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Photodissociation (also called photolysis or photodecomposition) is a chemical reaction using the energy of an incoming photon to break a chemical bond, i.e., to dissociate two parts of the molecule. The higher the energy, the more likely the photon has the energy to carry out the dissociation, thus gamma rays, X-rays and ultraviolet rays can dissociate the largest number of compounds.
A photodissociation region (or photon-dominated region or PDR) is an interstellar gas cloud in which there are sufficient ultraviolet photons to dissociate a significant number of molecules. Such regions occur in at the border between molecular clouds and HII regions, in the region where far ultraviolet can penetrate.
Emissive photodissociation (EP) is photodissociation that leaves an excited molecule, atom, or ion that subsequently emits a photon. The wavelength of such produced emissions can be a clue to the constituents of a gas.
The phrase nuclear photodissociation has occasionally been used for photofission, an unrelated reaction that is nuclear, consisting of fission triggered by a photon (rather than by, for example, an incoming neutron) which can occur in some internal stellar processes and some stellar events.