Near-threshold photoionization molecular beam mass spectrometry for identifying reactive intermediates and products in an atmospheric pressure non-thermal plasma operating in siloxane

In this study, we present near-threshold photoionization mass spectra that reveal organic species with mass-to-charge ratios ranging from 15 to 500, sampled during the plasma reformation of siloxane. The experiments utilized a tubular configuration of an atmospheric pressure dielectric barrier discharge (DBD) along with a helium-siloxane gas mixture, in which the siloxane concentration was maintained at approximately 100 ppm. We investigated two different siloxanes: hexamethyldisiloxane (HMDSO) and octamethyltetrasiloxane (D4). Near-threshold photoionization was achieved using tunable vacuum-ultraviolet (VUV) light generated by a synchrotron. This technique minimizes fragmentation, facilitating the identification of the parent molecule within complex mixtures. Additionally, the tunability of the VUV light allows for species identification based on photoionization efficiency (PIE) curves and enables isomer discrimination. For the two siloxanes studied, we observed variations in mass progression, peak intensities, and isomeric content as indicated by the PIE curves.