Kinetic studies of atmospherically relevant silicon chemistry. Part II: silicon monoxide reactions.

Silicon monoxide (SiO) is injected directly into the Earth's upper atmosphere by ablating meteoroids. SiO is also produced by the reaction of atomic Si (another ablation product) with O(2) and O(3). The reactions of SiO with several atmospherically relevant oxidants have been studied by the pulsed laser photolysis of a Si atom precursor in the presence of O(2), followed by time-resolved non-resonant laser-induced fluorescence of SiO at 282 nm. This yielded: k(SiO + O(3), 190-293 K) = (4.4 +/- 0.6) x 10(-13) cm(3) molecule(-1) s(-1); k(SiO + O(2) + He, 293 K) < or = 3 x 10(-32) cm(6) molecule(-2) s(-1), k(SiO + O + He, 293 K) < or = 1 x 10(-30) cm(6) molecule(-2) s(-1), k(SiO + H(2)O, 293 K, 4-20 Torr) < or = 4 x10(-14) cm(3) molecule(-1) s(-1), and k(SiO + OH, 293 K, 4-20 Torr) = (5.7 +/- 2.0) x 10(-12) cm(3) molecule(-1) s(-1). These results are explained by combining ab initio quantum chemistry calculations with transition state theory and RRKM theory. An upper limit of 5 x 10(-13) cm(3) molecule(-1) s(-1) for the reaction SiO(2) + O --> SiO + O(2) was determined, but calculations indicate the existence of a high barrier (104.7 kJ mol(-1)) which will make this reaction very slow at mesospheric temperatures.