Al atom was generated by discharge in pure N 2 O or Ar with an aluminum hollow-cathode containing several pieces of alumina. Five rotational transitions in the region 180–380 GHz were measured. The fine and hyperfine structure patterns of a certain rotational transition showed that the spin-rotation coupling constant changes anomalously by vibrational excitation. The rotational constant, centrifugal distortion constant, spin-rotation coupling constant with its centrifugal distortion term, and hyperfine coupling constants for the Al nucleus were precisely determined for the v = 1 and 2 states.