Spectral investigation of soot absorption properties during laser-induced incandescence measurements

The effect of rapid laser irradiation on the optical properties of mature soot particles is investigated by performing wavelength-resolved extinction measurements in the visible spectral region. In particular, the spectral behavior of the absorption properties is explored during laser irradiation, at the peak of the incandescence signal (prompt LII) and few nanoseconds after the peak, and finally when particles have reached an equilibrium condition with the surrounding gas. A significant variation of the absorption coefficient of the laser-irradiated soot particles compared to that of the pristine ones is observed already at the LII peak. Such variation keeps evolving with time until it reaches a final permanent value. Results are presented in relation to the laser fluence used for irradiation and discussed with the aim of stressing the need of knowing the entity of the modification of soot absorption properties during laser irradiation for a correct interpretation of LII data.