Within the 'Born-Oppenheimer' approximation, a reaction of an excited state species that occurs on a single 'potential-energy surface'.
See also: diabatic photoreaction
Source:
PAC, 1996, 68, 2223. 'Glossary of terms used in photochemistry (IUPAC Recommendations 1996)' on page 2227 (https://doi.org/10.1351/pac199668122223)