Term: spin conservation rule (Wigner rule)
https://doi.org/10.1351/goldbook.S05862

Definition:
For both radiative and radiationless transitions, the principle that transitions between terms of the same multiplicity are spin-allowed, while transitions between terms of different multiplicity are spin-forbidden.

Source: Orange Book, 2nd ed., p. 185 (https://media.iupac.org/publications/analytical_compendium/)

Citation: 'spin conservation rule (Wigner rule)' 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.S05862

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