What is "singlet oxygen"

n. (context physics chemistry English) A metastable state of molecular oxygen that has a slightly higher energy than normal triplet oxygen, and is more chemically reactive.

Singlet oxygen is a high energy form of oxygen. A gas with the formula O, its physical properties differ only subtly from those of the more prevalent triplet form of O. In terms of its chemical reactivity, however, singlet oxygen is far more reactive toward organic compounds. It is responsible for the photodegradation of many materials but can be put to constructive use in preparative organic chemistry and photodynamic therapy. Trace amounts of singlet oxygen are found in the upper atmosphere and also in polluted urban atmospheres where it contributes to the formation of lung damaging nitrogen dioxide. It often appears and coexists confounded in environments that also generate ozone, such as pine forests with photodegradation of turpentine.

In spectroscopic notation, the singlet and triplet forms of O are labeled Δ and Σ, respectively. The terms 'singlet oxygen' and ' triplet oxygen' refer to the quantum state of the molecules, with singlet oxygen existing in the singlet state with a total quantum spin of 0.