A Fresh Look at Signatures of s-Wave Scattering: Symmetry and the Breakdown of the Born-Oppenheimer Approximation

The peculiar behavior of σ-type anionic states is analyzed in terms of the interaction between bound and continuum states. Within a simple Landau-Zener model, strong diabatic coupling leads to a broad avoided crossing and strongly mixed adiabatic states, whereas weak diabatic coupling leads to a narrow avoided crossing and an abrupt change in the character of the adiabatic states. By using symmetry analysis and ab initio calculations, we show that σ-type anionic states such as those in HCl, methyl chloride, and pyrrole interact more strongly with the continuum than π*-type anionic states such as the N₂^- resonance. Strong interaction leads to the breakdown of the Born-Oppenheimer approximation, which results in a finite probability to decay into the detachment continuum even in regions where the anionic state is energetically below the neutral state. We show that calculations with a complex absorbing potential capture this special character of σ-type anions through removal of the detachment continuum, affording diabatization of the anionic state while describing the decay into the continuum via the imaginary energy.