Theoretical study of the negative ions of HBr and HI

Relativistic-configuration-interaction (RCI) and complete-active-space (CAS) multiconfiguration self-consistent-field (MCSCF)/CI calculations are performed for the low-lying anion states of HBr^- and HI^-. At both the RCI and CASSCF/CI levels of theory, the ²Σ⁺ anion states are found to be repulsive in the Franck-Condon regions of the HX ¹Σ⁺ states and bound relative to the H+X^- asymptotes at r>rₑ of the parent neutral states. The HI molecule is predicted to have a slightly positive adiabatic electron affinity, and electron attachment in this system is considered in a Langevin model. Electronic transition moments between the ²Σ1/2⁺ and ²Π1/2 states of HBr^- and HI^- are calculated in order to examine the origins of nonadiabatic H^- fragment production observed in dissociative-attachment experiments on HBr.