Photochemistry of the airglow continuum

Recent observations of the airglow continuum and related phenomena are discussed in an effort to identify the photochemical processes responsible for the emission. After an exhaustive examination it is suggested that most of the continuum is due to collision-induced transitions of the O2 molecule from bound states close to the dissociation limit, |${\text{O}}_2{}^{*}{(}^1{\Sigma }_g+)\to {\text{O}}_2{}^{*}{(}^3{\Sigma }_g-)+hv$| (5000–9000 Å) |${\text{O}}_2{}^{*}{(}^1{\Delta }_g+)\to {\text{O}}_2{}^{*}{(}^3{\Sigma }_g-)+hv$| (7000 Å–2·2 µ) with estimated emission rates of I and 150 kR respectively. Present ideas about the mechanism of recombination of atoms cannot explain the existence of this emission and need revising. There is also a continuum from the reaction |$\text{NO}+\text{O}\to {\text{NO}}_2+hv$| (4000–5500 Å) with an emission rate of about 200 R which can vary independently from that of the O2 continuum.