The fission of a bond originating at an atom which is adjacent to one assumed to bear the charge; the definition of β-, γ-cleavage then follows automatically. The process:
A00004.png
would thus be described as α-fission of a ketone with expulsion of a radical R1·. The carbon atoms of the radical R1· are called the α-, β-, γ-carbons, starting with the atom nearest the functional group.
Source: PAC, 1991, 63, 1541. 'Recommendations for nomenclature and symbolism for mass spectroscopy (including an appendix of terms used in vacuum technology). (Recommendations 1991)' on page 1558 (https://doi.org/10.1351/pac199163101541)
Homolytic cleavage of a bond connecting an atom or group to an excited chromophore. Often applied to a bond connected to a carbonyl group, in which case it is called a Norrish Type I photoreaction.
Note: This reaction should be distinguished from an @A00007@.
Source: PAC, 2007, 79, 293. 'Glossary of terms used in photochemistry, 3rd edition (IUPAC Recommendations 2006)' on page 302 (https://doi.org/10.1351/pac200779030293)