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