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.
PAC, 1996, 68, 2223
(Glossary of terms used in photochemistry (IUPAC Recommendations 1996))
on page 2248
See also: Orange Book, p. 187