Editing Ion source (section)

===Penning ionization===
{{Main|Penning ionization}}
[[Penning ionization]] is a form of chemi-ionization involving reactions between neutral atoms or molecules.<ref name="pmid17155624">{{cite journal |vauthors=Arango CA, Shapiro M, Brumer P |title=Cold atomic collisions: coherent control of penning and associative ionization |journal=Phys. Rev. Lett. |volume=97 |issue=19 |page=193202 |date=2006 |pmid=17155624 |doi=10.1103/PhysRevLett.97.193202 |bibcode=2006PhRvL..97s3202A|arxiv = physics/0610131 |s2cid=1480148 }}</ref><ref name="pmid17016831">{{cite journal |vauthors=Hiraoka K, Furuya H, Kambara S, Suzuki S, Hashimoto Y, Takamizawa A |title=Atmospheric-pressure Penning ionization of aliphatic hydrocarbons |journal=Rapid Commun. Mass Spectrom. |volume=20 |issue=21 |pages=3213–22 |date=2006 |pmid=17016831 |doi=10.1002/rcm.2706|bibcode=2006RCMS...20.3213H }}</ref> The process is named after the Dutch physicist [[Frans Michel Penning]] who first reported it in 1927.<ref>Penning, F. M. ''Die Naturwissenschaften'', 1927, '''15''', 818.  ''Über Ionisation durch metastabile Atome.''</ref> Penning ionization involves a reaction between a gas-phase excited-state atom or molecule G<sup>*</sup> and a target molecule M resulting in the formation of a radical molecular cation M<sup>+.</sup>, an electron e<sup>−</sup>, and a neutral gas molecule G:<ref>{{GoldBookRef|title=Penning gas mixture|file= P04476}}</ref>

:<chem>G^\ast{} + M -> G{} + M^{+\bullet}{} + e^- </chem>

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

Associative Penning ionization can proceed via

:<chem>G^\ast{} + M -> MG^{+\bullet}{} + e^-</chem>

Surface Penning ionization (also known as Auger deexcitation) refers to the interaction of the excited-state gas with a bulk surface S, resulting in the release of an electron according to

:<chem>G^\ast{} + S -> G{} + S{} + e^-</chem>.