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Photoexcitation is the first step in a photochemical process where the reactant is elevated to a state of higher energy, an excited state.The first law of photochemistry, known as the Grotthuss–Draper law (for chemists Theodor Grotthuss and John W. Draper), states that light must be absorbed by a chemical substance in order for a photochemical reaction to take place.
Photolysis occurs in the atmosphere as part of a series of reactions by which primary pollutants such as hydrocarbons and nitrogen oxides react to form secondary pollutants such as peroxyacyl nitrates. See Photochemical smog. The two most important photodissociation reactions in the troposphere are firstly:
Atmospheric chemistry is a branch of atmospheric science that studies the chemistry of the Earth's atmosphere and that of other planets. This multidisciplinary approach of research draws on environmental chemistry, physics, meteorology, computer modeling, oceanography, geology and volcanology, climatology and other disciplines to understand both natural and human-induced changes in atmospheric ...
Photogeochemical reactions are described by the same principles used to describe photochemical reactions in general, and may be classified similarly: Photosynthesis: in the most general sense, photosynthesis refers to any light-activated reaction for which the change in free energy (ΔG o ) is positive for the reaction itself (without ...
In a reversible photochemical reaction between compounds A and B, there will therefore be a "forwards" reaction of at a rate proportional to and a "backwards" reaction of at a rate proportional to . The ratio of the rates of the forward and backwards reactions determines where the equilibrium lies, and thus the photostationary state is found at:
Their contributions led to the development of actinometric measurements, measurements that provide the basis of determining photon flux in photochemical reactions. [ 4 ] [ 6 ] After a hiatus, in 1921, Baly et al. used ferric hydroxides and colloidal uranium salts as catalysts for the creation of formaldehyde under visible light.
Photochemical reactions are initiated by the absorption of a photon, typically in the wavelength range 290–700 nm (at the surface of the Earth). The energy of an absorbed photon is transferred to electrons in the molecule and briefly changes their configuration (i.e., promotes the molecule from a ground state to an excited state ).
This discovery of oxygen's reduction by a photosensitizer led to chemists studying photosensitizers as photoredox catalysts for their roles in the catalysis of pericyclic reactions and other reduction and oxidation reactions. Photosensitizers in synthetic chemistry allow for the manipulation of electronic transitions within molecules through an ...