View source for Tarjan's strongly connected components algorithm

'''Penning ionization''' is a form of [[chemi-ionization]], an ionization process involving reactions between neutral atoms or molecules.<ref name="pmid17155624">{{cite journal |author=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 |pages=193202 |year=2006 |pmid=17155624 |doi=10.1103/PhysRevLett.97.193202 |bibcode=2006PhRvL..97s3202A|arxiv = physics/0610131 }}</ref><ref name="pmid17016831">{{cite journal |author=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 |year=2006 |pmid=17016831 |doi=10.1002/rcm.2706}}</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>
The Penning effect is put to practical use in applications such as gas-discharge [[neon lamp]]s and [[fluorescent lamp]]s, 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<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>

:<math>G^* + M \to M^{+\bullet} + e^- + G</math>

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 ionization|Associative]] Penning ionization can also occur:

:<math>G^* + M \to MG^{+\bullet} + e^-</math>

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:

:<math>G^* + S \to G + S + e^-</math>

The positive charge symbol <math>S^+</math> 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]]

==References==
{{reflist}}

[[Category:Ion source]]