Phosphorescence
- In common use, phosphorescence also refers to the emission of light by bioluminescent plankton, and some other forms of chemoluminescence.
Phosphorescence 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 mechanics. As these transitions occur less often in certain materials, absorbed radiation may be re-emitted at a lower intensity for up to several hours.
In simpler terms, phosphorescence is a process in which energy absorbed by a substance is released relatively slowly in the form of light. This is in some cases the mechanism used for "glow-in-the-dark" materials which are "charged" by exposure to light. Unlike the relatively swift reactions in a common fluorescent tube, phosphorescent materials used for these materials absorb the energy and "store" it for a longer time as the subatomic reactions required to re-emit the light occur less often.
Overview
Photoluminescence (abbreviated as PL) is a process in which a substance absorbs photons (electromagnetic radiation) and then radiates photons back out. Quantum mechanically, this can be described as an excitation to a higher energy state and then a return to a lower energy state acompanied by the emission of a photon. This is one of many forms of luminescence (light emission) and is distinguished by photoexcitation (excitation by photons), hence the prefix photo-.[1] 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 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 configurations and molecular orbitals of simple atoms and molecules. More complicated molecules and advanced subtleties are treated in the field of computational 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.
Photoluminescence is an important technique for measuring the purity and crystalline quality of semiconductors such as GaAs and InP. 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 undergoes intersystem crossing into a state of higher spin multiplicity (see term symbol), usually a triplet 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.
ReferencesISBN links support NWE through referral fees
- ↑ International Union of Pure and Applied Chemistry. "photochemistry". Compendium of Chemical Terminology Internet edition.
Further reading
Donald A. McQuarrie, John D. Simon. Physical Chemistry, a molecular approach. University Science Books, 1997.
Notes on bioluminescence
Bioluminescence is the production and emission of light by a living organism as the result of a chemical reaction during which chemical energy is converted to light energy. It is widespread in the marine environment, but rare in terrestrial and especially freshwater environments. Examples include emission of visible light by dinoflagellates, jellyfish, squid, copepods, fireflies, and many other organisms belonging to a wide diversity of taxonomic groups.
Bioluminescence aids the survival and reproduction of individual organisms through such means as camouflage and defense, attraction of prey and mates, and communication. At the same time, the phenomena also contributes to human beings' visual experience and enjoyment of nature and its diversity. Furthermore, the creativity with which human beings are endowed has been applied to bioluminescence as well, employing it in biomedical and genetic research, while exploring potential new applications, such as creating plants that luminesce when they need water.
Characteristics of the phenomenon
Simply defined, bioluminescence is "light produced by a chemical reaction" that "originates in an organism" (Haddock et al. 2006). The term bioluminescence originates from the Greek bios for "living" and the Latin lumen for "light."
Bioluminescence is a form of luminescence, or "cold light" emission; less than 20 percent of the light generates thermal radiation. Bioluminescence should not be confused with fluorescence or phosphorescence. In fluorescence, the molecular absorption of a photon triggers the emission of another photon with a longer wavelength. In other words, the energy originates from an external source of light, which is absorbed and almost immediately emitted (Haddock et al. 2006). In phosphorescence, the material absorbs an external source of light as well, but does not immediately re-emit the radiation it absorbs. The absorbed radiation may be re-emitted at a lower intensity for up to several hours.
Chimiluminescence (or chemoluminescence) is the general term for production of light via a chemical reaction, and thus bioluminescence is a subset of chemiluminescence, but where the light-producing chemical reaction occurs inside an organism (Haddock et al. 2006).
Bioluminescence is generated by an enzyme-catalyzed chemoluminescence reaction, wherein the pigment luciferin is oxidized by the enzyme luciferase. In other words, the chemical luciferin is the one that produces light and the chemical luciferase is the one that drives, or catalyzes, the reaction (Haddock et al. 2006). Adenosine triphosphate (ATP) is involved in most instances. The chemical reaction can occur either within or outside of the cell.
In bacteria, the expression of genes related to bioluminescence is controlled by an operon (key nucleotide sequence) called the Lux operon.
Most photoluminescent events, in which a chemical substrate absorbs and then re-emits a photon of light, are fast, on the order of 10 nanoseconds. However, for light to be absorbed and emitted at these fast time scales, the energy of the photons involved (i.e. the wavelength of the light) must be carefully tuned according to the rules of quantum mechanics to match the available energy states and allowed transitions of the substrate. In the special case of phosphorescence, the absorbed photon energy undergoes an unusual intersystem crossing into an energy state of higher spin multiplicity (see term symbol), usually a triplet state. As a result, the energy can become trapped in the triplet state with only quantum mechanically "forbidden" transitions available to return to the lower energy state. These transistions, although "forbidden", will still occur but are kinetically unfavored and thus progress at significantly slower time scales. Most phosphorescent compounds are still relatively fast emitters, with triplet lifetimes on the order of milliseconds. However, some compounds have triplet lifetimes up to minutes or even hours, allowing these substances to effectively store light energy in the form of very slowly degrading excited electron states. If the phosphorescent quantum yield is high, these substances will release significant amounts of light over long time scales, creating so-called "glow-in-the-dark" materials.
Some examples of "glow-in-the-dark" materials do not glow because they are phosphorescent. For example, "glow sticks" glow due to a chemiluminescent process which is commonly mistaken for phosphorescence. In chemi-luminescence, an excited state is created via a chemical reaction. The excited state will then transfer to a "dye" molecule, also known as a (sensitizer, or fluorophor), and subsequently fluoresce back to the ground state.
Common pigments used in phosphorescent materials include zinc sulfide and strontium aluminate. Use of zinc sulfide for safety related products dates back to the 1930s. However, the development of strontium oxide aluminate, with a luminance approximately 10 times greater than zinc sulfide, has relegated most zinc sulfide based products to the novelty category. Strontium oxide aluminate based pigments are now used in exit signs, pathway marking, and other safety related signage.
The study of phosphorescent materials led to the discovery of radioactivity in 1896.
Equation
Where S is a singlet and T a triplet whose subscripts denote states (0 is the ground state, and 1 the excited state). Transitions can also occur to higher energy levels, but the first excited state is denoted for simplicity.
Literature
Phosphorescence is a recurrent symbol used by the writer D H Lawrence.
See Also
References
- Haddock, S. H. D., C. M. McDougall, and J. F. Case. 2006 (created 1997). The bioluminescence web page. University of California, Santa Barbara. Retrieved April 6, 2007.
External links
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