According to the principle of photochemical equivalence, there is a one-to-one relationship
between the number of photons absorbed by a system and the number of excited species
produced. The principle often fails for light of high intensity (e.g.
laser beams), when molecules may absorb more than one photon.
Source:
PAC, 1996, 68, 149
(A glossary of terms used in chemical kinetics, including reaction dynamics (IUPAC
Recommendations 1996))
on page 176