Spectroscopic and theoretical studies of the low-lying states of BaO(+).

The BaO(+) cation is of interest from the perspectives of electronic structure and the potential for cooling to ultra-cold temperatures. Spectroscopic data for the ion have been obtained using a two-color photoionization technique. The ionization energy for BaO was found to be 6.8123(3) eV. The ground state of BaO(+) was identified as X(2)Σ(+), and both vibrational and rotational constants were determined. Vibrationally resolved spectra were recorded for A(2)Π, the first electronically excited state. These data yielded the term energy, vibrational frequency, and the spin-orbit interaction constant. Relativistic electronic structure calculations were carried out using multi-reference configuration interaction (MRCI), coupled cluster and density functional theory methods. Transition moments for the pure vibrational and A(2)Π-X(2)Σ(+) transitions were predicted using the MRCI method.