# Phosphorescence

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

Phosphorescence is commonly defined as the delayed emission of light from a substance after exposure to and removal of the exciting radiation. It is a specific type of photoluminescence related to fluorescence. Unlike fluorescence, a phosphorescent material does not immediately re-emit the radiation it absorbs, and in some cases the "afterglow" might persist even for several hours or longer after the radiation source ceases. 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 long time periods.

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 that 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.

While in common use, phosphorescence often refers to the emission of light by bioluminescent plankton, this is actually a different phenomenon.

The phenomenon of phosphorescence—just as with fluorescence and bioluminescence—adds to the variety and wonder of nature for humans. Furthermore, human creativity has taken this property and created objects of both entertainment value (glowsticks for parties or marching bands) and vital use (highway exit signs, pathway markings, and other safety related signage as well as glowsticks used for military purposes). The development of strontium oxide aluminate, with a luminance approximately ten times greater than zinc sulfide, has been an important development in this regard.

## Overview

Phosphorescence is a form of photoluminescence. Photoluminescence 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 accompanied 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- (IUPAC). The period between absorption and emission is typically extremely short, in the order of ten nanoseconds. Under special circumstances, however, this period can be extended into minutes or hours.

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 ten 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.

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. Once the energy is trapped in this 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.

(Some less popular, broader definitions of fluorescence and narrower definitions of phosphorescence include the persistence of certain afterglows as fluorescence, but as "slow fluorescence" or "delayed fluorescence.")

### Phosphorescence versus bioluminescence and chemoluminescence

In common use, phosphorescence sometimes is used to refer to the emission of light by bioluminescent organisms (dinoflagellates, copepods, jellyfish, squid, etc.), and some other forms of chemoluminescence. However, while all are forms of luminescence, or "cold light," bioluminescence and chemoluminescence are different phenomena than phosphorescence.

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. Simply defined, bioluminescence is "light produced by a chemical reaction" that "originates in an organism" (Haddock et al. 2006). 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.

Chemoluminescence or chimiluminescence 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).

## Mechanism

Most photoluminescent events, in which a chemical substrate absorbs and then re-emits a photon of light, are fast, on the order of ten 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, 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 transitions, 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 that is commonly mistaken for phosphorescence, where the 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 ten 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.