The collision of an electron with a small molecular ion is dominated by resonant processes. Every state of the ion can serve as a parent ion for a series of neutral states. Below the ground state, these are a Rydberg series of neutral states converging to the ion. However, if the parent state is an excited state of the ion, these states are doubly excited (Feshbach) resonances that lie below the direct dissociation threshold, and are formed when an incoming electron excites the target ion and attaches to a Rydberg orbital. Capture into these states can initiate a number of resonant processes. As the neutral molecule evolves in time, the system can autoionize, meaning the electron can be re-emitted, returning the molecule to its original electronic state. When the ion returns to its ground state, it may have enough energy to dissociate. If it does not have enough energy to dissociate to products, the target ion is left in some state of vibrational excitation. This process provides an efficient path to dissociation at energies below the direct excitation level. If, while evolving in time, the neutral fragments reach an internuclear separation beyond which autoionization is no longer possible, the states are considered electronically bound and the result is dissociative recombination. Depending on the electron affinity and ionization potential of the final products, it may be energetically possible to have ion-pair formation. The study of these processes requires both the accurate treatment of the electron scattering processes, but must also include an accurate representation of the potential energy curves, both for electronically bound states and the resonant states. In addition, the couplings between these states, both the coupling between the resonant states and the scattering continuum (the autoionization width) and the non-adiabatic coupling between all states are needed to complete describe the cross section including the branching ratios into final states. These are obtained using structure calculations as well as scattering calculations, using the complex Kohn variational method. We will illustrate our results with examples on HeH+, O2+ and H2O+. AEO acknowledges support by the National Science Foundation, with some of this material based on work while serving at NSF. ÅL acknowledges support from the Swedish Research Council and the Carl-Trygger Foundation.

Resonances in electron-molecular ion scattering

251st National Meeting (2016)

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