In a section on ionization efficiency and ionization cross section, my mass spectrometry textbook, Mass Spectrometry by Jürgen Gross [1, pp. 38–39], says the following:
The ionization energy represents the absolute minimum energy required for ionization of the neutral concerned. This means in turn that in order to effect ionization, the impacting electrons need to carry at least this amount of energy. If this energy were then to be quantitatively transferred during the collision, ionization would take place. Obviously, such an event is of rather low probability and therefore, the ionization efficiency is close to zero with electrons carrying just the IE of the pertinent neutral. However, a slight increase in electron energy brings about a steady increase in ionization efficiency.
Why is such an event of "rather low probability"? It seems to me that if the impacting electrons are carrying at least the necessary ionization energy, as the author suggests, then shouldn't ionization occur (be a certainty or, at least, a high probability event)?
I would greatly appreciate it if people could please take the time to clarify this.
- Gross, J. H. Mass Spectrometry: A Textbook, 3rd ed.; Springer International Publishing: Cham, Switzerland, 2017. ISBN 978-3-319-54397-0.