Elementary reactions of formyl (HCO) radical studied by laser photolysis-transient absorption spectroscopy

Several elementary reactions of formyl radical of combustion importance were studied using pulsed laser photolysis coupled to transient UV-Vis absorption spectroscopy: HCO → H + CO (1), HCO + HCO → products (2), and HCO + CH₃ → products (3). One-pass UV absorption, multi-pass UV absorption as well as cavity ring-down spectroscopy in the red spectral region were used to monitor temporal profiles of HCO radical. Reaction (1) was studied over the buffer gas (He) pressure range 0.8-100 bar and the temperature range 498-769 K. Reactions (2) and (3) as well as the UV absorption spectrum of HCO, were studied at 298 and 588 K, and the buffer gas (He) pressure of 1 bar. Pulsed laser photolysis (308, 320, and 193 nm) of acetaldehyde, propionaldehyde, and acetone was used to prepare mixtures of free radicals. The second-order rate constant of reaction (1) obtained from the data at 1 bar is: κ₁(He) = (0.8 ± 0.4) × 10^-10exp(-(66.0 ± 3.4) kJ mol^-1//RT) cm³ molecule^-1 s^-1. The HCO dissociation rate constants measured in this work are lower than those reported in the previous direct work. The difference is a factor of 2.2 at the highest temperature of the experiments and a factor of 3.5 at the low end. The experimental data indicate pressure dependence of the rate constant of dissociation of formyl radical 1, which was attributed to the early pressure fall-off expected based on the theory of isolated resonances. The UV absorption spectrum of HCO was revised. The maximum absorption cross-section of HCO is (7.3 ± 1.2) 10^-18 cm² molecule^-1 at 230 nm (temperature independent within the experimental error). The measured rate constants for reactions (2) and (3) are: κ₂ = (3.6 ± 0.8) × 10^-11 cm³ molecule^-1 s^-1 (298 K); κ₃ = (9.3 ± 2.3) × 10^-11 cm³ molecule^-1 s^-1(298 and 588 K).