Seminars Archive

Threshold Photoelectron Spectroscopy with VUV Synchrotron Radiation to Trace Reactive Intermediates in Combustion, Pyrolysis and Catalysis

Abstract
In the last two decades soft photoionization with tunable vacuum ultraviolet (VUV) synchrotron radiation has evolved into a versatile workhorse for the identification and quantification of reactive intermediates and stable molecules in processes of energy conversion such as combustion and catalysis. Photoionization mass spectra (PIMS), which can be obtained by scanning the photon energy and recording the ion signal, can be useful to distinguish between constitutional isomers, when compared with calculated or experimental data. However, this approach is limited if the ionization energies of different isomers are close, since PI spectra do not show sharp peaks and vibrational fine structure as conventional photoelectron spectra do. Imaging Photoelectron Photoion Coincidence (iPEPICO) spectroscopy is an excellent extension to mass spectrometry, since it allows measuring (threshold) photoelectron spectra mass-selectively. These spectra show selective vibrational transitions into ion states, which mostly follow the Franck-Condon principle and thus serve as vibrational fingerprints to identify the complex composition of isomers. We will show several examples from the VUV beamline located at the Swiss Light Source, which profited greatly from this additional analytical dimension: Xylenes (dimethyl benzenes), are used as octane boosters in fuels. Although the first deactivation channel is the formation of the fuel radical (xylyl), further reactions are not very well understood. The combustion chemistry of xylyl radicals was thus investigated unimolecularly using a pyrolytic micro reactor.nTo simulate real combustion conditions an mxylene flame was studied. Apart from similar chemistry as compared to the micro reactor, we were able to identify up to four different isomers of the composition C9H8 and C10H10, which are important findings in terms of soot formation chemistry. Ring-expansion reactions (RER) of N heterocyclic carbenes (NHC) with main-group-element hydrides like silanes, boranes and beryllium compounds , were recently discovered as a new reaction class in the solvent phase. We will show that 1,3-di-methylimidazolin-2-ylidene, one of the smallest NHCs, also performs RERs in an intramolecular fashion upon excitation with VUV photons. We have determined the barrier for this reaction, which may give implications for catalysis.