Phosphorescence
Phosphorescence is a type of photoluminescence related to fluorescence. When exposed to light (radiation) of a shorter wavelength, a phosphorescent substance will glow, absorbing the light and reemitting it at a longer wavelength. Unlike fluorescence, a phosphorescent material does not immediately reemit the radiation it absorbs. Instead, a phosphorescent material absorbs some of the radiation energy and reemits it for a much longer time after the radiation source is removed.
In a general sense, there is no distinct boundary between the emission times of fluorescence and phosphorescence (i.e.: if a substance glows under a
There are two separate mechanisms that may produce phosphorescence, called triplet phosphorescence (or simply phosphorescence) and persistent phosphorescence (or
On the other hand, persistent phosphorescence occurs when a high-energy photon is absorbed by an atom and its electron becomes trapped in a defect in the
Everyday examples of phosphorescent materials are the glow-in-the-dark toys, stickers, paint, and clock dials that glow after being charged with a bright light such as in any normal reading or room light. Typically, the glow slowly fades out, sometimes within a few minutes or up to a few hours in a dark room.[5]
The study of phosphorescent materials led to the discovery of radioactive decay.
Etymology
The term phosphorescence comes from the ancient Greek word φῶς (phos), meaning "light", and the Greek suffix -φόρος (-phoros), meaning "to bear", combined with the Latin suffix -escentem, meaning "becoming of", "having a tendency towards", or "with the essence of".[6] Thus, phosphorescence literally means "having a tendency to bear light". It was first recorded in 1766.[7]
The term phosphor had been used since the Middle Ages to describe minerals that glowed in the dark. One of the most famous, but not the first, was Bolognian phosphor. Around 1604, Vincenzo Casciarolo discovered a "lapis solaris" near Bologna, Italy. Once heated in an oxygen-rich furnace, it thereafter absorbed sunlight and glowed in the dark. In 1677, Hennig Brand isolated a new element that glowed due to a chemiluminescent reaction when exposed to air, and named it "phosphorus".[8]
In contrast, the term luminescence (from the Latin lumen for "light"), was coined by Eilhardt Wiedemann in 1888 as a term to refer to "light without heat", while "fluorescence" by
There was much confusion between the meanings of these terms throughout the late nineteenth to mid-twentieth centuries. Whereas the term "fluorescence" tended to refer to luminescence that ceased immediately (by human-eye standards) when removed from excitation, "phosphorescence" referred to virtually any substance that glowed for appreciable periods in darkness, sometimes to include even chemiluminescence (which occasionally produced substantial amounts of heat). Only after the 1950s and 1960s did advances in
Introduction
In simple 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 fluorescence, such as those seen in
When the stored energy becomes locked in by the spin of the atomic electrons, a triplet state can occur, slowing the emission of light, sometimes by several orders of magnitude. Because the atoms usually begin in a singlet state of spin, favoring fluorescence, these types of phosphors typically produce both types of emission during illumination, and then a dimmer afterglow of strictly phosphorescent light typically lasting less than a second after the illumination is switched off.
Conversely, when the stored energy is due to persistent phosphorescence, an entirely different process occurs without a fluorescence precursor. When electrons become trapped within a defect in the atomic or molecular lattice, light is prevented from reemitting until the electron can escape. To escape, the electron needs a boost of thermal energy to help spring it out of the trap and back into orbit around the atom. Only then can the atom emit a photon. Thus, persistent phosphorescence is highly dependent on the temperature of the material.[12]
Triplet phosphorescence
Most photoluminescent events, in which a chemical substrate absorbs and then re-emits a photon of light, are fast, in the order of 10 nanoseconds. Light is absorbed and emitted at these fast time scales in cases where the energy of the photons involved matches the available energy states and allowed transitions of the substrate. In the special case of phosphorescence, the electron which absorbed the photon (energy) undergoes an unusual intersystem crossing into an energy state of different (usually higher) spin multiplicity (see term symbol), usually a triplet state. As a result, the excited electron can become trapped in the triplet state with only "forbidden" transitions available to return to the lower energy singlet state. These transitions, although "forbidden", will still occur in quantum mechanics but are kinetically unfavored and thus progress at significantly slower time scales. Most phosphorescent compounds are still relatively fast emitters, with triplet decay-times in the order of milliseconds.
Common examples include the phosphor coatings used in
Equation
Persistent phosphorescence
Solid materials typically come in two main types: crystalline and amorphous. In either case, a lattice or network of
When a defect occurs, depending on the type and material, it can create a hole, or a "trap". For example, a missing
The release of energy in this way is a completely random process, governed mostly by the average temperature of the material versus the "depth" of the trap, or how many electron-volts it exerts. A trap that has a depth of 2.0 electron-volts would require a great amount of thermal energy (very high temperatures) to overcome the attraction, while at a depth of 0.1 electron-volts very little heat (very cold temperatures) are needed for the trap to even hold an electron. Higher temperatures may cause the faster release of energy, resulting in a brighter yet short-lived emission, while lower temperatures may produce dimmer but longer-lasting glows. Temperatures that are too hot or cold, depending on the substance, may not allow the accumulation or release of energy at all. The ideal depth of trap for persistent phosphorescence at room temperature is typically between 0.6 and 0.7 electron-volts.[17] If the phosphorescent quantum yield is high, that is, if the substance has a large number of traps of the correct depth, these substances will release significant amounts of light over long time scales, creating so-called "glow in the dark" materials.
Persistent phosphorescence is the mechanism of most anything commonly referred to as glow in the dark. Typical uses include toys, frisbees and balls, safety signs, paints and markings, make-ups, art and décor, and a variety of other uses.
Chemiluminescence
Some examples of glow-in-the-dark materials do not glow by phosphorescence. For example,
Materials
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.
The development of strontium aluminate pigments in 1993 was spurred on by the need to find a substitute for glow-in-the-dark materials with high luminance and long phosphorescence, especially those that used promethium.[18][19] This led to the discovery by Yasumitsu Aoki (Nemoto & Co.) of materials with luminance approximately 10 times greater than zinc sulfide and phosphorescence approximately 10 times longer.[20][21] This has relegated most zinc sulfide based products to the novelty category. Strontium aluminate based pigments are now used in exit signs, pathway marking, and other safety related signage.[22]
-
Zinc sulfide (left) and strontium aluminate (right), in visible light, in darkness, and after 4 minutes in the dark.
-
Calcium sulfide (left) and metal-earth silicate (right) phosphoresce in red and blue respectively.
Since both phosphorescence (transition from T1 to S0) and the generation of T1 from an excited singlet state (e.g., S1) via intersystem crossing (ISC) are spin-forbidden processes, most organic materials exhibit insignificant phosphorescence as they mostly fail to populate the excited triplet state, and, even if T1 is formed, phosphorescence is most frequently outcompeted by non-radiative pathways. One strategy to enhance the ISC and phosphorescence is the incorporation of heavy atoms, which increase spin-orbit coupling (SOC).[23] Additionally, the SOC (and therefore the ISC) can be promoted by coupling n-π* and π-π* transitions with different angular momenta, also known as Mostafa El-Sayed's rule. Such transitions are typically exhibited by carbonyl or triazine derivatives, and most organic room-temperature phosphorescent (ORTP) materials incorporate such moieties. [24][25] In turn, to inhibit competitive non-radiative deactivation pathways, including vibrational relaxation and oxygen quenching and triplet-triplet annihilations, organic phosphors have to be embedded in rigid matrices such as polymers, and molecular solids (crystals,[26] covalent organic frameworks,[27] and others).
Uses
In 1974 Becky Schroeder was given a US patent for her invention of the "Glow Sheet" which used phosphorescent lines under writing paper to help people write in low-light conditions.[28]
Glow in the dark material is added to the plastic blend used in injection molds to make some disc golf discs, which allow the game to be played at night.
Often clock faces of watches are painted with phosphorescent colours. Therefore, they can be used in absolute dark environments for several hours after having been exposed to bright light.
A common use of phosphorescence is decoration. Stars made of glow-in-the-dark plastic are placed on walls, ceilings, or hanging from strings make a room look like the night sky.
Shadow wall
A shadow wall is created when a light flashes upon a person or object in front of a phosphorescent screen which temporarily captures the shadow. The screen or wall is painted with a glow-in-the-dark product that contains phosphorescent compounds.[33] Publicly, these shadow walls can be found at certain science museums.[34][35]
-
Before image of capturing a shadow on a phosphorescent wall.
-
After image of capturing a shadow on a phosphorescent wall.
See also
- Luminous gemstones
- Luminous paint
- Microsphere
- Persistent luminescence
- Phosphor
- Phosphoroscope
- Tritium
References
- ^ Illuminating Engineering -- Illuminating Engineering Society 1954 Page 228
- ^ Persistent Phosphors: From Fundamentals to Applications by Jianrong Qiu, Yang Li, Yongchao Jia -- Elsevier 2020 Page 1--25
- ^ Persistent Phosphors: From Fundamentals to Applications by Jianrong Qiu, Yang Li, Yongchao Jia -- Elsevier 2020 Page 1--25
- ^ Persistent Phosphors: From Fundamentals to Applications by Jianrong Qiu, Yang Li, Yongchao Jia -- Elsevier 2020 Page 1--25
- ^ "-escent". Online Etymology Dictionary.
- ^ "Phosphorescent". Online Etymology Dictionary.
- ^ New Trends in Fluorescence Spectroscopy by B Valeur -- Springer Page 1--6
- ^ New Trends in Fluorescence Spectroscopy by B Valeur -- Springer Page 1--6
- ^ New Trends in Fluorescence Spectroscopy by B Valeur -- Springer Page 1--6
- ^ New Trends in Fluorescence Spectroscopy by B Valeur -- Springer Page 5--6
- ^ Persistent Phosphors: From Fundamentals to Applications by Jianrong Qiu, Yang Li, Yongchao Jia -- Elsevier 2020 Page 1--25
- ^ Illuminating Engineering -- Illuminating Engineering Society 1954 Page 228
- ^ Philips Technical Library - Fluorescent Lamps By J. L. Ouweltjes -- The MacMillan Press 1971 Page 32--40
- ^ Principles of Lasers by Orazio Svelto -- Springer 2010
- ^ Practical Applications of Phosphors by William M. Yen, Shigeo Shionoya, Hajime Yamamoto -- CRC Press 2018 Page 453--474
- ^ Persistent Phosphors: From Fundamentals to Applications by Jianrong Qiu, Yang Li, Yongchao Jia -- Elsevier 2020 Page 1--25
- ^ Glow in the Dark Pigments - Japan's Top Inventions - TV | NHK WORLD-JAPAN Live & Programs, retrieved 2021-03-25
- ^ Kanji, Takamasu (May–June 2006). "Shining in the Niche Market withLuminous Pigment and IPRs Strategy" (PDF). Japan Spotlight.
- ISSN 0013-4651.
- ^ US5424006A, "Phosphorescent phosphor", issued 1994-02-25
- doi:10.1021/ed086p72
- S2CID 50788897.
- PMID 25849370.
- S2CID 208019093.
- .
- S2CID 253183290.
- ISSN 0362-4331. Retrieved 2020-08-16.
- ^ Helmenstine, Anne Marie. "Phosphorescence Definition and Examples". ThoughtCo. Retrieved 21 December 2022.
- ^ Shelton, Jacob. "Why Were Blacklight Posters So Popular in the '70s?". Groovy History. Retrieved 21 December 2022.
- ^ Bunting, Geoffrey (19 February 2015). "Glowing in the Dark". Historical Association. Retrieved 21 December 2022.
- ^ "Phosphorescent Light Examples in Daily Life". Studious Guy. Retrieved 21 December 2022.
- ^ Chiaverina, Chris. "Experimenting with Phosphorescence" (PDF). DiscoverieScience.com. Retrieved 3 November 2023.
- ^ "Shadow Box | Exploratorium Museum Exhibits". 29 November 2017.
- ^ "Shadow Wall".