Quantitative detection of HCO behind shock waves: The thermal decomposition of HCO

Abstract

Using FM spectroscopy formyl radicals were detected for the first time behind shock waves. HCO radicals have been generated by 308 nm photolysis of mixtures of formaldehyde in argon. The HCO spectrum of the (òA″ ← ²A′) (09⁰0 ← 00¹0) transition was measured at room temperature with high resolution and the predissociative linewidths Γ of the individual rotational lines were fitted to Γ = X + ZN′²(N′ + 1)², where X = 0.22 cm⁻¹ and Z = 1.0 × 10⁻⁵ cm⁻¹. Since FM spectroscopy is very sensitive to small line shape variations the spin splitting in the Q-branch could be resolved.

Time resolved measurements of HCO profiles at temperatures below 820 K provided the temperature independent rates of reaction (4), H + HCO → H₂ + CO, and reaction (5), HCO + HCO → CH₂O + CO, k₄ = 1.1 × 10¹⁴ cm³ mol⁻¹ s⁻¹k₅ = 2.7 × 10¹³ cm³ mol⁻¹ s⁻¹and the low pressure room temperature absorption cross section of the Q(9)P(2) line at 614.872 nm, α_c = (1.5 ± 0.4) × 10⁶ cm² mol⁻¹ (base e).

Measurements of the unimolecular decomposition of HCO, reaction (3) HCO + M → H + CO + M, were performed at temperatures from 835 to 1230 K and at total densities from 3.3 × 10⁻⁶ to 2.5 × 10⁻⁵ mol cm⁻³. They can be represented by the following Arrhenius expression. k₃ = 4.0 × 10¹³·exp(−65 kJ mol⁻¹/RT) cm³ mol⁻¹ s⁻¹ (Δ log k₃ = ±0.23)The corresponding RRKM fit, 4.8 × 10¹⁷·(T/K)^−1.2·exp(−74.2 kJ mol⁻¹/RT) cm³ mol⁻¹ s⁻¹ (600 < T/K < 2500), supports the lower range of previously reported high temperature rate expressions.