From a phenomenological point of view, the term is used to describe the process by
which a molecular entity absorbs light and a phenomenon originates from the excited state
of another molecular entity. In mechanistic photochemistry
the term has been reserved for the photophysical process in which an excited state
of one molecular entity (the donor) is deactivated to a lower-lying state by transferring
energy to a second molecular entity (the acceptor) which is thereby raised to a higher
energy state. The excitation may be electronic, vibrational, rotational or translational.
The donor and acceptor may be two parts of the same molecular entity, in which case
the process is called intramolecular
PAC, 1996, 68, 2223
(Glossary of terms used in photochemistry (IUPAC Recommendations 1996))
on page 2240
IUPAC. Compendium of Chemical Terminology, 2nd ed. (the "Gold Book"). Compiled by
A. D. McNaught and A. Wilkinson. Blackwell Scientific Publications, Oxford (1997).
XML on-line corrected version: http://goldbook.iupac.org (2006-) created by M. Nic,
J. Jirat, B. Kosata; updates compiled by A. Jenkins. ISBN 0-9678550-9-8. https://doi.org/10.1351/goldbook