A kinetic study of Mg+ and Mg-containing ions reacting with O3, O2, N2, CO2, N2O and H2O: implications for magnesium ion chemistry in the upper atmosphere

Abstract

Reactions between Mg⁺ and O₃, O₂, N₂, CO₂ and N₂O were studied using the pulsed laser photo-dissociation at 193 nm of Mg(C₅H₇O₂)₂ vapour, followed by time-resolved laser-induced fluorescence of Mg⁺ at 279.6 nm (Mg⁺(3²P_3/2–3²S_1/2)). The rate coefficient for the reaction Mg⁺ + O₃ is at the Langevin capture rate coefficient and independent of temperature, k(190–340 K) = (1.17 ± 0.19) × 10⁻⁹ cm³ molecule⁻¹ s⁻¹ (1σ error). The reaction MgO⁺ + O₃ is also fast, k(295 K) = (8.5 ± 1.5) × 10⁻¹⁰ cm³ molecule⁻¹ s⁻¹, and produces Mg⁺ + 2O₂ with a branching ratio of (0.35 ± 0.21), the major channel forming MgO₂⁺ + O₂. Rate data for Mg⁺ recombination reactions yielded the following low-pressure limiting rate coefficients: k(Mg⁺ + N₂) = 2.7 × 10⁻³¹ (T/300 K)^−1.88; k(Mg⁺ + O₂) = 4.1 × 10⁻³¹ (T/300 K)^−1.65; k(Mg⁺ + CO₂) = 7.3 × 10⁻³⁰ (T/300 K)^−1.59; k(Mg⁺ + N₂O) = 1.9 × 10⁻³⁰ (T/300 K)^−2.51 cm⁶ molecule⁻² s⁻¹, with 1σ errors of ±15%. Reactions involving molecular Mg-containing ions were then studied at 295 K by the pulsed laser ablation of a magnesite target in a fast flow tube, with mass spectrometric detection. Rate coefficients for the following ligand-switching reactions were measured: k(Mg⁺·CO₂ + H₂O → Mg⁺·H₂O + CO₂) = (5.1 ± 0.9) × 10⁻¹¹; k(MgO₂⁺ + H₂O → Mg⁺·H₂O + O₂) = (1.9 ± 0.6) × 10⁻¹¹; k(Mg⁺·N₂ + O₂ → Mg⁺·O₂ + N₂) = (3.5 ± 1.5) × 10⁻¹² cm³ molecule⁻¹ s⁻¹. Low-pressure limiting rate coefficients were obtained for the following recombination reactions in He: k(MgO₂⁺ + O₂) = 9.0 × 10⁻³⁰ (T/300 K)^−3.80; k(Mg⁺·CO₂ + CO₂) = 2.3 × 10⁻²⁹ (T/300 K)^−5.08; k(Mg⁺·H₂O + H₂O) = 3.0 × 10⁻²⁸ (T/300 K)^−3.96; k(MgO₂⁺ + N₂) = 4.7 × 10⁻³⁰ (T/300 K)^−3.75; k(MgO₂⁺ + CO₂) = 6.6 × 10⁻²⁹ (T/300 K)^−4.18; k(Mg⁺·H₂O + O₂) = 1.2 × 10⁻²⁷ (T/300 K)^−4.13 cm⁶ molecule⁻² s⁻¹. The implications of these results for magnesium ion chemistry in the atmosphere are discussed.