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 H(2)O. Here we report rate coefficients and branching ratios of the reactions of Si(+) and SiO(+) with O(3), 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(+) + O(3), 298 K) = (6.5 ± 2.1) × 10(-10) cm(3) molecule(-1) s(-1), with three product channels (branching ratios): SiO(+) + O(2) (0.52 ± 0.24), SiO + O(2)(+) (0.48 ± 0.24), and SiO(2)(+) + O (<0.1); k(SiO(+) + O(3), 298 K) = (6 ± 4) × 10(-10) cm(3) molecule(-1) s(-1), where the major products (branching ratio ≥ 0.95) are SiO(2) + O(2)(+). 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 O(2)((1)Δ(g)), 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(+) + O(2) + He reaction was measured to be k(298 K) = (9.0±1.3) × 10(-30) cm(6) molecule(-2) s(-1), in agreement with previous measurements. The SiO(2)(+) species produced from this reaction, which could be vibrationally excited, is observed to charge transfer at a relatively slow rate with O(2), with a rate constant of k(298 K) = (1.5 ± 1.0) × 10(-13) cm(3) molecule(-1) s(-1).

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.

Kinetic studies of atmospherically relevant silicon chemistry part I: silicon atom reactions.

Atomic silicon is generated by meteoric ablation in the Earth's upper atmosphere (70-110 km). The reactions of Si(3P(J)) atoms with several atmospherically relevant species were studied by the pulsed laser photolysis of a Si atom precursor (typically PheSiH3), followed by time-resolved laser induced fluorescence at 251.43 nm (Si(3p2 3P0 --> 4s 3P1)). This yielded: k(Si + O2, 190-500 K) = 9.49 x 10(-11) + 1.80 x 10(-10) x exp(-T/115 K) cm3 molecule(-1) s(-1) (uncertainty < or = +/- 15%), in good accord with recent high-level theoretical calculations but in marked disagreement with previous experimental work; k(Si + O3, 190-293 K) = (4.0 +/- 0.5) x 10(-10) cm3 molecule(-1) s(-1); k(Si + CO2, 293 K) < or = 1.2 x 10(-14) cm3 molecule(-1) s(-1); and k(Si + H2O, 293 K) < or = 2.6 x 10(-13) cm3 molecule(-1) s(-1). These results are explained using a combination of quantum chemistry calculations and long-range capture theory. The quenching rate coefficients k(Si(1D2) + N2, 293 K) = (4.0 +/- 0.7) x 10(-11) cm3 molecule(-1) s(-1) and k(Si(1D2) + H2O, 293 K) = (2.3 +/- 0.3) x 10(-10) cm3 molecule(-1) s(-1) were also determined.