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

摘要

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~+(3~2P_(3/2)-3~2S_(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) x 10~(-9) cm~3 molecule~(-1) s~(-1) (la error). The reaction MgO~+ + O3 is also fast, k(295 K) = (8.5 ± 1.5) x 10~(-10) cm~3 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 x 10~(-31) (T/300 K)~(-1); k(Mg~+ + O2) = 4.1 x 10~(-31) (T/300 K)~(-1.65); k(Mg~+ + CO2) = 7.3 x 10~(-30) (r/300 K)~(-1.59); k(Mg~+ + N2O) = 1.9 x 10~(-30) (r/300 K)~(-2.51) cm~6 molecule~(-2) s~(-1) with 1ct 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) x 10~(-11); k(MgO2~+ + H2O -> Mg~+H2O + O2) = (1.9 ± 0.6) x 10~(-1); k(Mg~+-N2 + O2 -> Mg~+-O2 + N2) = (3.5 ± 1.5) x 10~(-12) cm~3 molecule~(-1) s~(-1). Low-pressure limiting rate coefficients were obtained for the following recombination reactions in He: k(MgO2~+ + O2) = 9.0 x 10~(-30) (T/300 K)~(-3.80); k(Mg~+·CO2 + CO2) = 2.3 x 10~(-29) (T/300 K)~(-5.08); k(Mg~+·H2O + H2O) = 3.0 x 10~(-28) (T/300 K)~(-3.96); k(MgO2~+ + N2) = 4.7 x 10~(-30) (T/300 K)~(-3.75); k(MgO2~+ + CO2) = 6.6 x 10~(-29) (T/300 K)~(-4.18); k(Mg~+H2O + O2) = 1.2 x 10~(-27) (T/300 K)~(-4.13) cm~6 molecule~(-2) s~(-1). The implications of these results for magnesium ion chemistry in the atmosphere are discussed.