The quantum yield of a radiation-induced process
is the number of times a specific event occurs
per photon absorbed by the system.
The "event" is typically a kind of chemical
reaction.
Applications
The quantum yield for the decomposition of
a reactant molecule in a decomposition reaction
is defined as:
Quantum yield can also be defined for other
events, such as fluorescence:
Here, quantum yield is the emission efficiency
of a given fluorophore.
Examples
Quantum yield is used in modeling photosynthesis:
In a chemical photodegradation process, when
a molecule falls apart after absorbing a light
quantum, the quantum yield is the number of
destroyed molecules divided by the number
of photons absorbed by the system.
Since not all photons are absorbed productively,
the typical quantum yield will be less than
1.
Quantum yields greater than 1 are possible
for photo-induced or radiation-induced chain
reactions, in which a single photon may trigger
a long chain of transformations.
One example is the reaction of hydrogen with
chlorine, in which a few hundred molecules
of hydrochloric acid are typically formed
per quantum of blue light absorbed.
In optical spectroscopy, the quantum yield
is the probability that a given quantum state
is formed from the system initially prepared
in some other quantum state.
For example, a singlet to triplet transition
quantum yield is the fraction of molecules
that, after being photoexcited into a singlet
state, cross over to the triplet state.
The fluorescence quantum yield is defined
as the ratio of the number of photons emitted
to the number of photons absorbed.
Experimentally, relative fluorescence quantum
yields can be determined by measuring fluorescence
of a fluorophore of known quantum yield with
the same experimental parameters as the substance
in question.
The quantum yield is then calculated by:
where is the quantum yield, Int is the area
under the emission peak, A is absorbance at
the excitation wavelength, and n is the refractive
index of the solvent.
The subscript R denotes the respective values
of the reference substance.
See also
Quantum Efficiency
References
