View source for Chemi-ionization

{{Use American English|date = February 2019}}
{{Short description|Ionization from excited state bonding}}
{{Distinguish|chemical ionization}}
'''Chemi-ionization''' is the formation of an [[ion]] through the reaction of a gas phase [[atom]] or [[molecule]] with another atom or molecule when the collision energy is below the [[Ionization energy|energy]] required to ionize the reagents.<ref name=":0">{{Cite Q|Q51494633}}</ref><ref>{{Cite Q|Q29541795}}</ref> The reaction may involve a reagent in an [[excited state]]<ref name=":1">{{Cite Q|Q55872037}}</ref> or may result in the formation of a new [[chemical bond]].<ref name=":0" /><ref name=Klucharev1993>{{citation | last = Klucharev | first =  A. N. | year = 1993 | title = Chemi-ionization processes | journal = Physics-Uspekhi | volume = 36 | pages = 486–512 | doi = 10.1070/PU1993v036n06ABEH002162 |bibcode = 1993PhyU...36..486K | issue = 6 }}</ref> Chemi-ionization can proceed through the [[Penning ionization|Penning]], associative, dissociative or rearrangement ionization reactions. Includes reactions that produce a free [[electron]] or a pair of [[Ion|ions]] (positive and negative).<ref>{{Cite Q|Q125499439}}</ref>{{sfn|Leonas, Kalinin|1977|loc=Introduction|p=280}}

This process is helpful in [[mass spectrometry]] because it creates unique bands that can be used to identify molecules.<ref name="DykeShaw1994">{{cite journal|last1=Dyke|first1=John M.|last2=Shaw|first2=Andrew M.|last3=Wright|first3=Timothy G.|title=Study of Chemiionization Reactions in the O + 2-Butyne Reaction Mixture|journal=The Journal of Physical Chemistry|volume=98|issue=25|year=1994|pages=6327–6331|issn=0022-3654|doi=10.1021/j100076a016}}</ref> This process is extremely common in nature as it is considered the primary initial reaction in flames.{{fact|date=December 2023}}

== Definitions ==
In the literature, the term "chemi-ionization" is used inconsistently.<ref>{{Cite Q|Q125693344}}</ref> [[R. Stephen Berry|Berry]] broadly defined chemi-ionization as ''"processes that lead to the formation of free charges, electrons and ions under the conditions of chemical reactions"''. Fontijn defined chemi-ionization more narrowly as reactions ''"in which the number of elementary charge carriers is increased as a direct result of the formation of new chemical bonds"''. Fontijn explicitly specified that the number of charge carriers increases, but Berry's definition includes the [[Penning ionization]].<ref>{{Cite Q|Q57948440}}</ref> In the 1977 review of ionization in collisions of atomic particles at low kinetic energies, Leonas and Kalinin stated that the ionization processes in which collisional energies are below the [[Ionization potential|ionization potentials]] are called chemi-ionization processes.{{sfn|Leonas, Kalinin|1977|loc=Abstract|p=279}}

The [[IUPAC]] defined chemi-ionization in the context of [[mass spectrometry]] as ''"ionization of an atom or molecule by interaction with another internally excited atom or molecule"''. The [[IUPAC]] definition includes only reactions that involve an atom or a molecule in an [[excited state]]. Also, IUPAC mentioned that chemi-ionization includes reactions in which chemical bonds are not changed.<ref name=":1" /> The older IUPAC definition (1973) didn't require the interaction to be with an atom or a molecule in an excited state, but mentioned that it is generally excited.<ref name=":3">{{Cite Q|Q125952233}}</ref> Also, the older definition stated that the ionization is the result of a collision,<ref name=":3" /> while the new definition refers to the ionization of one of the interacting species.<ref name=":1" />

== Energy requirements ==
A certain amount of [[Ionization energy|energy]], which may be large enough, is required to remove an electron from an atom or a molecule in its ground state.<ref name=":2" /><ref>{{Britannica|id=293063|title=Ionization energy|author=The Editors of Encyclopaedia Britannica}}</ref> In chemi-ionization processes, the energy consumed by the ionization must be stored in atoms or molecules in a form of potencial energy or can be obtained from an accompanying [[Exothermic reaction|exothermic]] [[chemical change]] (for example, from a formation of a new [[chemical bond]]). In atoms or molecules, the energy can be stored in the form of an [[excited state]]. In molecules, it can alternatively be stored in the form of vibrational excitation.<ref name=":2">{{Cite Q|Q125491026|chapter=Molecular beam studies of the dynamics of elementary chemical processes|pages=67−81|author1=V. Aquilanti|author2=G. G. Volpi|chapter-url=https://books.google.com/books?id=7YNIDwAAQBAJ&pg=PA71}}</ref> In exothermic chemical reactions, the released energy can be acquired by the molecule in the form of internal vibrational excitation and then cause ionization if the released energy is large enough.<ref>{{Cite Q|Q125738588|chapter=Chapter 3. Instrumental Aspects of Gas Chromatography|chapter-url=https://books.google.com/books?id=ULOd8AdW5McC&pg=PA262|p=262}}</ref>

==Reactions==
Chemi-ionization reactions include:{{sfn|Leonas, Kalinin|1977|loc=Introduction|p=280}}

<chem display="block">A^\ast{} + B{} -> A{} + B^{+}{} + e^{-}{} (Penning\ ionization)</chem><chem display="block">A{} + B{} -> AB^{+}{} + e^{-}{} (associative\ ionization)</chem><chem display="block">A{} + B{} -> A^{+}{} + B^{-}{} (ionization\ by\ electron\ transfer)</chem><chem display="block">A{} + BC{} -> AB^{+}{} + C{} + e^{-}{} (rearrangement\ ionization)</chem><chem display="block">A{} + BC{} -> A{} + B{} + C^{+}{} + e^{-}{} (dissociative\ ionization)</chem>

=== Reactions involving a reagent in an excited state ===
Chemi-ionization can be represented by
:<chem>G^\ast{} + M -> M^{+\bullet}{} + e^-{} + G</chem>
where G is the [[excited state]] species (indicated by the super-scripted asterisk), and M is the species that is ionized by the loss of an [[electron]] to form the [[Radical (chemistry)|radical]] [[cation]] (indicated by the super-scripted "plus-dot").

==Astrophysical implications==
Chemi-ionization has been postulated to occur in the hydrogen rich atmospheres surrounding stars. This type of reaction would lead to many more excited hydrogen atoms than some models account for. This affects our ability to determine the proper optical qualities of solar atmospheres with modeling.<ref name="MihajlovIgnjatović2011">{{cite journal|last1=Mihajlov|first1=Anatolij A.|last2=Ignjatović|first2=Ljubinko M.|last3=Srećković|first3=Vladimir A.|last4=Dimitrijević|first4=Milan S.|title=Chemi-Ionization in Solar Photosphere: Influence on the Hydrogen Atom Excited States Population|journal=The Astrophysical Journal Supplement Series|volume=193|issue=1|year=2011|pages=2|issn=0067-0049|doi=10.1088/0067-0049/193/1/2|arxiv = 1105.2134 |bibcode = 2011ApJS..193....2M }}</ref>

== In flames ==
The most common example of chemi-ionization occurs in hydrocarbon flame. The reaction can be represented as
:<chem>O + CH -> HCO+ + e^-</chem> <ref name="VinckierGardner1977">{{cite journal |last1=Vinckier |first1=C. |last2=Gardner |first2=Michael P. |last3=Bayes |first3=Kyle D. |year=1977 |title=A study of chemi-ionization in the reaction of oxygen atoms with acetylene |journal=The Journal of Physical Chemistry |volume=81 |issue=23 |pages=2137–2143 |doi=10.1021/j100538a001 |issn=0022-3654}}</ref>

This reaction is present in any hydrocarbon flame and can account for deviation in the amount of expected ions from [[thermodynamic equilibrium]].<ref name="FontijnMiller1965">{{cite journal |last1=Fontijn |first1=A. |last2=Miller |first2=W.J. |last3=Hogan |first3=J.M. |year=1965 |title=Chemi-ionization and chemiluminescence in the reaction of atomic oxygen with C2H2, C2D2, and C2H4 |journal=Symposium (International) on Combustion |volume=10 |issue=1 |pages=545–560 |doi=10.1016/S0082-0784(65)80201-6 |issn=0082-0784}}</ref>

== History ==
The term chemi-ionization was coined by Hartwell F. Calcote in 1948 in the Third Symposium on Combustion and Flame, and Explosion Phenomena.<ref name="Calcote1948">{{cite journal |last1=Calcote |first1=Hartwell F. |year=1948 |title=Electrical properties of flames |journal=Symposium on Combustion and Flame, and Explosion Phenomena |volume=3 |issue=1 |pages=245–253 |doi=10.1016/S1062-2896(49)80033-X |issn=1062-2896 |hdl-access=free |hdl=2027/uva.x030313059}}</ref>  The Symposium performed much of the early investigation into this phenomenon in the 1950s.{{Citation needed|date=May 2024}} The majority of the research on this topic was performed in the 1960s and '70s.{{Citation needed|date=May 2024}} It is currently seen in many different ionization techniques used for mass spectrometry.<ref name="ChenYu2010">{{cite journal |last1=Chen |first1=Lee Chuin |last2=Yu |first2=Zhan |last3=Hiraoka |first3=Kenzo |year=2010 |title=Vapor phase detection of hydrogen peroxide with ambient sampling chemi/chemical ionization mass spectrometry |journal=Analytical Methods |volume=2 |issue=7 |pages=897 |doi=10.1039/c0ay00170h |issn=1759-9660}}</ref><ref name="MasonWilliams2004">{{cite journal |last1=Mason |first1=Rod S. |last2=Williams |first2=Dylan R. |last3=Mortimer |first3=Ifor P. |last4=Mitchell |first4=David J. |last5=Newman |first5=Karla |year=2004 |title=Ion formation at the boundary between a fast flow glow discharge ion source and a quadrupole mass spectrometer |journal=Journal of Analytical Atomic Spectrometry |volume=19 |issue=9 |pages=1177 |doi=10.1039/b400563p |issn=0267-9477}}</ref>

==See also==
*[[Penning ionization]]
*[[Associative ionization]]
*[[Charge-exchange ionization]]
*[[Auger effect]]

==References==
{{Reflist}}

== Bibliography ==
* {{Cite Q|Q125875406
 |ref = {{harvid|Leonas, Kalinin|1977}}
}}
* {{Cite Q|Q56226700
 |ref = {{harvid|Klucharev|1993}}
}}

[[Category:Ion source]]