Coupled-channel study of the Rydberg-valence interaction in HBr

We report an ab initio study of the low-lying valence and Rydberg states of HBr. The calculations are carried out employing the multireference single- and double-excitation configuration interaction method including the spin-orbit interaction. The first excited adiabatic potential of ¹Σ⁺ symmetry presents two minima which correspond to the Rydberg E¹Σ⁺ and valence V¹Σ⁺ observed states. We calculate the vibrational levels of these two states using a coupled-channel treatment based on the two diabatic potentials deduced from the ab initio adiabatic potentials and the Rydberg-valence interaction. The chaotic energy separations between the observed levels are well reproduced in the calculations. We have also obtained for the first time theoretical data for numerous Rydberg states of HBr lying in the (66-79) × 10³ cm^-1 excitation energy interval. The calculated spectroscopic parameters are found to be in good agreement with experiment and provide a basis for future studies of radiative and non-radiative processes in the HBr molecule.