Definition of Quantum Yield

The number of defined events which occur per photon absorbed by the system.

The integral quantum yield is

For a photochemical reaction,

The differential quantum yield is

where d[X]/dt is the rate of change of the amount of (substance) concentration of a measurable quantity, and n the amount of photons (mol or its equivalent einsteins) absorbed per unit time.

can be used for photophysical processes or photochemical reactions.

IUPAC PHOTOCHEMICAL GLOSSARY (1992).