Penning ionization is a form of chemi-ionization, an ionization process involving reactions between neutral atoms or molecules.^[1][2] 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.

The process is named after the Dutch physicist Frans Michel Penning who first reported it in 1927. Penning started to work at the Philips Natuurkundig Laboratorium at Eindhoven to continue the investigation of electric discharge on rare gases. Later, he started measurements on the liberation of electrons from metal surfaces by positive ions and metastable atoms, and especially on the effects related to ionization by metastable atoms.^[3]

Penning ionization refers to the interaction between an electronically excited gas-phase atom G^* and a target molecule M. The collision results in the ionization of the molecule yielding a cation M^+., an electron e⁻, and a neutral gas molecule, G, in the ground state.^[4] Penning ionization occurs via formation of a high energy collision complex, evolving toward the formation of a cationic species, by ejecting a high energy electron.^[5]

${\displaystyle {\ce {{G^{\ast }}+M->{M^{+\bullet }}+{e^{-}}+G}}}$

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

When the total electron excitation energy of colliding particles is sufficient, then the bonding energy of two particles that bonded together can also be contributed into the associative penning ionization act.^[6][7][8] Associative Penning ionization can also occur:

${\displaystyle {\ce {{G^{\ast }}+M->{MG^{+\bullet }}+e^{-}}}}$

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:

${\displaystyle {\ce {{G^{\ast }}+S->{G}+{S}+e^{-}}}}$

The positive charge symbol ${\displaystyle {\ce {S+}}}$ 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.

• Chemical ionization