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photoinitiator

n. (context chemistry English) Any substance that initiates a chemical reaction by the absorption a photon to form an excited state

Wikipedia
Photoinitiator

A photoinitiator is a molecule that creates reactive species ( free radicals, cations or anions) when exposed to radiation (UV or visible). Synthetic photoinitiators are key components in photopolymers (i.e. photo-curable coatings, adhesives and dental restoratives).

Some small molecules in the atmosphere can also act as photoinitiators by decomposing to give free radicals (in photochemical smog). For instance, nitrogen dioxide is produced in large quantities by gasoline-burning internal combustion engines. NO in the troposphere gives smog its brown coloration and catalyzes production of toxic ground-level ozone. Molecular oxygen (O) also serves as a photoinitiator in the stratosphere, breaking down into atomic oxygen and in order to form the ozone in the ozone layer.