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Penning ionization is a form of chemi-ionization, an ionization process involving reactions between neutral atoms or molecules. The process is named after the Dutch physicist Frans Michel Penning who first reported it in 1927. 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 in electronically excited-state G* and a target molecule M. It is a resulting in the formation of a radical molecular cation M+., an electron e−, and a neutral gas molecule G:


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

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

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.

Electron spectroscopy
Penning ionization has been applied to Penning ionization electron spectroscopy for gas chromatography detector in glow discharge by using the reaction for He$$ or Ne$$. The kinetic energy of electron ejected is analyzed by the collisions between target (gas or solid) and metastable atoms by scanning the retarding field in a flight tube of the analyzer in the presence of a weak magnetic field. The electron produced by reaction has a kinetic energy E determined by:


 * $$E = E_\text{m} + IE$$

The Penning ionization electron energy does not depend on the conditions of the experiments or any other species since both E$m$ and IE are atomic or molecular constants of the energy of He$$ and the ionization energy for the species.

Glow Discharge Mass Spectrometry
Glow discharge mass spectrometry is the direct determination of trace element in solid samples. It occurs with two ionization mechanisms: the direct electron impact ionization and Penning ionization. Processes inherent to the glow discharge, namely cathodic sputtering coupled with Penning ionization, yield an ion population from which semi-quantitative results can be directly obtained.