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  130, 4745 – 4758 (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.

Download this paper (PDF)

Related Research

EOM-CC methods

Resonances