fotolyse

Fotolyse, or photolysis, is the decomposition of a chemical species induced by the absorption of light. When a molecule absorbs a photon, an electron is promoted to an excited electronic state. In this excited state the bonds can weaken and cleave, producing reactive fragments such as radicals, ions, or smaller molecules. The breakup can be homolytic, yielding two radicals, or heterolytic, yielding a cation and an anion. The outcome depends on the molecular structure, the light wavelength (photon energy), the surrounding medium, and the presence of other reactive species. The process can occur directly on the absorbing molecule or via photosensitized pathways where energy or electron transfer from another species enables bond rupture.

In atmospheric chemistry, photolysis governs the lifetimes of many compounds. Photodissociation cross-sections and actinic flux determine rates. Examples include the photodissociation of nitrogen dioxide (NO2 + hv → NO + O), which participates in ozone chemistry, and ozone photolysis (O3 + hv → O2 + O), which affects stratospheric composition.

In industry and research, photolysis underlies processes such as UV curing and photoinitiated polymerization. It is also used in laboratory studies to generate reactive intermediates and to probe reaction mechanisms. Kinetic analysis often relies on absorption spectroscopy and actinometry to quantify light exposure and quantum yields.

Safety aspects include the formation of reactive radicals and potentially hazardous species, so appropriate shielding and ventilation are important when conducting photolytic experiments.