A transition between two states of a molecular entity, the energy difference being emitted or absorbed as photons. In principle, radiative and radiationless transitions can be distinguished in molecules. The first occur by absorption or emission of light quanta, and the latter is the result of the transformation of electronic excitation energy into vibrational/rotational energy.
See also: radiationless transition
Orange Book, 2nd ed., p. 185 (http://media.iupac.org/publications/analytical_compendium/)
PAC, 1984, 56, 231. 'Nomenclature, symbols, units and their usage in spectrochemical analysis-Part VI: molecular luminescence spectroscopy' on page 234 (https://doi.org/10.1351/pac198456020231)
PAC, 1996, 68, 2223. 'Glossary of terms used in photochemistry (IUPAC Recommendations 1996)' on page 2270 (https://doi.org/10.1351/pac199668122223)