Quantum Yield

by Scott Prahl, Oregon Medical Laser Center

First Law of Photochemistry

Quantum yield has at least two basic definitions, but both are based on the actual numbers of photons absorbed. This is a consequence of the first law of photochemistry (the Grotthus-Draper law) that states

Only light absorbed by a molecule can produce photochemical change.
By basing the definitions on the number of photons absorbed, the specifics of chemical concentration and illumination become of less importance.

Fluorescence Quantum Yield

The definition usually used in spectroscopy is that

Quantum yield is the number of photons emitted per photon absorbed.
This quantum yield is between zero and one. The quantum yield should be constant for a particular molecule, but not surprisingly is not always. The quantum yield can vary with solvent, with temperature, and with concentration.

Photochemical Quantum Yield

There is a secondary definition of quantum yield that is popular in photochemistry:

Quantum yield is the number of molecules of reactant consumed per photon of light absorbed.
Another definition is
The quantum yield of a given photochemical product is the probability that the absorption of one photon by the system leads to one molecule of that product.
Clearly if the emitted photons are considered product, then this definition encompasses the fluorescence definition above. This number ranges from 10-6 to more than 106 depending on the photochemical reaction. When the photochemical quantum yield exceeds one then secondary reactions take place; a value greater than two suggests a chain reaction. The quantum yield is often separated into primary quantum yield and secondary quantum yields.

© 1998 OMLC. contact: Scott Prahl