A photophysical process. An isoenergetic radiationless transition between two electronic states having different multiplicities. It often results in a vibrationally excited molecular entity in the lower electronic state, which then usually deactivates to its lowest vibrational level.
Source:
PAC, 1996, 68, 2223. 'Glossary of terms used in photochemistry (IUPAC Recommendations 1996)' on page 2248 (https://doi.org/10.1351/pac199668122223)
See also:
Orange Book, 2nd ed., p. 187 (https://media.iupac.org/publications/analytical_compendium/)