Kinetic study of the reaction between Fe and O3 under mesospheric conditions

Abstract

Detailed observations of the layer of atomic iron in the mesosphere have been made in the last four years. However, there is almost no information about the gas-phase chemistry of neutral iron which is necessary to understand the characteristic features of the layer. In this paper, a study of the reaction Fe + O₃→ FeO + O₂ is reported. The reaction was investigated by the pulsed photodissociation at 193.3 nm of ferrocene vapour to produce Fe atoms in an excess of O₃ and N₂ bath gas, followed either by time-resolved laser-induced fluorescence spectroscopy of atomic Fe at 248.3 nm [Fe(x ⁵F⁰₅–a ⁵D₄)], or by time-resolved chemiluminescence from the ‘orange bands’ of FeO at λ= 590 ± 5 nm [FeO(⁵Δ_i–x ⁵Δ_i)]. The rate coefficient is given in the Arrhenius form by k(189 < T/K < 359)=(3.44 ± 0.76)× 10^–10 exp[–(1210 ± 420)J mol^–1/RT] cm³ molecule^–1 s^–1, where the quoted uncertainties are 2σ. This result is compared with the predictions of long-range capture theory, and contrasted with the analogous reactions of other metal atoms with O₃. The title reaction is then demonstrated to be the most rapid oxidation process of atomic iron in the atmosphere between 65 and 100 km. Furthermore, it is shown that this reaction may make a significant contribution to the night-time production of O₂(b ¹Σ⁺_g) in the upper atmosphere below 90 km.