Photoluminescence (abbreviated as
PL) is a process in which a substance absorbs photons (electromagnetic radiation) and then re-radiates photons.
Quantum mechanicallyQuantum mechanics is a set of principles describing the physical reality at the atomic level of matter and the subatomic . These descriptions include the simultaneous wave-like and particle-like behavior of both matter and radiation...
, this can be described as an
excitationQuantum Jump was a 1970s British band, consisting of keyboard player and singer Rupert Hine, guitarist Mark Warner, bass player John G. Perry and drummer Trevor Morais ....
to a higher
energy stateA quantum mechanical system or particle that is bound, confined spatially, can only take on certain discrete values of energy, as opposed to classical particles, which can have any energy. These values are called energy levels. The term is most commonly used for the energy levels of electrons in...
and then a return to a lower energy state accompanied by the emission of a photon. This is one of many forms of
luminescenceLuminescence is light that usually occurs at low temperatures, and is thus a form of cold body radiation. It can be caused by chemical reactions, electrical energy, subatomic motions, or stress on a crystal. This distinguishes luminescence from incandescence, which is light generated by high...
(light emission) and is distinguished by
photoexcitationPhotoexcitation is the mechanism of electron excitation by photon absorption, when the energy of the photon is too low to cause photoionization.The absorption of photon takes place in accordance to the Planck's Quantum Theory....
(excitation by photons), hence the prefix
photo-.
Photoluminescence (abbreviated as
PL) is a process in which a substance absorbs photons (electromagnetic radiation) and then re-radiates photons.
Quantum mechanicallyQuantum mechanics is a set of principles describing the physical reality at the atomic level of matter and the subatomic . These descriptions include the simultaneous wave-like and particle-like behavior of both matter and radiation...
, this can be described as an
excitationQuantum Jump was a 1970s British band, consisting of keyboard player and singer Rupert Hine, guitarist Mark Warner, bass player John G. Perry and drummer Trevor Morais ....
to a higher
energy stateA quantum mechanical system or particle that is bound, confined spatially, can only take on certain discrete values of energy, as opposed to classical particles, which can have any energy. These values are called energy levels. The term is most commonly used for the energy levels of electrons in...
and then a return to a lower energy state accompanied by the emission of a photon. This is one of many forms of
luminescenceLuminescence is light that usually occurs at low temperatures, and is thus a form of cold body radiation. It can be caused by chemical reactions, electrical energy, subatomic motions, or stress on a crystal. This distinguishes luminescence from incandescence, which is light generated by high...
(light emission) and is distinguished by
photoexcitationPhotoexcitation is the mechanism of electron excitation by photon absorption, when the energy of the photon is too low to cause photoionization.The absorption of photon takes place in accordance to the Planck's Quantum Theory....
(excitation by photons), hence the prefix
photo-. The period between absorption and emission is typically extremely short, in the order of 10 nanoseconds. Under special circumstances, however, this period can be extended into minutes or hours.
Ultimately, available energy states and allowed transitions between states (and therefore wavelengths of light preferentially absorbed and emitted) are determined by the rules of quantum mechanics. A basic understanding of the principles involved can be gained by studying the
electron configurationIn atomic physics and quantum chemistry, electron configuration is the arrangement of electrons of an atom, a molecule, or other physical structure...
s and
molecular orbitalIn chemistry, a molecular orbital is a mathematical function that describes the wave-like behavior of an electron in a molecule. This function can be used to calculate chemical and physical properties such as the probability of finding an electron in any specific region. The use of the term...
s of simple atoms and molecules. More complicated molecules and advanced subtleties are treated in the field of
computational chemistryComputational chemistry is a branch of chemistry that uses computers to assist in solving chemical problems. It uses the results of theoretical chemistry, incorporated into efficient computer programs, to calculate the structures and properties of molecules and solids...
.
Forms of photoluminescence
The simplest photoluminescent processes are
resonant radiations, in which a photon of a particular wavelength is absorbed and an equivalent photon is immediately emitted. This process involves no significant internal energy transitions of the chemical substrate between absorption and emission and is extremely fast, of the order of 10 nanoseconds.
More interesting processes occur when the chemical substrate undergoes internal energy transitions before re-emitting the energy from the absorption event. The most familiar such effect is
fluorescenceFluorescence is the emission of visible light by a substance that has absorbed light of a differing, usually invisible, wavelength. Absorption of a photon triggers the emission of a photon with a longer wavelength. A shorter wavelength emission is sometimes observed from multiple photon absorption...
, which is also typically a fast process, but in which some of the original energy is dissipated so that the emitted light photons are of lower energy than those absorbed. The generated photon in this case is said to be red shifted, referring to the loss of energy as
Jablonski diagramA Jablonski diagram, named after the Polish physicist Aleksander Jabłoński, is a diagram that illustrates the electronic states of a molecule and the transitions between them. The states are arranged vertically by energy and grouped horizontally by spin multiplicity. Nonradiative transitions are...
shows.
Photoluminescence is an important technique for measuring the purity and crystalline quality of semiconductors such as
GaAsGaas is a commune in the Landes department in Aquitaine in south-western France....
and
InPINP may stand for:*International Network for Peace is a global network of organizations. They work together to break the cycles of violence.*INP, a chemical company headquartered in Bucheon Gyeonggi, Korea....
. Several variations of photoluminescence exist, including photoluminescence excitation (PLE).
Time-resolved photoluminescence (TRPL) is where you excite luminescence in a sample with a light pulse and then look at the decay in photoluminescence with respect to time. This technique is useful in measuring the minority carrier lifetime of III-V semiconductors like Gallium arsenide (
GaAsGaas is a commune in the Landes department in Aquitaine in south-western France....
.)
An even more specialized form of photoluminescence is
phosphorescencePhosphorescence is a specific type of photoluminescence related to fluorescence. Unlike fluorescence, a phosphorescent material does not immediately re-emit the radiation it absorbs. The slower time scales of the re-emission are associated with "forbidden" energy state transitions in quantum...
, in which the energy from absorbed photons undergoes
intersystem crossingIntersystem crossing is a radiationless process involving a transition between two electronic states with different spin multiplicity.-Singlet and triplet states:...
into a state of higher spin multiplicity (
see term symbolIn quantum mechanics, the Russell-Saunders term symbol is an abbreviated description of the angular momentum quantum numbers in a multi-electron atom. It is related with the energy level of a given electron configuration. LS coupling is assumed...
), usually a
triplet stateIn physics, spin is the angular momentum intrinsic to a body, as opposed to orbital angular momentum, which is the motion of its center of mass about an external point. In quantum mechanics, spin is particularly important for systems at atomic length scales, such as individual atoms, protons, or...
. Once the energy is trapped in the triplet state, transition back to the lower singlet energy states is quantum mechanically forbidden, meaning that it happens much more slowly than other transitions. The result is a slow process of radiative transition back to the singlet state, sometimes lasting minutes or hours. This is the basis for "glow in the dark" substances.
Photoluminescent material in safety applications
One of the major uses of photoluminescent material is for safety and egress marking. It is most commonly seen in the form of "fire exit" signage. The industry is governed by a number of international standards and guidelines that stipulate performance criteria under certain conditions of excitement. A guide to these standards can be found at
http://www.photoluminescent.co.uk/standards
Photoluminescent material for temperature detection
- Phosphor thermometry
Phosphor thermometry is an optical method for surface temperature measurement. The method exploits luminescence emitted by phosphor material. Phosphors are fine white or pastel-colored inorganic powders which may be stimulated by any of a variety of means to luminesce, i.e. emit light...
, phosphorescence can be used to detect the temperature of an object eg a gas turbine component ,,,
Further reading