is a form of chemi-ionization
, an ionization process involving reactions between neutral atoms and/or molecules. The process is named after the Dutch physicist Frans Michel Penning
who first reported it in 1927.Penning, F. M. Die Naturwissenschaften
, 1927, 15
, 818. Über Ionisation durch metastabile Atome.
Penning ionization refers to the interaction between a gas-phase excited-state atom or molecule G<sup>*</sup> and a target molecule M resulting in the formation of a radical molecular cation M<sup>+.</sup>, an electron e<sup>−</sup>, and a neutral gas molecule G:
- <math>G^* + M to M^ + e^- + G</math>
Penning ionization occurs when the target molecule has an ionization potential
lower than the internal energy of the excited-state atom or molecule. Associative
Penning ionization can also occur:
- <math>G^* + M to MG^ + e^-</math>
Surface Penning ionization refers to the interaction of the excited-state gas with a surface S, resulting in the release of an electron.
- <math>G^* + S to G + S + e^-</math>
(The positive charge symbol <math>S^+</math> that would appear to be required for charge conservation is omitted, because S is a macroscopic surface and the loss of one electron has a negligible effect.)