Since 2013, experimental investigations of chemical reaction networks in premixed, laminar low-pressure flames have been carried out at the Swiss Light Source (SLS) in cooperation with the University of Duisburg-Essen (UDE) and the Paul Scherrer Institute (PSI). For the first time, photoelectron photoion coincidence (PEPICO) spectroscopy was used in a flame experiment to provide detailed species data for validation of kinetic reaction models and to address specific chemical reaction kinetics issues, e.g., the isomerization of the m-xylyl radical. The unambiguous spectral identification of isomeric species in reactive environments (e.g., flames) as well as the high sensitivity to radical species (especially fuel radicals) are highlights of the experiment.
For using PEPICO spectroscopy in flame diagnostics, a specially designed combustion chamber for model flames with a molecular-beam sampling probe was set into operation at the VUV beamline of the SLS. The established molecular-beam mass spectrometry (MBMS) in combination with the analytical capabilities of the PEPICO spectroscopy and the high energy resolution of tunable vacuum ultraviolet (VUV) radiation, which can only be provided by specific particle accelerators, allows a comprehensive, isomer-selective analysis of stable and reactive combustion species. Cations and electrons, which are generated during the ionization process, can only be measured in coincidence by means of PEPICO spectroscopy. In addition to the established detection strategy by photoionization efficiency (PIE) curves, an additional, previously unmatched, analytical dimension is achieved. Electrons are velocity map imaged onto a position sensitive detector. By considering only electrons having no kinetic energy, i.e., threshold electrons, information about the internal energy of the cation is obtained. The resulting threshold photoelectron (TPE) spectra are spectroscopic fingerprints of each molecule and a clear identification of combustion species is feasible as exemplary demonstrated for the C3H4 isomers propadiene and propyne.
A further advantage of the flame experiment at the SLS is its outstanding detection sensitivity for measuring radicals, which is achieved by the short distance between sampling and ionization volume. This allows even the detection of isomeric radicals, which play a key role in combustion networks and are only present in low concentrations in specific regions of a flame. For example, the butyl radicals (C4H9), which are formed in an isobutane flame and were not detectable in previous flame experiments, could be clearly identified with the new PEPICO flame experiment. A further development of the PEPICO spectrometer is the double-imaging setup which also allows the velocity map imaging of photoions onto a position sensitive detector. The double-imaging setup enables the optimization of the molecular beam to further increase the sensitivity of the experiment.
The PEPICO experiment is a significant addition to the core activities of the institute of combustion technology, i.e., fuels and pollutants, and combines scientific fundamental research with wider generic societal issues. For example, hydrogen abstraction reactions and soot precursor chemistry are investigated within the DFG project "PEPICO in combustion chemistry research II". To investigate the pressure effect on the reactivity of the fuel mixtures, the PEPICO experiment was successfully extended for the use with atmospheric flow and jet-stirred reactors and even with a high-pressure (6 bar) reactor.
P. Oßwald, P. Hemberger, T. Bierkandt, E. Akyildiz, M. Köhler, A. Bodi, T. Gerber, T. Kasper, In situ flame chemistry tracing by imaging photoelectron photoion coincidence spectroscopy, Review of Scientific Instruments 85 (2014) 025101.
D. Krüger, P. Oßwald, M. Köhler, P. Hemberger, T. Bierkandt, Y. Karakaya, T. Kasper, Hydrogen abstraction ratios: A systematic iPEPICO spectroscopic investigation in laminar flames, Combustion and Flame 191 (2018) 343-352.
T, Bierkandt, P. Hemberger, P. Oßwald, D. Krüger, M. Köhler, T. Kasper, Flame structure of laminar premixed anisole flames investigated by photoionization mass spectrometry and photoelectron spectroscopy, Proceedings of the Combustion Institute 37 (2019) 1579-1587.
D. Krüger, P. Oßwald, M. Köhler, P. Hemberger, T. Bierkandt, T. Kasper, The fate of the OH radical in molecular beam sampling experiments, Proceedings of the Combustion Institute 37 (2019) 1563-1570.