Polyatomic molecular entities luminesce with appreciable yield only from the lowest excited state of a given multiplicity. There are exceptions to this rule.
The concept has been extended to the reaction of excited species, i.e., polyatomic molecular entities react with appreciable yield only from the lowest @E02257@ of a given multiplicity.
PAC, 2007, 79, 293. 'Glossary of terms used in photochemistry, 3rd edition (IUPAC Recommendations 2006)' on page 360 (https://doi.org/10.1351/pac200779030293)