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.