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 GrotthusDraper 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 10^{6} 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.
