Monitoring carbonaceous species in the transition from molecules to particles in shock-tube pyrolysis of toluene by laser induced emission spectroscopy

Particle inception remains the most enigmatic stage of the formation process of carbonaceous particles. Detailed knowledge of the evolution of optical properties during the transition from molecular species to particles is essential for unraveling this phenomenon and enabling accurate particle volume fraction measurements of freshly formed particles in combustion environments. This study monitors the transition from molecular precursors to incipient soot particles during toluene pyrolysis behind reflected shock waves by laser-induced emission spectroscopy. Time-resolved and spectrally-resolved measurements of laser-induced emission were performed with excitation at 266, 355, 532, or 1064 nm. Microsecond time resolution was provided upon laser-pulse excitation via simultaneous measurements at various spatial locations behind the reflected shock wave, using the reaction-time-resolved detection concept. These measurements trace the evolution of different stages of the carbonaceous species evolving from red-shifted laser-induced fluorescence (LIF) progressing from toluene decomposition and polycyclic aromatic hydrocarbon (PAH) formation to the onset of incipient soot and subsequent laser-induced incandescence (LII) from refractory soot. LII signals recorded after 1064-nm excitation were utilized to identify initial particle formation, while time-resolved LII measurements provided insight into particle-size evolution. These findings contribute to a deeper understanding of soot inception and provide optical properties of the early stage of soot particles.