Electron-Impact Ionization

diagram of ionization process

Electron-impact ionization can be dissociative, leading to charged and neutral fragments, or non-dissociative, leading to the parent ion. Both processes are illustrated here. The impact event indicated by the red arrow produces an ionic state at an energy above its dissociation limit, and the ion immediately fragments. The event indicated by the blue arrow produces a bound state of the parent cation. However, in many hydrocarbons and fluorocarbons, electron-impact ionization is completely or almost completely dissociative, even if the parent ion is thermodynamically stable. The diagram shows how this comes about: because the ionization process is practically instantaneous (the electron is moving far faster than the nuclei), the arrows are vertical. Thus the red arrow intersects the topmost potential curve in a repulsive region, above the dissociation limit, rather than near the minimum of that curve. The middle curve corresponds to an atypical situation in which the ionic state has a geometry similar to the neutral molecule's.