Photochemical reaction

A chemical reaction initiated or altered when a molecule or material absorbs light. The absorbed energy can drive pathways unavailable through heating alone.

Photon: A quantum of electromagnetic radiation that carries energy and momentum. Its energy is absorbed to initiate or alter the reaction.
Electromagnetic spectrum: The full range of electromagnetic radiation, ordered by frequency or wavelength. Photochemical outcomes depend on where absorbed light falls in this spectrum.
Photocatalysis: The acceleration of a chemical reaction by a catalyst activated by light. It uses light-generated charge carriers or excited states to drive chemical transformations.
Norrish reaction: A photochemical reaction in which an excited carbonyl compound undergoes characteristic bond cleavage or rearrangement. It illustrates how excited carbonyl groups can fragment or rearrange.
Electronic excited state: A higher-energy electronic configuration reached when an atom or molecule absorbs energy. Light absorption commonly promotes a molecule into this reactive state.
Absorption spectrum: A record of how strongly a substance absorbs light across wavelengths. Its bands identify wavelengths capable of exciting the reactant.
Photopolymerization: Polymer formation initiated by light, often through radical or cationic reactions. It turns photochemical initiation into rapid curing of coatings and resins.
Paternò–Büchi reaction: A photochemical [2+2] cycloaddition between a carbonyl compound and an alkene. It uses an excited carbonyl group to form an oxetane ring.
Internal conversion: A radiationless transition between electronic states of the same spin multiplicity. It can redirect excited-state energy into vibrations rather than chemical change.
Molecular orbital: A quantum-mechanical description of an electron's spatial distribution in a molecule. Electronic transitions between molecular orbitals account for many light absorptions.

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