photodissociation

Photodissociation, also known as photolysis in some contexts, is the process by which a chemical bond is broken as a result of absorbing a photon. When a molecule absorbs light with energy exceeding its bond dissociation energy, it can dissociate into two or more fragments. This process can occur in the gas phase or in condensed matter, and the specific products depend on the electronic excited state accessed and the shape of the molecular potential-energy surfaces. Photodissociation is distinct from photoionization, in which the absorbed energy ejects an electron.

Mechanisms of photodissociation include direct photodissociation, where absorption leads to a repulsive excited-state surface and immediate fragmentation, and predissociation, where a bound excited state couples to a repulsive state, allowing dissociation with a finite lifetime. Indirect pathways and multiphoton processes can also contribute, particularly under high-intensity light or complex environments. The likelihood of dissociation is described by the photodissociation cross section and the quantum yield, which depend on wavelength and molecular state.

Occurrence and significance: In Earth's atmosphere, photodissociation drives many chemical cycles, including ozone formation and destruction and the removal of pollutants like nitrogen dioxide. In interstellar and circumstellar environments, photodissociation rates influence the chemical composition of gas clouds and photodissociation regions. Rates depend on the radiation field, molecular absorption cross sections, and shielding by dust and other gases. In laboratories and industry, photodissociation is used to study reaction mechanisms and to generate reactive fragments for chemical synthesis.

Measurement and modeling: Photodissociation cross sections are determined by spectroscopy and used in atmospheric, planetary, and astrochemical models. Calculations require accurate quantum-mechanical potential-energy surfaces and considerations of vibrational and rotational state distributions.