Oxygen is produced on a net basis by photodissociation of water in the upper atmosphere coupled with hydrogen loss. (realclimate.org)
In its stratosphere I conclude the important photodissociation reaction that of carbon dioxide with ultraviolet photons which produces oxygen atoms and carbon monoxide molecules. (principia-scientific.org)
Experimentalists have been most successful with reactions involving photons, such as the photoionization and photodissociation of molecules. (sciencemag.org)