UNUSUAL KINETIC-BEHAVIOR OF THE REACTIONS MG+O2+M AND CA+O2+M(M=N2,HE) OVER EXTENDED TEMPERATURE RANGES

The reaction Mg + O2 + N2 Was studied by a static-flow technique in which the rate coefficient was determined relative to the diffusion coefficient of (Mg(S))-S-1 atoms in N2. This yielded k(349K<T<625K) = (2.15 +/- 0.56) X 10(-30) exp[-(19.5 +/- 0.7) kJ mol-1/RT], where the quoted uncertainty is 2sigma. This result is in very good accord with the results of a recent flow-tube study which found the rate coefficient to be about 3 times slower when He was the third body. The reaction Ca + O2 + M (M = N2, He) was then studied by the pulsed laser photolysis of a Ca atom precursor, followed by time-resolved laser-induced fluorescence of the resulting Ca(1S) atoms. The reaction with N2 as the third body is between 2 and 3 times faster than the reaction with He, and the rate coefficients for both third bodies increase between 218 and 340 K and then decrease up to 1100 K. Ab initio calculations and RRKM theory are then employed to demonstrate that the unusual temperature dependences of both the Mg and Ca reactions are most probably caused by barriers in the entrance channels of these reactions. The recommended rate coefficients between 120 and 2000 K are then given by log10(k(Mg+O2+N2)) = -392.7 + 439.1x - 203.2x2 + 42.56x3 - 3.413x4, log10(k(Mg+O2+He)) = -394.5 + 442.4x - 205.0x2 + 42.94x3 -3.436x4, log10(k(Ca+O2+N2)) = -97.75 + 84.66x - 39.85x2 + 8.522x3 - 0.7093x4, and log10(k(Ca+O2+He)) = -93.67 + 78.02x - 35.76x2 + 7.369x3 - 0.5899x4, where k is in units of cm6 molecule-2 s-1, x = log10(T), and the uncertainty in k is estimated to be +/-30%. Finally, the implications of these results for the chemistry of meteor-ablated Mg and Ca in the upper atmosphere are considered.