A photophysical process. An isoenergetic @R05057@ between two electronic states having different multiplicities. It often results in a vibrationally excited molecular entity in the lower @ET07026@, 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 [Terms] [Paper]
See also:
Orange Book, 2nd ed., p. 187 [Terms] [Book]