# Penning ionization

Penning ionization is a form of chemi-ionization, an ionization process involving reactions between neutral atoms or molecules.[1][2] The process is named after the Dutch physicist Frans Michel Penning who first reported it in 1927.[3] The Penning effect is put to practical use in applications such as gas-discharge neon lamps and fluorescent lamps, where the lamp is filled with a Penning mixture to improve the electrical characteristics of the lamps.

## Reaction

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:[4]

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

Penning ionization occurs when the target molecule has an ionization potential lower than the internal energy of the excited-state atom or molecule.

## Variants

Associative Penning ionization can also occur:

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

Surface Penning ionization (Auger Deexcitation) refers to the interaction of the excited-state gas with a surface S, resulting in the release of an electron:

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

The positive charge symbol $S^+$ 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.