O( 1S) In the lower thermosphere - Chapman vs Barth

New data on the production and quenching of O( ¹S) in the lower thermosphere have been combined with current altitude profiles of 5577 Å intensities and O( ³P) concentrations to show that a persuasive case can be made for the idea that O( ¹S) is generated by the sequence of steps O( ³P) + O( ³P) + M → O ₂^∗ + M, O ₂^∗ + O( ³P) → O ₂ + O( ¹S) (the Barth mechanism), rather than by the reaction O( ³P) + O( ³P) + O( ³P) → O ₂+ O( ¹S) (the Chapman mechanism). The Chapman reaction is not entirely excluded, but now must be considered hypothetical, since the laboratory data correlate with the atmospheric observations only through a mechanism involving an intermediate O ₂^∗. Further work is needed to establish the identity of this intermediate, which could well be the O₂( A³Σ⁺_u) state, and to obtain the necessary kinetic data for a complete description of the system.