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)

Abstract

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: k_rec,0 = 1.0 × 10⁻²⁹ (T/300 K)^−2.31 cm⁶ molecule⁻² s⁻¹; k_rec,∞ = 8.1 × 10⁻¹⁰ cm³ molecule⁻¹ s⁻¹. For Fe⁺ + H₂O: k_rec,0 = 5.3 × 10⁻²⁹ (T/300 K)^−2.02 cm⁶ molecule⁻² s⁻¹; k_rec,∞ = 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.