Room temperature and shock tube study of the reaction HCO+O2 using the photolysis of glyoxal as an efficient HCO source.

The rate of the reaction 1, HCO+O2-->HO2+CO, has been determined (i) at room temperature using a slow flow reactor setup (20 mbar<p<500 mbar) and (ii) in the temperature range 739 K<T<1108 K behind reflected shock waves (0.82 bar<p<1.84 bar) employing a perturbation approach. Following the 193 nm excimer laser photolysis of mixtures of glyoxal in Ar, concentration-time profiles were measured using frequency modulation (FM) detection of HCO at a wavelength of lambda=614.752 nm. Observed differences between HCO concentration-time profiles measured with and without O2 added to the reaction mixtures could be almost exclusively attributed to reaction 1. The determined rate constants, k1(295 K)=(3.55+/-0.05)x10(12) cm3 mol-1 s-1, k1(739-1108 K)=3.7x10(13) exp(-13 kJ mol-1/RT) cm3 mol-1 s-1 (Delta log k1=+/-0.16), reveal a slightly positive temperature dependence of reaction 1 at high temperatures. Furthermore, the 193 nm photolysis of glyoxal, (CHO)2, has been proven to be an efficient HCO source. Besides HCO, photolysis of the precursor also produces H atoms. The ratio of initially generated H atoms and HCO radicals, f=[H]0/[HCO]0total, was found to depend on the total density rho. At room temperature, it varies from f=1.6 at rho=8x10(-7) mol cm-3 to f=3.0 at rho=2x10(-5) mol cm-3. H atoms are transformed via reaction 4, H+(CHO)2-->H2+HCO+CO, into additional HCO radicals. The rate constants of reaction 4 were determined from unperturbed photolysis experiments to be k4(295 K)=(3.6+/-0.3)x10(10) cm3 mol-1 s-1 and k4(769-1107 K)=5.4x10(13)exp(-18 kJ mol-1/RT) cm3 mol-1 s-1(Delta log k4=+/-0.12).