Penning ionization

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 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:

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

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.

See also

• Chemical ionization