The flame structure of a fuel-rich (ϕ = 2.4), laminar premixed, and lightly sooting acetylene flame at 40 mbar and the influence of ethanol addition on the species pool was investigated. Special emphasis was put on the analysis of important soot precursors like propargyl, benzene, and the polyynes. The mole fractions of more than 50 stable and radical species up to m/z = 170 are obtained experimentally in the flames by molecular-beam mass spectrometry (MBMS) in combination with single-photon ionization (SPI) by vacuum ultraviolet (VUV) radiation from the Advanced Light Source (ALS) in Berkeley, CA, USA. For the neat acetylene flame, successful measurements were performed with a combination of MBMS and imaging photoelectron photoion coincidence spectrometry (iPEPICO) at the VUV beamline at the Swiss Light Source (SLS) in Villigen, Switzerland and adding additional species information to the data set. Some interesting isomers (C3H2, C4H5, C4H2O) can be clearly identified by comparison of measured photoionization efficiency (PIE) curves or threshold photoelectron (TPE) spectra with Franck–Condon simulations or literature spectra, respectively. Because of apparatus improvements, the chemical resolution in this study goes beyond prior work and provides a high-quality data set for the development of reaction mechanisms at fuel-rich, low-pressure conditions.

Quantitative species data for the development and critical examination of combustion mechanisms are in high demand regarding the need for predictive combustion models that may assess the emission potential of current and emerging fuels. Mass spectrometric investigation is one of the often-used techniques to provide mole fractions of stable and reactive intermediates including radicals from specifically designed laboratory experiments. Molecular-beam mass spectrometry (MBMS) has been coupled with electron ionization (EI) and photoionization (PI) to determine the species compositions, and combinations of these techniques have been successful in the investigation of the combustion pathways in flames of numerous hydrocarbon, oxygenated and nitrogenated fuels. Photoelectron/photoion coincidence spectroscopy (PEPICO) has recently emerged as a novel diagnostics to be combined with flame-sampling mass spectrometry, and its potential as a complement of existing techniques is just about being explored. In a multi-laboratory investigation, the present study has thus combined four different MBMS spectrometers (in Bielefeld, Germany, the Advanced Light Source in Berkeley, USA, the Swiss Light Source in Villigen, Switzerland, and the SOLEIL synchrotron in St. Aubin, France) to study a rich premixed argon-diluted low-pressure (40 mbar) ethylene–oxygen flame under comparable conditions. This was done with the aim of illustrating the respective properties and capabilities of the methods under these conditions, with an emphasis on the power offered by the synchrotron-based techniques, including PEPICO, for combustion chemistry studies. Examples include comparisons of selected species quantification as well as PEPICO spectra measured at different instruments.

Fuel-rich flames (φ=1.8) of ethanol, 1,3-butadiene, and m-xy lene were investigated at low-pressure (40-50 mbar) with special emphasis on the identificat ion of reactive radicals, especially fuel radicals (C2H5O, C4H5, and C8H9) and some other important isomers (e.g. CH3O, C8H8, C11H10). Measurements were performed at the Swiss Light Source (SLS), where single-photon ionizat ion with VUV radiation offers a soft ionization technique for the sampled species. Isomer-selective detection is achieved by imaging photoelectron photoion coincidence (iPEPICO) techniques which comprises time-of-flight mass spectrometry and photoelectron spectroscopy. Measured photoionization efficiency curves and threshold photoelectron spectra are compared with reference spectra from the literature or calculated Franck-Condon simulations. Quantification of the major species as well as several intermediate species for all flames yields data sets for model validation. ∗ Corresponding author: thomas.bierkandt@uni-due.de Proceedings of the European Combustion Meeting 2015 Introduction Although the amount of alternative energy carriers is increasing, currently more than 80% of the world energy consumption is covered by fossil fuels (oil, natural gas, and coal) and fossil fuels will also be the most important primary energy carrier in the next years [1]. So, a further understanding in combustion processes is still necessary to control the emissions of pollutants like polycyclic aromat ic hydrocarbons (PAHs), soot, oxygenates, and NOx. Especially, the investigation of reactive combustion intermediates is important because they influence pollutant formation. The use of biofuels can reduce the format ion of aromat ic compounds and soot and ethanol is one of widely used biofuels worldwide. Ethanol was studied in detail in prev ious work under different conditions as single fuel [2,3] and additive [4,5], respectively. Particularly, the ability for soot reduction is often shown. 1,3-Butadiene is suited to investigate the formation of benzene as the first aromatic ring which is important for the formation o f all further PAHs and therefore for soot [6]. The formation of the propargyl radical plays here a key role. It is an important soot precursor because of the recombination reaction (C3H3 + C3H3 ⇌ C6H6) to benzene. Laminar flames of xy lenes [7,8] were also studied before but less extensive than ethanol. An established technique to identify and quantify combustion species, especially react ive rad icals, in premixed laminar low-pressure flames is molecu larbeam mass spectrometry (MBMS). It can be combined with different ionization techniques, e.g. electron ionization, single photon ionization, and REMPI (resonance enhanced multiphoton ionization). MBMS measurements typically result in speciation data for the validation of chemical kinetic reaction mechanisms [9]. One of the main experimental challenges is the transfer of the gas sample from the flame into the mass analyzer without changing the gas composition. It is typically accomplished through quenching reactions by either a rapid reduction in pressure or by dilution with an inert gas. Another challenge is the discrimination between isomers. In princip le, this can be done by comparison of measured photoionization efficiency (PIE) curves with already known ionizat ion energies or calculated values of possible species. This procedure works well for the isomer with the lowest ionization energy. The assignment of h igher thresholds can become difficu lt because changes in the slopes of the ionization efficiency curve do not necessarily correlate with ionization thresholds of other isomers. Photoelectron photoion coincidence (PEPICO) spectroscopy, which detects the electrons that are produced in the ionization process in coincidence with the ions, enables the measurement of mass-selected threshold photoelectron spectra (ms-TPES). These spectra can help to facilitate species detection because vibrational transitions from the neutral to specific ionic states can be observed yielding a fingerprint of the molecule [10]. Experiments We have investigated fuel-rich flames of ethanol, 1,3-butadiene, and m-xy lene at low-pressure with a stoichiometry of 1.8 for all three flames. The exact flame conditions can be found in Tab le 1. Conditions for 1,3-butadiene and m-xy lene are the same as in the work of Hansen et al. [6] and Li et al. [7], respectively. All measurements are performed at the Swiss Light Source of the Paul Scherrer Institute in Villigen, Switzerland where photoionization (PI) is achieved by tunable vacuum ultraviolet synchrotron radiation. Liquid fuels are continuously evaporated in a heated vaporizer and the steam is then introduced with the argon flow as carrier gas into the mixing chamber o f the

Adaptation of a low-pressure flat flame burner with a flame-sampling interface to the imaging photoelectron photoion coincidence spectrometer (iPEPICO) of the VUV beamline at the Swiss Light Source is presented. The combination of molecular-beam mass spectrometry and iPEPICO provides a new powerful analytical tool for the detailed investigation of reaction networks in flames. First results demonstrate the applicability of the new instrument to comprehensive flame diagnostics and the potentially high impact for reaction mechanism development for conventional and alternative fuels. Isomer specific identification of stable and radical flame species is demonstrated with unrivaled precision. Radical detection and identification is achieved for the initial H-abstraction products of fuel molecules as well as for the reaction controlling H, O, and OH radicals. Furthermore, quantitative evaluation of changing species concentrations during the combustion process and the applicability of respective results for kinetic model validation are demonstrated. Utilization of mass-selected threshold photoelectron spectra is shown to ensure precise signal assignment and highly reliable spatial profiles.