The microwave spectrum of CuO, X 2Π, measured with optical detection

The microwave pure rotational transitions J=3/2←1/2 and 5/2←3/2 have been measured for gas phase CuO in its X 2Π state, using microwave-optical double resonance. The copper hyperfine structure was observed, and has been analyzed for the first time for the ground state of an orbitally degenerate transition metal-containing molecule in the gas phase. Rotational, lambda-doubling, and magnetic and electric hyperfine parameters have been evaluated; the Fermi contact parameter bF has been found to be very large and negative. The problem of determining the signs of the ‘‘off-diagonal’’ hyperfine parameters in 2Π electronic states is discussed.