The radicals produced by hydrogen abstraction in the initial fuel decomposition step are essential in combustion kinetics, but their experimental detection is very challenging. Imaging photoelectron photoion coincidence spectroscopy enables the detection and identification of even these isomeric radicals. Laminar low-pressure (40 mbar) hydrogen flames doped with different alkanes and alkenes are investigated systematically with the goal to identify the formation pathways and the fate of fuel radicals formed in hydrogen abstraction reactions. The abstraction reactions of primary, secondary, tertiary, and vinylic H atoms were never target of a systematic, direct semiquantitative investigation in a flame environment and this paper describes such a study for the first time. Performing the measurements at the vacuum ultraviolet beamline located at the Swiss Light Source enables isomer-selective detection of reactive radical species by imaging photoelectron photoion coincidence spectroscopy. For unambiguous identification of several isomeric radicals, threshold photoelectron spectra were compared with reference photoelectron spectra. H-abstraction ratios of isomeric radicals were determined and compared to literature reaction barriers and rate coefficients. In addition to the quantitative information, the peak positions of the profiles of radicals formed by hydrogen abstraction or addition to the fuel molecules as function of distance from the burner show faster H-abstraction for unbranched alkanes and alkenes than for branched fuels and faster H-addition than H-abstraction, respectively. 1

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This work investigates the decomposition of tetramethylsilane and the formation of silicon oxide clusters in a laminar premixed low-pressure hydrogen flame using molecular-beam mass spectrometry (MBMS). A comprehensive list of the species that exist in the gas phase was compiled and spatially resolved mole fraction profiles of species in the flame were obtained. Quantitative data in dependence of height above the burner were obtained for all major species and intermediates. The MBMS detection technique allowed to monitor Si-C-O-H, and Si-O-H-containing compounds as well as C-C species. The measured data show that the reaction of tetramethylsilane is initiated by H-abstraction from a methyl group and subsequent formation of oxygenated species. The measurements suggest that combustion of tetramethylsilane in a hydrogen flame proceeds mainly by a stepwise substitution of the methyl ligands by hydroxyl groups. Molecular and radical intermediates like Si(CH)OH, Si(OH), and Si(OH) are formed in the reaction zone. Significant amounts of Si(OH) are present at large distances above the burner. A repetitive growth pattern suggests that the monomer Si(OH) is a likely species initiating the formation and growth of larger silicon oxide clusters, e.g., SiOH, SiOH, and SiOH that can form nanoparticles in subsequent reactions.

The formation of typical low‐temperature oxidation products is observed in laminar premixed low‐pressure flames investigated by photoionization molecular‐beam mass spectrometry at the Swiss Light Source. The C1–C4 alkyl hydroperoxides can be identified in n‐butane‐ and 2‐butene‐doped hydrogen flames by their photoionization efficiency spectra at m/z 48, 62, 76, and 90. C1–C3 alkyl hydroperoxides are also observed in a propane‐doped hydrogen flame and in a neat propane flame. In addition, threshold photoelectron spectra reveal the presence of the alkyl hydroperoxides. In the 2‐butene/H2 flame, the photoionization spectrum at m/z 88 also enables the identification of butenyl hydroperoxides by comparison with calculated ionization energies of the alkenyl hydroperoxides and a literature spectrum. The low‐temperature species are formed close to the burner surface with maximum mole fractions at 0.25–0.75 mm above the burner. At 0.5 mm, even the methylperoxy radical (CH3OO) is measured for the first time in a laminar premixed flame. The rate of production analyses show that consumption of the hydroperoxyalkyl radicals results in the formation of cyclic ethers. In the n‐butane/H2 flame, ethylene oxide, oxetane, and methyloxirane are identified. Besides expected small oxygenated species, for example, formaldehyde or acetaldehyde, the larger C4 oxygenates butanone (C2H5COCH3) and 2,3‐butanedione (C4H6O2) are formed in the two C4 hydrocarbon‐doped hydrogen flames. Quantification of alkyl hydroperoxides with estimated photoionization cross sections based on the corresponding alcohols, which have similar photoelectron structures to the alkyl hydroperoxides, shows that mole fractions are on the order of 10−5–10−6 in the n‐butane/H2 flame. Measurements are corroborated by simulations, which also predict the presence of some peroxides in detectable concentrations, that is, mole fractions larger than 10−7, under the investigated conditions. The observation of peroxide species and cyclic ethers in the investigated laminar premixed flames give new insights into the contribution of low‐temperature combustion chemistry in a flame.