Watching soot inception via online Raman spectroscopy

In this work, online spontaneous Raman spectroscopy was applied to study the soot inception and growth zones of a low pressure premixed ethylene/oxygen flame. Firstly, we measured online Raman spectrum of aerosol soot extracted from the flame. The spectrum was compared to ex situ Raman measurements of the same soot after being deposited on a window. In the aerosol soot particles, the presence of an abundant sp hybridized carbon chain component is inferred accompanying a polycyclic aromatic component. Both cumulenic and polyynic chains are affected by the deposition under vacuum and by the pressure as it is raised to atmospheric, revealing that the sp phase is no longer visible in the ex situ characterization of the sampled soot. Secondly, we monitored the Raman spectra of aerosol soot as a function of the height above the burner. Comparing spectral differences of by-products along the blue to orange zone of the flame permitted the soot formation and evolution to be probed. The online spectra, by avoiding structural modifications of the real soot structures by deposition, highlight the role of unsaturated carbon chains in the chemical condensation scenario.