Theoretical investigation of the AΠ-XΣ, BΣ-XΣ, CΣ-XΣ, and EΠ-XΣ transitions of the CO molecule.

The spectrum of carbon monoxide is important for astrophysical media, such as planetary atmospheres, interstellar space, exoplanetary and stellar atmospheres; it also important in plasma physics, laser physics and combustion. Interpreting its spectral signature requires a deep and thorough understanding of its absorption and emission properties. A new accurate spectroscopic model for the ground and electronically-excited states of the CO molecule computed at the aug-cc-pV5Z CASSCF/MRCI+Q level is reported. Detailed investigation of the AΠ-XΣ, BΣ-XΣ, CΣ-XΣ, and EΠ-XΣ band systems is presented consisting of calculated potential energy curves as well as permanent and transition dipole moment curves. The BΣ and CΣ states are characterized by having multiple avoided crossings which are diabatized to obtain an accurate electronic structure model. The results are validated by comparing our computed spectra with various high-resolution spectroscopy experiments. To the best of our knowledge, this is the first systematic theoretical spectroscopic study of highly excited states of the CO molecule.