Phosphorescence

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

Phosphorescence is a specific type of photoluminescence related to fluorescence. Photoluminescence (abbreviated as PL) is a process in which a substance absorbs Photons (electromagnetic radiation and then re-radiates photons Fluorescence is a Luminescence that is mostly found as an 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. A quantum mechanical system or particle that is bound, confined spacially can only take on certain discrete values of energy as opposed to classical particles which Quantum mechanics is the study of mechanical systems whose dimensions are close to the Atomic scale such as Molecules Atoms Electrons 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.

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. In Physics, the photon is the Elementary particle responsible for electromagnetic phenomena A nanosecond ( ns) is one billionth of a second See also times of other orders of magnitude. 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. Intersystem crossing is a photophysical process An isoenergetic non-radiative transition between two Electronic states having different multiplicities. In Quantum mechanics, the term symbol is an abbreviated description of the Angular momentum quantum numbers in a multi- Electron Atom. In Physics, '''spin''' is the Angular momentum intrinsic to a body as opposed to Orbital angular momentum, which is the motion of its Center of mass 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. Chemical kinetics, also known as reaction kinetics is the study of rates of chemical processes 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. The quantum yield of a Radiation -induced process is the number of times that a defined event occurs per Photon absorbed by the system

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. A glow stick is a single-use translucent Plastic tube containing isolated substances which when combined are capable of producing Light through a Chemical reaction Chemiluminescence (sometimes " chemoluminescence " is the emission of Light ( Luminescence) with limited emission of heat as the result of a chemical 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. A sensitiser in Chemoluminescence is a chemical compound capable of Light emission after it has received energy from a molecule which became excited previously A fluorophore, in analogy to a Chromophore, is a component of a molecule which causes a molecule to be Fluorescent.

Common pigments used in phosphorescent materials include zinc sulfide and strontium aluminate. Zinc sulfide (or zinc sulphide is a Chemical compound with the formula Zn[[Sulfur S]] Strontium aluminate ( SRA, SrAl, 24 is a solid odorless nonflammable pale yellow powder heavier than water 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. Strontium aluminate based afterglow pigments are marketed under brandnames like Super-LumiNova^[1][2] or NoctiLumina. Super-LumiNova is a brand name under which Strontium aluminate based non- Radioactive and non-toxic photolumnescent or afterglow pigments for illuminating ^[3]

The study of phosphorescent materials led to the discovery of radioactivity in 1896. Radioactive decay is the process in which an unstable Atomic nucleus loses energy by emitting ionizing particles and Radiation.

Equation

Where S is a singlet and T a triplet whose subscripts denote states (0 is the ground state, and 1 the excited state). In Theoretical physics, a singlet usually refers to a one-dimensional representation (e In Physics, '''spin''' is the Angular momentum intrinsic to a body as opposed to Orbital angular momentum, which is the motion of its Center of mass 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. David Herbert Richards Lawrence (11 September 1885 – 2 March 1930 was an English writer of the 20th century whose prolific and diverse output included Novels short
• Phosphorescence occurs in the book: Carry on Mr. Bowditch.

See also

• Phosphor

References

External links

• Phosphorescence on Scienceworld
• The Cathode Ray Tube site
• Glow in the Dark Paint and Powder
• Glow in the Dark (Phosphorescent) Technologies
• Glow in the Dark Project Forum