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 ${\mathrm{HBr}}^{\mathrm{-}}$ and ${\mathrm{HI}}^{\mathrm{-}}$. At both the RCI and CASSCF/CI levels of theory, the ${}_{}{}^2{}_{}{}^{}\mathrm{\Sigma }_{}^{+}{}_{}{}^{}$ anion states are found to be repulsive in the Franck-Condon regions of the HX ${}_{}{}^1{}_{}{}^{}\Sigma _{}^{+}{}_{}{}^{}$ states and bound relative to the H+$X^{\mathrm{-}}$ asymptotes at r>$r_e$ 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 ${}_{}{}^2{}_{}{}^{}\mathrm{\Sigma }_{1/2}^{+}{}_{}{}^{}$ and ${}_{}{}^2{}_{}{}^{}\mathrm{\Pi }_{1/2}^{}{}_{}{}^{}$ states of ${\mathrm{HBr}}^{\mathrm{-}}$ and ${\mathrm{HI}}^{\mathrm{-}}$ are calculated in order to examine the origins of nonadiabatic ${\mathrm{H}}^{\mathrm{-}}$ fragment production observed in dissociative-attachment experiments on HBr.