Optical and microscopy investigations of soot structure alterations by laser-induced incandescence

Understanding the physical process of LII is central to practical implementation and accurate theoretical modelling of LII. The LII dependence upon laser fluence is shown to depend upon detection conditions thereby not providing direct information about the soot temperature or structural changes. Transmission electron microscopy, used to investigate the morphological changes induced in the soot at different laser fluences, shows increasing graphitization of the soot with increasing laser fluence. For laser fluences above 0.45 ± 0.05 J/cm² at 1064 nm, vaporization/fragmentation of soot primary particles and aggregates occurs. Optical measurements are performed using a second laser pulse to probe the effects of these changes upon the LII signal. With the exception of very low fluences, the structural changes induced in the soot lead to a decreased LII intensity produced by the second laser pulse. These two-pulse experiments also show that these changes do not alter the LII signal on timescales less than 1 μs for fluences below the vaporization threshold.