Direct observation and identification of resonance-stabilized radicals in a sooting flame

Understanding the molecular mechanisms governing soot inception is critical for developing accurate models of particulate formation in combustion systems. Polycyclic aromatic hydrocarbons (PAHs) are well established as key precursors/intermediates that participate to soot formation, but the growth reactions towards the particulate phase are still the subject of active debate. Among the proposed chemical mechanisms, aromatic resonance-stabilized radicals (RSRs) offer a promising alternative pathway, but their in situ detection in flames has proven challenging, limiting further mechanistic insight. Here, we report the direct observation and identification of several polycyclic aromatic RSRs, using a combination of synchrotron-based mass-selected vacuum ultraviolet photoelectron spectroscopy and quantum chemical calculations. The existence of both delocalized and localized π-radicals and their specific implication on the different stages of soot formation, via radical-driven PAH reaction mechanisms, is discussed in the context of combustion chemistry. Polycyclic aromatic hydrocarbons (PAHs) are key precursors in soot formation, but the reactions leading to clustering in the particulate phase remain under debate. Here, the authors study PAH radical-driven pathways using synchrotron-based and mass-selected vacuum ultraviolet photoelectron spectroscopy and show that early clustering might be driven by π-delocalized radicals, while more localized radicals might contribute to covalent cross-linking during later stages of soot inception.