Atomic oxygen

Atomic oxygen is oxygen present as individual atoms rather than the molecules ${\mathrm{O}}_2$ or ${\mathrm{O}}_3$. Its open-shell electronic structure makes many forms of atomic oxygen highly reactive.

Photodissociation and electrical excitation can generate it in gases. In the upper atmosphere, atomic oxygen participates in reaction networks and can interact strongly with exposed spacecraft materials. Its chemical behavior cannot be inferred by treating it as ordinary molecular oxygen at a lower concentration.

An electronically excited atomic-oxygen state that can be quenched collisionally or react with species such as water. Quenching back to the ground state and radical production compete, so the photochemical source rate need not equal the radical source rate.