On the possible role of the reaction O + HO₂ → OH + O₂ in OH airglow

Several workers have concluded that analysis of the OH infrared emission from the upper mesosphere and lower thermosphere requires population of OH(υ = 5, 6) by the reaction O + HO 2 → OH(υ ≤ 6) + O 2 . It has been stated previously that this may be unlikely if the reaction proceeds by O atom transfer rather than H atom transfer. We show here that recent laboratory evidence argues convincingly that the mechanism of this reaction involves O atom transfer after formation of an HO 3 complex and that little or no population of highly vibrational levels (υ ≥ 4) of OH are expected, although nonnegligible population of lower levels seems plausible. On this basis it appears that this reaction should not be contributing substantially to emission from OH(υ = 5, 6). The reverse reaction, OH(υ ≥ 6) + O 2 → HO 2 + O, which has been suggested to be a potential sink for OH(υ ≥ 6), is considered and is expected to be slow unless an HO 3 complex is formed, in which case, multiple quantum deactivation of OH is also a likely process. Recent experimental evidence suggesting that OH(υ = 4, 5) may be populated by the H + O 3 → OH + O 2 reaction is discussed, although no firm conclusions about its importance can be reached.