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# Penning ionization

Penning ionization is a form of chemi-ionization, an ionization process involving reactions between neutral atoms and/or molecules.[1][2][3] The term Penning ionization refers to the interaction between a gas-phase excited-state atom or molecule G* and a target molecule M resulting in the formation of a radical molecular cation M+., an electron e- and a neutral gas molecule G.

## Reactions

$G^* + M \to M^{+\bullet} + e^- + G$

Associative Penning ionization can occur:

$G^* + M \to MG^{+\bullet} + e^-$

Surface Penning ionization refers to the interaction of the excited-state gas with a surface S, resulting in the release of an electron.

$G^* + S \to G + S + e^-$

Penning ionization occurs when the target molecule has an ionization potential lower than the internal energy of the excited-state atom or molecule. The process was first reported by F.M. Penning in 1927.