A fresh look at signatures of s-wave scattering: Symmetry and the breakdown of the Born–Oppenheimer approximation

R. Moorby, M. Mukherjee, C. Utku, T.-C. Jagau, and A.I. Krylov
J. Phys. Chem. A , submitted (2026)

The peculiar behavior of sigma-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 numeric calculations, we show that sigma-type anionic states such as those in HCl, methyl chloride, and pyrrole interact much stronger with the continuum than pi-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 the 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 sigma-type anions through removal of the detachment continuum, affording diabatization of the anionic state, while describing the decay into the continuum via imaginary energy Gamma.

Related Research

EOM-CC methods

Resonances