A kinetic study of the reactions of MgO with H2O, CO2 and O2: implications for magnesium chemistry in the mesosphere

Abstract

The recombination reactions of MgO with H₂O, CO₂ and O₂ were studied by the pulsed photodissociation at 193.3 nm of magnesium acetyl acetonate [Mg(C₅H₇O₂)₂] vapour in the presence of O₃, producing MgO in an excess of the reactant and N₂ bath gas. MgO was monitored by time-resolved non-resonant LIF by pumping the MgO((B¹Σ⁺–X¹Σ⁺), Δ = 0) transition at 499.4 nm and detecting MgO(B¹Σ⁺–A¹Σ⁺) emission at λ>600 nm. All three recombination reactions were found to be in the fall-off region over the experimental pressure range (2–60 Torr). The data were fitted by RRKM theory combined with ab initio quantum calculations on Mg(OH)₂, MgCO₃, OMgO₂ and MgO₃, yielding the following results (180–600 K, 10⁻⁶–10³ Torr). For MgO + H₂O: log₁₀(k_rec,0/cm⁶ molecule⁻² s⁻¹) = − 32.75 + 7.894log₁₀T − 2.127log₁₀²T, k_rec,∞ = 3.52 × 10⁻¹⁰exp(−334/T) cm³ molecule⁻¹ s⁻¹, F_c = 0.28. For MgO + CO₂: log₁₀(k_rec,0/cm⁶ molecule⁻² s⁻¹) = − 33.70 + 5.827log₁₀T − 1.494log₁₀²T, k_rec,∞ = 6.79 × 10⁻¹⁰exp(−310/T) cm³ molecule⁻¹ s⁻¹, F_c = 0.37. For MgO + O₂: log₁₀(k_rec,0/cm⁶ molecule⁻² s⁻¹) = − 28.05 + 1.423log₁₀T − 0.683log₁₀²T; k_rec,∞ = 1.16 × 10⁻¹⁰exp(−219/T) cm³ molecule⁻¹ s⁻¹, F_c = 0.34 (F_c is the broadening factor). The uncertainty in extrapolating to the mesospheric temperature range (120–250 K) was determined using a Monte Carlo procedure. Finally, the implications of these results for magnesium chemistry in the mesosphere are discussed.