Solar induced chemical reactions can be divided into thermochemical or photochemical. Photoelectrochemical oxidation may be thought of as a special case of photochemical oxidation (PCO). Summer smog is primarily associated with the photochemical formation of ozone. Photochemical degradation is negligible compared to photocatalytic degradation. This leads to a significantly larger photochemical reactivity and more ground level ozone. The tautomerization can also be catalyzed via photochemical process. They are created via photolithography or photochemical machining. The composition and chemical reactions involved in photochemical smog were not understood until the 1950s. It can be synthesized by chemical or photochemical reactions. Photoreactivation is a repair process in which photolyase enzymes directly reverse CPDs via photochemical reactions. Many sources of deterioration in celluloid exist, such as thermal, chemical, photochemical, and physical. The pentacarbonyl species are produced by photochemical reactions. The table below shows the selectivity rules for thermal and photochemical electrocyclic reactions. In the case of photochemical reactions, light provides the activation energy. Most photochemical transformations occur through a series of simple steps known as primary photochemical processes.