Electron-induced reactions

When an electron is attached or transferred to a molecular system, it is as a rule put into an anti-bonding orbital. This leads to forces on the nuclei favoring bond-breaking processes, and in this way the energy of the incoming electron is channeled into nuclear motions. Electron-induced processes play prominent roles in many contexts ranging from astro- and plasma physics over the SNR1 reaction mechanism and the chemistry of the atmosphere, to radiation damage of living tissue.

As a simple example, consider electron attachment to the nitrobenzene molecule: Ph-NO2. Putting an electron into one of the pi*-orbitals of the benzene ring yields a metastable radical anion, a so called resonance state. This resonance is the key intermediate in the displayed reaction scheme. It can decay by

  1. electron autodetachment possibly leaving the molecule in an excited state
  2. dissociation into a phenyl radical and NO2-, or any other energetically accessible radical/anion pair
  3. a stable Ph-NO2- radical anion can be formed, if there is a way to dissipate energy to an environment, say a cluster, or if the molecule sits on a surface

Note that the double arrows do not symbolize an equilibrium, but rather that both directions are possible. The intermediate resonance state can decay in several ways, but it can also be made in different experiments: electron scattering, ion-neutral collisions, or photo-excitation. In other words, there are many pathways through this reaction scheme, and many come with their own names: photo-detachment, induced excitation, dissociative attachment, associative detachment, etc.

Note also that analogous reaction schemes exist for total charges different from -1. Examples for processes in the analogous schemes are photo-ionization and dissociative recombination for a total charge of 0, Auger decay for a total charge of +1, and for multiply charged species there are various Coulomb explosion pathways.

Studying the full dynamics of an electron-induced processes is today, even for small systems, still completely out of question. Typically we concentrate on the properties of a specific intermediate, on a single reaction step, or on building models for more complex processes. So studying electron-induced reactions is intimately related to having methods for resonance states. Another way to induce reactions is capture in a dipole-bound state followed by electron transfer into a valence state.

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