The rovibronic spectrum of carbon monoxide (CO) including its excited states

The spectroscopic nature of CO plays an essential part in astrophysical applications and the radiation transfer process. While existing databases provide high-accuracy data for ground-state infrared transitions, there is a lack of information on the transitions in the vacuum ultraviolet (VUV) region and high-temperature electronic excitations. We present a comprehensive spectroscopic model for CO that explicitly accounts for eight electronic states (X ¹Σ⁺, A ¹Π, I ¹Σ^-, D ¹Δ, a' ³Σ⁺, a ³Π, d ³Δ and e ³Σ^-), incorporating spin-orbit couplings and electronic angular momentum couplings. The potential energy curves (PECs) of the electronic states are reconstructed by combining the Rydberg-Klein-Rees method and ab initio calculations, providing an effective description for the entire internuclear distances. We demonstrate that while the X ¹Σ⁺-X ¹Σ⁺ transition is the primary transition process for transition frequencies below 40 000 cm^-1, the X ¹Σ⁺-A ¹Π system becomes the dominant radiation transition in the VUV region (above 60 000 cm^-1) as temperatures exceed 3000 K, remaining the same up to 8000 K. By extending the spectral coverage, this work enables more spectroscopic characterization and radiative transfer modeling for high-temperature stellar atmospheres and interstellar VUV observations.