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

Reactions between Mg+ and O-3, O-2, N-2, 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(2)P(3/2)-3(2)S(1/2))). The rate coefficient for the reaction Mg+ + O-3 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) (1 sigma error). The reaction MgO+ + O-3 is also fast, k(295 K) = (8.5 +/- 1.5) x 10(-10) cm(3) molecule(-1) s(-1), and produces Mg+ + 2O(2) with a branching ratio of (0.35 perpendicular to 0.21), the major channel forming MgO2+ + O-2. Rate data for Mg+ recombination reactions yielded the following low-pressure limiting rate coefficients: k(Mg+ + N-2) = 2.7 x 10(-31) (T/300 K)(-1.88); k(Mg+ + O-2) = 4.1 x 10(-31) (T/300 K)(-1.65); k(Mg+ + CO2) = 7.3 x 10(-30) (T/300 K)(-1.59); k(Mg+ + N2O) = 1.9 x 10(-30) (T/300 K)(-2.51) cm 6 molecule(-2) s(-1), with 1s 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+center dot CO2 + H2O -> Mg+center dot H2O + CO2) = (5.1 +/- 0.9) x 10(-11); k(MgO2+ + H2O -> Mg+center dot H2O + O-2) = (1.9 +/- 0.6) x 10(-11); k(Mg+center dot N-2 + O-2 -> Mg+center dot O-2 + N-2) = (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+ + O-2) = 9.0 x 10(-30) (T/300 K)(-3.80); k(Mg+center dot CO2 + CO2) - 2.3 x 10(-29) (T/300 K)(-5.08); k(Mg+center dot H2O + H2O) - 3.0 x 10(-28) (T/300 K)(-3.96); k(MgO2+ + N-2) = 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+center dot H2O + O-2) = 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.