= Photochemistry
<Photochemistry> studies chemical changes initiated or influenced by <light>. Absorption of a <photon> can prepare an electronically excited <molecule>, opening reaction pathways different from those available on its ground-state <potential energy surface>.<IUPAC Compendium of Chemical Terminology, fifth edition (Gold Book)>[Photochemistry P04588 and photochemical reaction P04585]{ref}
The absorbed energy is $E=h\nu$, so changing <wavelength> changes both which species absorb and which states are reached. <Photodissociation> of <molecular oxygen> is an atmospheric example. Absorption alone does not guarantee chemical conversion: relaxation and competing reactions affect the <quantum yield>. A photon-initiated chain can produce more than one product event per absorbed <photon>, so a chemical <quantum yield> need not be at most one.
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