[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][6] This process is helpful in mass spectrometry because it creates unique bands that can be used to identify molecules.
[7] This process is extremely common in nature as it is considered the primary initial reaction in flames.
[8] 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.
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] A certain amount of energy, which may be large enough, is required to remove an electron from an atom or a molecule in its ground state.
[12][13] 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 chemical change (for example, from a formation of a new chemical bond).
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.
[17] The term chemi-ionization was coined by Hartwell F. Calcote in 1948 in the Third Symposium on Combustion and Flame, and Explosion Phenomena.