Ab initio study on the electronic structure of the

The electronic structure of the 4 2Σ+ and 5 2Σ+ excited states of CO+ are studied by using state-averaged complete-active-space self-consistent-field/multireference and configuration interaction calculations at and around the equilibrium internuclear distance Re = 2.132 au of the CO 1Σ+ ground state. The results reveal that the 4 2Σ+ and the 5 2Σ+ state are located at 16.0 and 18.6 eV above the CO+ 2Σ+ ground state at Re = 2.132 au, respectively. The 5 2Σ+ state in the internuclear distance region R = 1.930−2.132 au and the 4 2Σ+ state around R ≈ 1.850 au are identified as the lowest Rydberg-type 2Σ+ state. This state has a predominant Rydberg-type configuration 4σ26σ1π4 which is composed of a C2+O ion core (4σ21π4) plus one electron in an s-type Rydberg orbital (6σ).