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 sampled from nitrogen-quenched diffusion flames of ethylene and methane are irradiated on-line 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 variation of the extinction coefficient of the irradiated particles compared to the extinction coefficient of the pristine ones is observed, especially 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. The results indicate that both soot nanostructure and optical properties are strongly dependent on the laser energy density when irradiated by a laser source. This is significant for ethylene soot, while for methane soot the degree of variation of such properties is less pronounced, at least in the investigated heating conditions.