Phosphorescence Definition and Examples

Glowing phosphorescent face


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Phosphorescence is luminescence that occurs when energy is supplied by electromagnetic radiation, usually ultraviolet light. The energy source kicks an electron of an atom from a lower energy state into an "excited" higher energy state; then the electron releases the energy in the form of visible light (luminescence) when it falls back to a lower energy state.

Key Takeaways: Phosphorescence

  • Phosphorescence is a type of photoluminescence.
  • In phosphorescence, light is absorbed by a material, bumping up the energy levels of electrons into an excited state. However, the energy of the light doesn't quite match up with the energy of allowed excited states, so the absorbed photos get stuck in a triplet state. Transitions to a lower and more stable energy state take time, but when they occur, light is released. Because this release occurs slowly, a phosphorescent material appears to glow in the dark.
  • Examples of phosphorescent materials include glow-in-the-dark stars, some safety signs, and glowing paint. Unlike phosphorescent products, fluorescent pigments stop glowing once the light source is removed.
  • Although named for the green glow of the element phosphorus, phosphorus actually glows because of oxidation. It is not phosphorescent!

Simple Explanation

Phosphorescence releases the stored energy slowly over time. Basically, phosphorescent material is "charged" by exposing it to light. Then the energy is stored for a period of time and slowly released. When the energy is released immediately after absorbing the incident energy, the process is called fluorescence.

Quantum Mechanics Explanation

In fluorescence, a surface absorbs and re-emits a photon almost instantly (about 10 nanoseconds). Photoluminescence is quick because the energy of the absorbed photons matches energy states and allowed transitions of the material. Phosphorescence lasts much longer (milliseconds up to days) because the absorbed electron crosses into an excited state with higher spin multiplicity. The excited electrons get trapped in a triplet state and can only use "forbidden" transitions to drop to a lower energy singlet state. Quantum mechanics allows for forbidden transition, but they are not kinetically favorable, so they take longer to occur. If enough light is absorbed, the stored and released light becomes sufficiently significant for material to appear to "glow in the dark." For this reason, phosphorescent materials, like fluorescent materials, appear very bright under a black (ultraviolet) light. A Jablonski diagram is commonly used to display the difference between fluorescence and phosphorescence.

Jablonski diagram
This Jablonski diagram shows the difference between the mechanisms of fluorescence and phosphorescence. Smokefoot / Creative Commons Attribution-Share Alike 3.0

History

The study of phosphorescent materials dates back to at least 1602 when Italian Vincenzo Casciarolo described a "lapis solaris" (sun stone) or "lapis lunaris" (moon stone). The discovery was described in philosophy professor Giulio Cesare la Galla's 1612 book De Phenomenis in Orbe Lunae. La Galla reports Casciarolo's stone emitted light on it on after it had been calcified through heating. It received light from the Sun and then (like the Moon) gave out light in the darkness. The stone was impure barite, although other minerals also display phosphorescence. They include some diamonds (known to Indian king Bhoja as early as 1010-1055, rediscovered by Albertus Magnus and again rediscovered by Robert Boyle) and white topaz. The Chinese, in particular, valued a type of fluorite called chlorophane that would display luminescence from body heat, exposure to light, or being rubbed. Interest in the nature of phosphorescence and other types of luminescence eventually led to the discovery of radioactivity in 1896.

Materials

Besides a few natural minerals, phosphorescence is produced by chemical compounds. Probably the best-known of these is zinc sulfide, which has been used in products since the 1930s. Zinc sulfide usually emits a green phosphorescence, although phosphors may be added to change the color of light. Phosphors absorb the light emitted by phosphorescence and then release it as another color.

More recently, strontium aluminate is used for phosphorescence. This compound glows ten-time brighter than zinc sulfide and also stores its energy much longer.

Examples of Phosphorescence

Common examples of phosphorescence include stars people put on bedroom walls that glow for hours after the lights are turned out and paint used to make glowing star murals. Although the element phosphorus glows green, the light is released from oxidation (chemiluminescence) and is not an example of phosphorescence.

Sources

  • Franz, Karl A.; Kehr, Wolfgang G.; Siggel, Alfred; Wieczoreck, Jürgen; Adam, Waldemar (2002). "Luminescent Materials" in Ullmann's Encyclopedia of Industrial Chemistry. Wiley-VCH. Weinheim. doi:10.1002/14356007.a15_519
  • Roda, Aldo (2010). Chemiluminescence and Bioluminescence: Past, Present and Future. Royal Society of Chemistry.
  • Zitoun, D.; Bernaud, L.; Manteghetti, A. (2009). Microwave Synthesis of a Long-Lasting Phosphor. J. Chem. Educ. 86. 72-75. doi:10.1021/ed086p72