- Photoluminescence
Photoluminescence (abbreviated as PL) is a process in which a substance absorbs
photons (electromagnetic radiation) and then re-radiates photons. Quantum mechanically, this can be described as an excitation to a higher energy state and then a return to a lower energy state accompanied by the emission of a photon. This is one of many forms ofluminescence (light emission) and is distinguished byphotoexcitation (excitation by photons), hence the prefix "photo-". [GoldBookRef|title=photochemistry|file=P04588] The period between absorption and emission is typically extremely short, in the order of 10nanosecond s. Under special circumstances, however, this period can be extended into minutes or hours.Ultimately, available chemical 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 configuration s andmolecular orbital s of simpleatoms andmolecules . More complicated molecules and advanced subtleties are treated in the field ofcomputational chemistry .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
fluorescence , 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 asJablonski diagram shows.Photoluminescence is an important technique for measuring the purity and crystalline quality of semiconductors such as
GaAs andInP . Several variations of photoluminescence exist, including photoluminescence excitation (PLE).An even more specialized form of photoluminescence is
phosphorescence , in which the energy from absorbed photons undergoesintersystem crossing into a state of higher spin multiplicity ("seeterm symbol "), usually atriplet state . 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.References
Further reading
Wikimedia Foundation. 2010.