Phosphorescence

In common use, phosphorescence also refers to the emission of light by bioluminescent plankton, and some other forms of chemoluminescence.

Phosphorescent powder under visible light, ultraviolet light, and total darkness.

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.

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.

File:Red tide bioluminescence at midnight.jpg

Image of bioluminescent red tide event of 2005 at a beach in Carlsbad California showing brilliantly glowing crashing waves containing billions of Lingulodinium polyedrum dinoflagellates. The blue light is a result of a luciferase enzyme.

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

$S_{0}+h\nu \to S_{1}\to T_{1}\to S_{0}+h\nu ^{\prime }$

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

• Bioluminescence

References

ISBN links support NWE through referral fees

• 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

• Phosphorescence on Scienceworld
• The Cathode Ray Tube site
• Glow in the Dark Project Forum

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