The
quantum yield of a
radiationIn physics, radiation describes any process in which energy emitted by one body travels through a medium or through space, ultimately to be absorbed by another body...
-induced process is the number of times that a defined event occurs per
photonIn physics, a photon is an elementary particle, the quantum of the electromagnetic field and the basic "unit" of light and all other forms of electromagnetic radiation. It is also the force carrier for the electromagnetic force...
absorbed by the system. Thus, the quantum yield is a measure of the efficiency with which absorbed light produces some effect.
Quantum yield can be defined by the equation:
Q= photons emitted/photons absorbed.
Quantum yield is essentially the emission efficiency of a given fluorochrome.
For example, in a chemical
photodegradationPhotodegradation is degradation of a photodegradable molecule caused by the absorption of photons, particularly those wavelengths found in sunlight, such as infrared radiation, visible light and ultraviolet light. However, other forms of electromagnetic radiation can cause photodegradation...
process, when a molecule falls apart after absorbing a light
quantumIn physics, a quantum is the minimum unit of any physical entity involved in an interaction. An example of an entity that is quantized is the energy transfer of elementary particles of matter and of photons and other bosons...
, the quantum yield is the number of destroyed molecules divided by the number of photons absorbed by the system.
The
quantum yield of a
radiationIn physics, radiation describes any process in which energy emitted by one body travels through a medium or through space, ultimately to be absorbed by another body...
-induced process is the number of times that a defined event occurs per
photonIn physics, a photon is an elementary particle, the quantum of the electromagnetic field and the basic "unit" of light and all other forms of electromagnetic radiation. It is also the force carrier for the electromagnetic force...
absorbed by the system. Thus, the quantum yield is a measure of the efficiency with which absorbed light produces some effect.
Quantum yield can be defined by the equation:
Q= photons emitted/photons absorbed.
Quantum yield is essentially the emission efficiency of a given fluorochrome.
Example
For example, in a chemical
photodegradationPhotodegradation is degradation of a photodegradable molecule caused by the absorption of photons, particularly those wavelengths found in sunlight, such as infrared radiation, visible light and ultraviolet light. However, other forms of electromagnetic radiation can cause photodegradation...
process, when a molecule falls apart after absorbing a light
quantumIn physics, a quantum is the minimum unit of any physical entity involved in an interaction. An example of an entity that is quantized is the energy transfer of elementary particles of matter and of photons and other bosons...
, 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
hydrogenHydrogen is the chemical element with atomic number 1. It is represented by the symbol H. At standard temperature and pressure, hydrogen is a colorless, odorless, nonmetallic, tasteless, highly flammable diatomic gas with the molecular formula H
2...
with
chlorineChlorine Chlorine Chlorine ( , from the Greek word 'χλωρóς' (khlôros, meaning 'pale green'), is the chemical element with atomic number 17 and symbol Cl. It is a halogen, found in the periodic table in group 17 (formerly VII, VIIa, or VIIb). As the chloride ion, which is part of common salt and...
, in which a few hundred molecules of
hydrochloric acidHydrochloric acid is the solution of hydrogen chloride in water. It is a highly corrosive, strong mineral acid and has major industrial uses. It is found naturally in gastric 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
singletIn theoretical physics, a singlet usually refers to a one-dimensional representation . It may also refer to two or more particles prepared in a correlated state, such that the total angular momentum of the state is zero....
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 (excitation wavelength, slit widths, photomultiplier voltage etc.) 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 (on a wavelength scale),
A is absorbance at the excitation wavelength, and
n is the refractive index of the sample. The subscript R denotes the respective values of the reference substance.
An improved determination is done by measuring the emission intensities over a range of optical densities, usually with and using the slope of the resulting line instead. In this case, the equation becomes:
where is the slope of the integrated fluorescence intensity with respect to the absorbance at the excitation wavelength of the sample and the subscript
samp (as in ) denotes the values corresponding to the sample to be determined.