Kinetic studies of atmospherically relevant silicon chemistry. Part III: Reactions of Si+ and SiO+ with O3, and Si+ with O2

Silicon ions are generated in the Earth's upper atmosphere by hyperthermal collisions of material ablated from incoming meteoroids with atmospheric molecules, and from charge transfer of silicon-bearing neutral species with major atmospheric ions. Reported Si+ number density vs. height profiles show a sharp decrease below 95 km, which has been commonly attributed to the fast reaction with H2O. Here we report rate coefficients and branching ratios of the reactions of Si+ and SiO+ with O3, measured using a flow tube with a laser ablation source and detection of ions by quadrupole mass spectrometry. The results obtained are (2σ uncertainty): k(Si+ + O3, 298 K) = (6.5 ± 2.1) × 10−10 cm3 molecule−1 s−1, with three product channels (branching ratios): SiO+ + O2 (0.52 ± 0.24), SiO + O2+ (0.48 ± 0.24), and SiO2+ + O (<0.1); k(SiO+ + O3, 298 K) = (6 ± 4) × 10−10 cm3 molecule−1 s−1, where the major products (branching ratio ≥ 0.95) are SiO2 + O2+. Reactions (1) and (2) therefore have the unusual ability to neutralise silicon directly, as well as forming molecular ions which can undergo dissociative recombination with electrons. These reactions, along with the recently reported reaction between Si+ and O2(1Δg), largely explain the disappearance of Si+ below 95 km in the atmosphere, relative to other major meteoric ions such as Fe+ and Mg+. The rate coefficient of the Si+ + O2 + He reaction was measured to be k(298 K) = (9.0±1.3) × 10−30 cm6 molecule−2 s−1, in agreement with previous measurements. The SiO2+ species produced from this reaction, which could be vibrationally excited, is observed to charge transfer at a relatively slow rate with O2, with a rate constant of k(298 K) = (1.5 ± 1.0) × 10−13 cm3 molecule−1 s−1.