Low-Field Ion Extraction and Coincident Electron Detection in a Tabletop Setup Help Resolve Ambiguities in Identifying Combustion Intermediates*

Most combustion reactions are exceedingly complex and difficult to model due to hundreds of chemical intermediates, many of which have not even been identified. Photoionization mass spectrometry (PIMS) has become a common tool for identifying these intermediates for two reasons: this technique is very sensitive to low-concentration species, and very little prior knowledge of the reaction is required. Here, we improve on the traditional fixed-wavelength PIMS measurement in two ways. First, we use a low electric field to extract ions in order to distinguish thermally-induced fragmentation in the reactor from dissociative ionization caused by the laser. Second, we use electron-ion coincidence detection to measure a photoelectron spectrum for each observed mass simultaneously. We provide proof of principle that the electron spectrum can distinguish isomers that have sufficiently different ionization potentials. These improvements were completed in a tabletop setup, rather than at a national facility, enabling year-round data collection. Here, we use a pyrolysis microreactor jet to cause thermal fragmentation of fuels, but the detection techniques can be generalized to other reactors.