"The laser-induced incandescence (LII) signal is proportional to soot volume fraction" is an often used statement in scientific papers, and it has – within experimental uncertainties – been validated in comparisons with other diagnostic techniques in several investigations. In 1984 it was shown theoretically in a paper by Melton that there is a deviation from this statement in that the presence of larger particles leads to some overestimation of soot volume fractions. In the present paper we present a detailed theoretical investigation of how the soot particle size influences the relationship between LII signal and soot volume fraction for different experimental conditions. Several parameters have been varied; detection wavelength, time and delay of detection gate, ambient gas temperature and pressure, laser fluence, level of aggregation and spatial profile. Based on these results we are able, firstly, to understand how experimental conditions should be chosen in order to minimize the errors introduced when assuming a linear dependence between the signal and volume fraction and secondly, to obtain knowledge on how to use this information to obtain more accurate soot volume fraction data if the particle size is known.

 We have performed a comparison of ten models that predict the temporal behavior of laser-induced incandescence (LII) of soot.  In this paper we present a summary of the models and comparisons of calculated temperatures, diameters, signals, and energy-balance terms.  The models were run assuming laser heating at 532 nm at fluences of 0.05 and 0.70 J/cm2 with a laser temporal profile provided.  Calculations were performed for a single primary particle with a diameter of 30 nm at an ambient temperature of 1800 K and pressure of 1 bar.  Preliminary calculations were performed with a fully constrained model.  The comparison of unconstrained models demonstrates a wide spread in calculated LII signals.  Many of the differences can be attributed to the values of a few important parameters, such as the refractive index function E(m) and thermal and mass accommodation coefficients.  Constraining these parameters brings most of the models into much better agreement with each other, particularly for the low-fluence case.  Agreement among models is not as good for the high-fluence case, even when selected parameters are constrained.  The reason for greater variability in model results at high fluence appears to be related to solution approaches to mass and heat loss by sublimation.

 We investigated the physical and chemical changes induced in soot aggregates exposed to laser radiation using a scanning mobility particle sizer, a transmission electron microscope, and a scanning transmission x-ray microscope to perform near edge x-ray absorption fine structure spectroscopy.  Laser-induced nanoparticle production was observed at fluences above 0.12 J/cm2 at 532 nm and 0.22 J/cm2 at 1064 nm.  Our results indicate that new particle formation proceeds via (1) vaporization of small carbon clusters by thermal or photolytic mechanisms, followed by homogeneous nucleation, (2) heterogeneous nucleation of vaporized carbon clusters onto material ablated from primary particles, or (3) both processes. 

This paper reports on a study of laser-induced incandescence of carbon particles in free space within a high vacuum (<10-3 mbar) excited by an Nd:YAG laser pulse. We have conducted an experimental study using samples of carbon black placed within an evacuated, sealed glass vessel which is slowly tumbled to cause a cascade of carbon black particles in free space. Our experiments show that under a high vacuum two important phenomena are observed. Due to the absence of gaseous conduction, in comparison to particles in ambient air, incandescence lifetime in a vacuum is dramatically extended to more than 50 μs with a corresponding increase of a factor of over 104 in the integrated or total number of photons emitted by each soot primary particle. For large aggregates and/or agglomerates in a vacuum after a delay of the order of 2 to 10 μs, the large particles fragment into smaller entities. We have also modelled the incandescence behaviour using well established methods.