Unimolecular dissociation of formyl radical, HCO → H plus CO, studied over 1-100 bar pressure range

Unimolecular dissociation of formyl radical, HCO --> H + CO (1), was studied using pulsed laser photolysis coupled to transient UV-vis absorption spectroscopy. One-pass UV absorption, multipass UV absorption, and cavity ring down spectroscopy in the red spectral region were used to monitor temporal profiles of the HCO radical. A heatable high-pressure flow reactor of a new design was employed. Reaction I was studied over a buffer gas (He) pressure range 0.8-100 bar and a temperature range 498-769 K. Formyl radicals were prepared by pulsed photolysis of acetaldehyde and propionaldehyde (308 nm, XeCl excimer laser, 320 nm, doubled dye laser). In addition to formyl radicals monitored at 230 and 613.8 nm, methyl radicals were monitored via absorption at 216.5 nm. The initial concentrations of free radicals were varied between 7 x 10(10) and 8 x 10(13) molecules cm(-3). The obtained second-order rate constant at 1 bar is k(1)(He) = (0.8 +/- 0.4) x 10(-10) exp(-66.0 +/- 3.4 kJ mol(-1)/RT) cm(3) molecule(-1) s(-1) (498-769 K). The low-pressure data of this study were combined with those from a high-temperature shock tube study and the low-temperature data on the reverse reaction to yield k(1)(He) = (0.60 +/- 0.14) x 10(-10) exp(-64.2 +/- 1.4 kJ mol(-1)/RT) cm(3) molecule(-1) s(-1) over an extended temperature range, 298-1229 K. The dissociation rate constants measured in this work are lower than previously reported by a factor of 2.2 at the highest temperature of our measurements and a factor of 3.5 at the low end. Our experimental data indicate a pressure dependence of the second-order rate constant for the dissociation of formyl radical (1), which is attributed to pressure falloff expected from the theory of isolated resonances.