A kinetic study of the reactions of Fe+ with N2O, N2, O2, CO2 and H2O, and the ligand-switching reactions Fe+·X + Y → Fe+·Y + X (X = N2, O2, CO2; Y = O2, H2O)

A series of reactions involving Fe+ ions were studied by the pulsed laser ablation of an iron target, with detection of ions by quadrupole mass spectrometry at the downstream end of a fast flow tube. The reactions of Fe+ with N2O, N2 and O2 were studied in order to benchmark this new technique. Extending measurements of the rate coefficient for Fe+ + N2O from 773 K to 185 K shows that the reaction exhibits marked non-Arrhenius behaviour, which appears to be explained by excitation of the N2O bending vibrational modes. The recombination of Fe+ with CO2 and H2O in He was then studied over a range of pressure and temperature. The data were fitted by RRKM theory combined with ab initio quantum calculations on Fe+·CO2 and Fe+·H2O, yielding the following results (120–400 K and 0–103 Torr). For Fe+ + CO2: krec,0 = 1.0 × 10−29 (T/300 K)−2.31 cm6 molecule−2 s−1; krec,∞ = 8.1 × 10−10 cm3 molecule−1 s−1. For Fe+ + H2O: krec,0 = 5.3 × 10−29 (T/300 K)−2.02 cm6 molecule−2 s−1; krec,∞ = 2.1 × 10−9 (T/300 K)−0.41 cm3 molecule−1 s−1. The uncertainty in these rate coefficients is determined using a Monte Carlo procedure. A series of exothermic ligand-switching reactions were also studied at 294 K: k(Fe+·N2 + O2) = (3.17 ± 0.41) × 10−10, k(Fe+·CO2 + O2) = (2.16 ± 0.35) × 10−10, k(Fe+·N2 + H2O) = (1.25 ± 0.14) × 10−9 and k(Fe+·O2 + H2O) = (8.79 ± 1.30) × 10−10 cm3 molecule−1 s−1, which are all between 36 and 52% of their theoretical upper limits calculated from long-range capture theory. Finally, the role of these reactions in the chemistry of meteor-ablated iron in the upper atmosphere is discussed. The removal rates of Fe+ by N2, O2, CO2 and H2O at 90 km altitude are ∼0.1, 0.07, 3 × 10−4 and 1 × 10−6 s−1, respectively. The initially formed Fe+·N2 and Fe+·O2 are converted into the H2O complex at ∼0.05 s−1. Fe+·H2O should therefore be the most abundant single-ligand Fe+ complex in the mesosphere below 90 km.