Ro-vibronic Transitions Between the B²Σ⁺-A²Σ⁺ States of the OH Radical

The spectrum of the hydroxyl radical OH plays a crucial role in diverse environments, including planetary atmospheres, astrophysical media, and combustion processes. Accurate modeling of its spectral signatures requires reliable descriptions of rovibronic transitions. In this work, we present a new high-resolution spectroscopic study of the B²Σ⁺-A²Σ⁺ band system of the OH radical. Transition Dipole Moment Functions (TDMF) were computed using the multireference configuration interaction method with Davidson correction (MRCI+Q) and the aug-cc-pV5Z basis set. These TDMF, along with the potential energy curves from literature, were employed in LeRoy’s LEVEL program to generate Transition Dipole Moment Matrix Elements (TDMMEs). The TDMMEs were then transformed to Hund’s case (a) to account for electron spin effects. The resulting dataset provides a fully resolved set of rovibronic transitions, including line position, line strength, Einstein A coefficients and the detailed quantum numbers (e/f parity, F1/F2, v, J, N). To our knowledge, this represents the first systematic spectroscopic model for the B²Σ⁺-A²Σ⁺ system computed using post-Hartree-Fock methods and a quintuple-zeta basis. Importantly, it incorporates all rovibronic transitions within this band system, including weak lines often neglected in previous studies. This work is the most complete and accurate resource to date for modeling the B²Σ⁺-A²Σ⁺ band system of OH.