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:
PAC, 1996, 68, 2223
(Glossary of terms used in photochemistry (IUPAC Recommendations 1996))
on page 2277
PAC, 1984, 56, 231
(Nomenclature, symbols, units and their usage in spectrochemical analysis-Part VI:
molecular luminescence spectroscopy)
on page 234