Core-excited resonances

Dissociative electron attachment (DEA) occurs when the molecular transient anion state is dissociative in the Franck-Condon (FC) region, the localization time is of the order of or larger than the time required for dissociation along a particular nuclear coordinate, and one of the resulting fragments has positive electron affinity. In this case, a stable atomic or molecular anion is formed along with one or more neutral species. Dissociative electron attachment usually occurs via the formation of core-excited resonances since these possess sufficiently long lifetimes to allow for dissociation of the anion before autoionization. 

Temporary anion states may be broadly classified either as shape resonances or core-excited resonances (4). The former are well described by a configuration in which the impacting electron attaches to an atom or molecule in one of the originally unoccupied orbitals. In the latter, electron capture is accompanied by electronic excitation, giving rise to a temporary anion with a two-particle-one-hole (2p-lh) configuration. One can further distinguish core-excited resonances into those In which the resonance lies energetically below its parent state and those in which it lies above. The former are referred to as Feshbach resonances and the latter as core-excited... 

The spectrum serves to Illuminate several of the characteristics of core-excited resonances. The "doublet" structure, repeated at intervals of approximately 170 meV, is characteristic of the >2 C-C symmetric stretch. The spacing between the first and second features in each pair is 60 meV. We attribute these to two quanta of the CH2 torsional mode, l.e. 2V1,. These characteristic energies in the anion are quite close to those of the Rydberg "parent" state, as expected. The existence of the low frequency modes is a clear indication of the long lifetime of the Feshbach resonance relative to that of the B2g shape resonance discussed previously. 

From the above discussion we see that electron-molecule interactions can lead to transient negative ion (TNI) formation and subsequently to a variety of chemical changes. On TNI formation, an extra electron is captured into the unoccupied molecular orbital (UMO) of the neutral molecule, and a shape or core-excited resonance results (see O Fig. 34-4). 

Schematic diagram showing the electronic configuration of a neutral (a) and transient negative ion (TNI) (b, c). The interacting electron initially captures into the unoccupied MOs of the neutral molecule resulting in TNI formation via (a) shape resonance or (c) core-excited resonance. For a shape resonance, the electron can interact with any unoccupied MO. The SOMO was the empty LUMO before the LEE interaction. In core-excited resonance, on electron interaction an electronic transition takes place from an inner shell to the vacant MOs creating a "hole"(-i- charge) in the inner shell, shown by an arrow (c). The up and down arrows show the occupancy of the molecular orbitals (MOs) with electrons of ocand 3 spins. HOMO highest occupied molecular orbital, LUMO lowest unoccupied molecular orbital, SOMO singly occupied molecular orbital...

Finally, it bears mention that where anions are involved, solvation effects can often have a qualitative impact on the basic physical picture, especially in polar solvents where M is likely to be dramatically stabilized with respect to M. Polar solvation might, for example, convert an open channel, core-excited resonance (M ) into a core-excited Feshbach resonance by drawing the (M ) potential curve below that of M. In such a case, the solution-phase anion resonance would be expected to have a significantly longer lifetime as compared to its gas-phase analogue. ... 

---