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The Basic Laws Of Photochemistry, Photochemistry Induced By Visible And Ultraviolet Light, Reaction Pathways, DissociationIonization, Isomerization

Photochemistry is the study of light-induced chemical reactions and physical processes. A photochemical event involves the absorption of light to create an excited species that may subsequently undergo a number of different reactions. These include unimolecular reactions such as dissociation, ionization, and isomerization; bimolecular reactions, which involve a reaction with a second molecule or atom to form a new compound; and reactions producing an emission of light, or luminescence. A photochemical reaction differs notably from a thermally, or heat, induced reaction in that the rate of a photochemical reaction is frequently greatly accelerated, and the products of the photochemical reaction may be impossible to produce otherwise. With the advent of lasers (powerful, single-color light sources) the field of photochemistry has advanced tremendously over the past few decades. An increased understanding of photochemistry has great implications outside of the laboratory, as photochemical reactions are an extremely important aspect of everyday life, underlying the processes of vision, photosynthesis, photography, atmospheric chemistry, the production of smog, and the destruction of the ozone layer.

The absorption of light by atoms and molecules to create an excited species is studied in the field of spectroscopy. The study of the reactions of this excited species is the domain of photochemistry. However, the fields are closely related; spectroscopy is routinely used by photochemists as a tool for identifying reaction pathways and products and, recently, for following reactions as they occur in real time. Some lasers can produce a pulse of light that is only "on" for 1 femtosecond (10-15 seconds). A femtosecond laser can be used like an extremely high-speed strobe camera to spectroscopically "photograph" a photochemical reaction.

The separation of an electron from an atom or molecule, leaving a positively charged ion, is a special form of dissociation called ionization. Ionization following absorption of a photon (path ii) usually occurs with light of very short wavelengths (less than 100 nm) and therefore is usually not studied by photochemists, although it is of great importance in x-ray technology. X rays are also sometimes referred to as ionizing radiation.

An excited molecule may undergo a rearrangement of its bonds, forming a new molecule made up of the same atoms but connected in a different manner; this process is called isomerization (path iii). The first step in the vision process involves the light-induced isomerization of pigments in the retina that subsequently undergo a number of thermally and enzymatically driven reactions before ultimately producing a neural signal.

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