Photochemistry

emission of electrons when light hits a material

thumb|Photochemical immersion well reactor (50 mL) with a mercury-vapor lamp

thumb|Table-top spectrophotometer thumb|Beckman IR-1 Spectrophotometer, thumb|right | Beckman Model DB Spectrophotometer (a double beam model), 1960 thumb|Hand-held spectrophotometer used in graphic industry

Photodissociation, photolysis, photodecomposition, or photofragmentation is a chemical reaction in which molecules of a chemical compound are broken down by absorption of light (photons). It is defined as the interaction of one or more photons with one target molecule that dissociates into two fragments.

thumb|right|Triboluminescence of nicotine -salicylate Triboluminescence is a phenomenon in which light is generated when a material is mechanically pulled apart, ripped, scratched, crushed, or rubbed (see tribology). The phenomenon is not fully understood but appears in most cases to be caused by the separation and reunification of static electric charges, see also triboelectric effect. The term comes from the Greek τρίβειν ("to rub"; see tribology) and the Latin lumen (light). Triboluminescence can be observed when breaking sugar crystals and peeling adhesive tapes.

thumb|right|Scintillation crystal surrounded by various scintillation detector assemblies thumb|Extruded plastic scintillator material fluorescing under a UV inspection lamp at [[Fermilab for the MINERνA project]] thumb|Various scintillation crystals. The second crystal from the left is targeted by an UV source and shines brightly in visible light. A scintillator ( ) is a material that exhibits scintillation (also termed radioluminescence), a kind of luminescence, when excited by ionizing radiation. Luminescent materials, when struck by an incoming particle, absorb its energy and scintillate (

thumb|upright=1.5|Pump-probe techniques

process of photosynthesis

Radiolysis is the dissociation of molecules caused by ionizing radiation. The high-energy flux results in cleavage of one or more chemical bonds. Radiolysis is distinguished from other dissociations by the type of radiation involved: it is not photodissociation using lower-energy ranges of the electromagnetic spectrum or other high-energy processes such as an electric discharge. The chemistry of concentrated solutions under ionizing radiation is extremely complex. Radiolysis can locally modify redox conditions, and therefore the speciation and the solubility of the compounds.

right|Excimer energy diagram An excimer (originally short for excited dimer) is a short-lived polyatomic molecule formed from two species that do not form a stable molecule in the ground state. In this case, formation of molecules is possible only if such atom is in an electronic excited state. Heteronuclear molecules and molecules that have more than two species are also called exciplex molecules (originally short for excited complex). Excimers are often diatomic and are composed of two atoms or molecules that would not bond if both were in the ground state. The lifetime of an excimer is very

obsolete unit measuring either one mole of photons or the energy of one mole of photons

thumb|In the experiment above, photons from a light source (out of frame on the right hand side) are absorbed by the surface of the titanium dioxide () disc, exciting electrons within the material. These then react with the water molecules, splitting it into its constituents of hydrogen and oxygen. In this experiment, chemicals dissolved in the water prevent the formation of oxygen, which would otherwise recombine with the hydrogen.

Photochromism is the reversible change of color upon exposure to light. It is a transformation of a chemical species (photoswitch) between two forms through the absorption of electromagnetic radiation (photoisomerization), where each form has a different absorption spectrum. This reversible structural or geometric change in photochromic molecules affects their electronic configuration, molecular strain energy, and other properties.

diagram representing the electronic states of a molecule and the transitions between them

A photopolymer or light-activated resin is a polymer that changes its properties when exposed to light, often in the ultraviolet or visible region of the electromagnetic spectrum. These changes are often manifested structurally, for example hardening of the material occurs as a result of cross-linking when exposed to light. An example is shown below depicting a mixture of monomers, oligomers, and photoinitiators that conform into a hardened polymeric material through a process called curing.

sources of electricity or hydrogen via electrolysis

thumb|A plastic bucket used as an open-air flowerpot photodegraded after some years. Photodegradation is the alteration of materials by light. Commonly, the term is used loosely to refer to the combined action of sunlight and air, which cause oxidation and hydrolysis. Often photodegradation is intentionally avoided, since it destroys paintings and other artifacts. It is, however, partly responsible for remineralization of biomass and is used intentionally in some disinfection technologies. Photodegradation does not apply to how materials may be aged or degraded via infrared light or heat, but

In chemistry and materials science, a luminophore is the part of a molecule, coordination complex, or solid-state material that is responsible for its luminescence (light emission following excitation). In molecular photochemistry, the closely related IUPAC-recommended term lumiphore refers to "a part of a molecular entity (or atom or group of atoms) in which electronic excitation associated with a given emission band is approximately localized", by analogy with chromophore for absorption. In practice, the term luminophore is widely used across chemistry, physics, and engineering literature fo

photochemical reaction

thumb|100px|A generic diazirine

number of times a specific event occurs per photon absorbed by the system

chemical compound

In chemistry, chromism is a process that induces a change, often reversible, in the colors of compounds. In most cases, chromism is based on a change in the electron states of molecules, especially the π- or d-electron state, so this phenomenon is induced by various external stimuli which can alter the electron density of substances. It is known that there are many natural compounds that have chromism, and many artificial compounds with specific chromism have been synthesized to date. It is usually synonymous with chromotropism, the (reversible) change in color of a substance due to the physic

diagram showing energy levels of an atom

thumb|right|300 px|A photosensitizer being used in photodynamic therapy Photosensitizers are light absorbers that alter the course of a photochemical reaction. They usually are catalysts. They can function by many mechanisms; sometimes they abstract an electron from the substrate, and sometimes they abstract a hydrogen atom from the substrate. At the end of this process, the photosensitizer returns to its ground state, where it remains chemically intact, poised to absorb more light. One branch of chemistry which frequently utilizes photosensitizers is polymer chemistry, using photosensitizers

thumb|Clay earth pigments such as [[burnt sienna often have a high lightfastness]]

type of DNA damage

type of photochemical reaction of carbonyl compounds in organic chemistry

any form of optical data storage allowing three-dimensional recording or display

electrical dipole moment in quantum mechanics

thumb|403x403px|An illustration of electron excitation, showing excitation by photon (left) and by particle collision (right). This is the simplest case of photoexcitation, sinca a single photon excites a single quantum particle. Photoexcitation is a phenomenon in physics where an excited state of a quantum system (an atom or a molecule) is created by photon absorption. The excited state originates from the interaction between a photon and the quantum system when the energy of the photon is too low to cause photoionization. A very simple example of this process is electron excitation.

in chemistry, sulfoxidation refers to two distinct reactions: the radical oxidation of alkanes with sulfur dioxide and oxygen; see the oxygenation of a thioether to a sulfoxide or sulfone; see

Chemistry professor at the University of Wisconsin-Madison (1926-2012)

2-Phenylpyridine is an organic compound with the formula C6H5C5H4N (or C11H9N). It is a colourless viscous liquid. The compound and related derivatives have attracted interest as precursors to highly fluorescent metal complexes of possible value as organic light emitting diodes (OLEDs).

synthetic chemical fuel produced from solar energy

metabolism of ionizing radiation by living organisms

A spiropyran is a type of photochromic organic chemical compound, characterized by their ability to reversibly switch between two structural forms—spiropyran and merocyanine—upon exposure to light or other external stimuli. This reversible transformation alters their optical and electronic properties, making them valuable in various applications, including molecular switches, optical data storage, sensors, and smart materials.

type of mass spectrometry detector

In photochemistry, photohydrogen is hydrogen produced with the help of artificial or natural light. This is how the leaf of a tree splits water molecules into protons (hydrogen ions), electrons (to make carbohydrates) and oxygen (released into the air as a waste product). Photohydrogen may also be produced by the photodissociation of water by ultraviolet light.

Diarylethene is the general name of a class of chemical compounds that have aromatic functional groups bonded to each end of a carbon–carbon double bond. The simplest example is stilbene, which has two geometric isomers, E and Z.

transition from a higher to a lower electronic state in a molecule or atom

caused by UV-photons

Formation of pyrimidine dimers in DNA due to UVB exposure

Photoprotection is the biochemical process that helps organisms cope with molecular damage caused by sunlight. Plants and other oxygenic phototrophs have developed a suite of photoprotective mechanisms to prevent photoinhibition and oxidative stress caused by excess or fluctuating light conditions. Humans and other animals have also developed photoprotective mechanisms to avoid UV photodamage to the skin, prevent DNA damage, and minimize the downstream effects of oxidative stress.

technique for studying free-radical reactions in gases; an intense flash of light dissociates molecules in the sample creating free radicals, which can be detected spectroscopically