Term: appearance energy (appearance potential)
https://doi.org/10.1351/goldbook.A00421

Definition:
Refers to ionization of a molecule or atom by electron collision or photon absorption. In mass spectrometry it has often been reported as the voltage which corresponds to the minimum energy of the electrons in the ionizing beam necessary for the production of a given fragment ion. In photoionization it is the minimum energy of the quantum of light which produces ionization of the absorbing molecule.

Related Terms:
1) ionization (http://doi.org/10.1351/goldbook.I03183).
2) mass spectrometry (http://doi.org/10.1351/goldbook.M03746).
3) photoionization (http://doi.org/10.1351/goldbook.P04620).

Source: PAC, 1990, 62, 2167. 'Glossary of atmospheric chemistry terms (Recommendations 1990)' on page 2175 (https://doi.org/10.1351/pac199062112167)

Citation: 'appearance energy (appearance potential)' in IUPAC Compendium of Chemical Terminology, 5th ed. International Union of Pure and Applied Chemistry; 2025. Online version 5.0.0, 2025. https://doi.org/10.1351/goldbook.A00421

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