Phosphorescence is light released by matter after exposure to 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, more stable energy state.
Phosphorescence is one form of photoluminescence. Other common types of photoluminescence include chemiluminescence and fluorescence. The energy for chemiluminescence comes from a chemical reaction. Like phosphorescence, fluorescence releases light after exposure to electromagnetic radiation (like black light). However, fluorescence occurs much more quickly than phosphorescence and fades as soon as the light source is removed. Phosphorescent materials glow minutes, hours, or even days after the lights glow out, so they glow in the dark.
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 photons get stuck in a triplet state. Eventually, the excited electrons drop to a lower and more stable energy state and release the extra energy as light. The process occurs slowly, so phosphorescent material appears to glow in the dark.
- Examples of phosphorescent materials include glow-in-the-dark stars, certain safety signs, glowing paint, and some road markers.
- While phosophorescence takes its name from the green glow of the element phosphorus, phosphorus isn’t phosphorescent. The reason the element glows is because of oxidation (chemiluminescence).
How It Works – Simple Explanation
Basically, a phosphorescent material is “charged” by exposing it to light. The material absorbs light and releases the stored energy slowly and at a longer wavelength than the original light. So, a phosphorescent material might absorb ultraviolet light and release green light, but it can’t go the other way in the spectrum (e.g., green to blue). Sometimes fluorescent dyes are added to phosphorescent materials to change the color of the light. Fluorescent materials absorb energy and immediately release light. Phosphorescent objects glow more brightly under a black light than in the dark because they may contain fluorescent dyes and because some phosphorescent transitions occur quickly.
How It Works – Quantum Mechanics Explanation
In fluorescence, a surface absorbs and re-emits a photon almost instantly (about 10 nanoseconds). This type of photoluminescence is fast 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 gets trapped in a triplet state and can only use “forbidden” transitions to drop to a lower energy singlet state. Quantum mechanics allows for forbidden transitions, 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 a 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.
History
In 1602, 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 its 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. Other phosphorescent gems 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
In addition to natural minerals, phosphorescence is produced by chemical compounds. The best-known of these is zinc sulfide, which has been used in glow-in-the-dark stars and other 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.
Today, doped strontium aluminate is the phosphorescent compound of choice. It glows ten time brighter than zinc sulfide and stores its energy much longer. The brightest color released by strontium aluminate is green, but aqua and blue also glow brightly and for a long time. Red, yellow, orange, white, and violet also occur, but are either dimmer or fade faster.
Phosphorescence Examples
The stars people put on bedroom walls to glow at night are phosphorescent. Some watches have phosphorescent hands. There are also paving stones, lamps, and key rings that glow in the dark from this process. The phosphorus glow is chemiluminescence, so it is not an example of phosphorescence.
References
- 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
- McQuarrie, Donald A.; Simon, John D.; Choi, John (1997). Physical Chemistry: A Molecular Approach (1st ed.). University Science Books. ISBN: 9780935702996
- 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