Chemi-ionization

Source: Wikipedia, the free encyclopedia.

Not to be confused with 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 energy required to ionize the reagents.[1][2] The reaction may involve a reagent in an excited state[3] or may result in the formation of a new chemical bond.[1][4] Chemi-ionization can proceed through the Penning, associative, dissociative or rearrangement ionization reactions. Includes reactions that produce a free electron or a pair of ions (positive and negative).[5]

This process is helpful in mass spectrometry because it creates unique bands that can be used to identify molecules.[6] This process is extremely common in nature as it is considered the primary initial reaction in flames.[citation needed]

Definitions

In the literature, the term "chemi-ionization" is used inconsistently.[7] 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.[8]

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.[3]

Energy requirements

A certain amount of

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.[9] 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.[11]

Reactions

Chemi-ionization reactions include:[12]

Reactions involving a reagent in an excited state

Chemi-ionization can be represented by

where G is the

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.[13]

In flames

The most common example of chemi-ionization occurs in hydrocarbon flame. The reaction can be represented as

[14]

This reaction is present in any hydrocarbon flame and can account for deviation in the amount of expected ions from thermodynamic equilibrium.[15]

History

The term chemi-ionization was coined by Hartwell F. Calcote in 1948 in the Third Symposium on Combustion and Flame, and Explosion Phenomena.[16] The Symposium performed much of the early investigation into this phenomenon in the 1950s. The majority of the research on this topic was performed in the 1960s and '70s. It is currently seen in many different ionization techniques used for mass spectrometry.[17][18]

See also

References

  1. ^
    Wikidata Q51494633
    .
  2. Wikidata Q29541795
    .
  3. ^
    Wikidata Q55872037
    .
  4. doi:10.1070/PU1993v036n06ABEH002162
  5. Wikidata Q125499439
    .
  6. ISSN 0022-3654
    .
  7. Wikidata Q125693344
    .
  8. Wikidata Q57948440
    .
  9. ^
    Wikidata Q125491026
    .
  10. ^ The Editors of Encyclopaedia Britannica, Ionization energy at the Encyclopædia Britannica
  11. Wikidata Q125738588
    .
  12. Wikidata Q125875406
    .
  13. ISSN 0067-0049
    .
  14. ISSN 0022-3654
    .
  15. ISSN 0082-0784
    .
  16. ISSN 1062-2896
    .
  17. ISSN 1759-9660
    .
  18. ISSN 0267-9477
    .
Retrieved from "https://en.wikipedia.org/w/index.php?title=Chemi-ionization&oldid=1223256935"