KINETIC-STUDY OF THE REACTION BETWEEN FE AND O-3, UNDER MESOSPHERIC CONDITIONS

Detailed observations of the layer of atomic iron in the mesosphere ha ve been made in the last four years. However, there is almost no infor mation about the gas-phase chemistry of neutral iron which is necessar y to understand the characteristic features of the layer. In this pape r, a study of the reaction Fe + O-3, --> FeO + O-2 is reported. The re action was investigated by the pulsed photodissociation at 193.3 nm of ferrocene vapour to produce Fe atoms in an excess of O-3, and N-2, ba th gas, followed either by time-resolved laser-induced fluorescence sp ectroscopy of atomic Fe at 248.3 nm [Fe(x(5)F(5)(0)-a(5)D(4))], or by time-resolved chemiluminescence from the 'orange bands' of FeO at lamb da = 590 +/- 5 nm [FeO((5) Delta(i)-X(5) Delta(i))]. The rate coeffici ent is given in the Arrhenius form by k(189 < T/K < 359) = (3.44 +/- 0 .76) x 10(-10) exp[-(1210 +/- 420)J mol(-1)/RT] cm(3) molecule(-1) s(- 1), where the quoted uncertainties are 2 sigma. This result is compare d with the predictions of long-range capture theory, and contrasted wi th the analogous reactions of other metal atoms with O-3. The title re action is then demonstrated to be the most rapid oxidation process of atomic iron in the atmosphere between 65 and 100 km. Furthermore, it i s shown that this reaction may make a significant contribution to the night-time production of O-2(b1 Sigma(g)(+)) in the upper atmosphere b elow 90 km.