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How does photodissociation produce reactive free radicals?
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photochemical reaction
Chain of fluorescent tunicates.
In the context of a photochemical reaction, photodissociation—also known as photolysis—produces reactive free radicals through the following process:
- Excitation: When a molecule absorbs a photon, it is promoted to an excited state. This absorption causes an almost instantaneous change in the molecule's electron configuration.12
- Bond Cleavage: Because electrons are responsible for the bonding forces that hold atoms together, this drastic shift in electron distribution can cause a specific chemical bond to weaken or cease to exist entirely.2
- Radical Formation: The bond undergoes cleavage, resulting in the release of two fragments. These fragments are radicals because each retains enough electrons to constitute half of a chemical bond, leaving them with unpaired electrons that make them highly reactive and often short-lived.32
A primary example of this process is the fragmentation of molecular oxygen in the stratosphere into oxygen atom radicals, which subsequently react to form ozone.2