Nanosecond laser irradiation of soot particles: Insights on structure and optical properties

In spite of the advances in laser diagnostics in combustion, the effect of rapid laser irradiation on the physical/chemical properties of soot particles is far from being comprehensively understood. Optical properties, particle nanostructure and aggregate size of laser-irradiated soot particles are analyzed in this paper. Carbonaceous particles generated by nitrogen-quenched diffusion flames of ethylene and methane are irradiated by a 1064-nm short laser pulse. Wavelength-resolved extinction measurements in the visible range are used to follow their transformation by varying the laser energy density. A significant variation of the extinction coefficient of the irradiated particles compared to the extinction coefficient of the pristine ones is observed mainly for ethylene soot. The particle nanostructures are analyzed by Raman spectroscopy and the effect of laser irradiation on aggregate structure is evaluated by measuring particle size distributions. Results suggest that different mechanisms, namely fragmentation and vaporization are responsible for changes in the soot optical properties. However, further investigations are still necessary for drawing an exhaustive picture of the effect of intense laser radiation on soot particles and assessing the impact on the interpretation of laser diagnostics.