Kinetic studies of atmospherically relevant silicon chemistry Part I: Silicon atom reactions

Abstract

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(3p²³P₀→ 4s ³P₁)). This yielded: k(Si + O₂, 190–500 K) = 9.49 × 10⁻¹¹ + 1.80 × 10⁻¹⁰× exp(−T/115 K) cm³ molecule⁻¹ s⁻¹ (uncertainty ≤±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) × 10⁻¹⁰ cm³ molecule⁻¹ s⁻¹; k(Si + CO₂, 293 K) ≤ 1.2 × 10⁻¹⁴ cm³ molecule⁻¹ s⁻¹; and k(Si + H₂O, 293 K) ≤ 2.6 × 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) × 10⁻¹¹ cm³ molecule⁻¹ s⁻¹ and k(Si(¹D₂) + H₂O, 293 K) = (2.3 ± 0.3) × 10⁻¹⁰ cm³ molecule⁻¹ s⁻¹ were also determined.