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• Quantitative Detection of HCO behind Shock Waves: The Thermal Decomposition of HCO

G. Friedrichs, J.T. Herbon, D.F. Davidson, and R.K. Hanson
Phys. Chem. Chem. Phys. 4 (2002) 5778-5788

 

Using frequency modulation spectroscopy formyl radicals were detected behind shock waves for the first time. HCO radicals have been generated by 308 nm photolysis of mixtures of formaldehyde in argon. The HCO spectrum of the (A2A'' - X2A') (0900 - 0010) transition was measured at room temperature with high resolution and the predissociative linewidth Γ of the individual rotational lines were fitted to Γ = X + ZN'2(N'+1)2, where X = 0.22 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  provided the temperature independent rates of reaction (4), H+HCO → H2+CO, and reaction (5), HCO+HCO → CH2O+CO,

k4 = 1.1· 1014 cm3mol-1s-1
k5 = 2.7· 1013 cm3mol-1s-1

and the low pressure room temperature absorption cross section of the Q(9)P(2) line at 614.872 nm,αc = 1.51· 106 cm2mol-1 (base e).

Measurements of the unimolecular decomposition of HCO, (3) HCO + M → H + CO + M, were performed at temperatures from 830 to 1230 K and total densities from 3.3· 10-6 to 2.5· 10-5 mol/cm3. They can be represented by the following Arrhenius expression.

k3 = 4.0· 1013 · exp(-65 kJmol-1 / R T) cm3mol-1s-1 (Δ log k3 = ±0.23)

The corresponding RRKM fit, 4.8· 1017 · (T/K)-1.2 exp(-74.2 kJmol-1 / R T) cm3mol-1s-1 (600 < (T/K) < 2500), supports the lower range of previously reported high temperature rate expressions.

 

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