The characterisation of the A' 2Σ- state of copper oxide from its nuclear hyperfine structure

Lines in the (0, 0) band of the A' 2Σ-X 2PI transition of CuO at 650 nm have been recorded with Doppler-limited linewidths by laser excitation spectroscopy. Many of the lines show a characteristic quartet structure that is attributed to large hyperfine interactions involving the 63Cu nucleus (I = 3/2). However, the variation of the hyperfine splitting with rotational quantum number is not characteristic of a molecule in an isolated 2Σ state. The splittings have been reproduced quantitatively by a model in which the 2Σ state is heavily perturbed by the nearby γ2PI state. The same model also explains the large spin-rotation splittings observed for the 2Σ state. However, while the spin-rotation splittings can be fitted equally well by the assumption that the A' state is 2Σ+ or 2Σ-, the hyperfine structure is only consistent with the latter assignment. The explicit treatment of the perturbation of the 2Σ levels by those of the 2PI state provides parameter values which are more characteristic of the 2Σ state. In particular, the Fermi contact parameter is determined to be -0.053 cm-1, a result which by itself points directly to the state being a 2Σ- state. The sign of this hyperfine parameter provides a very simple method of distinguishing between 2Σ+ and 2Σ- states. (Auth.)