Optical properties of “young” carbonaceous particles

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Despite the importance of carbonaceous particle formation in combustion processes, there is a surprisingly large number of mysteries related to their formation, namely to the transition from molecular structures to particles. The objective of this project is to investigate and clarify the optical properties of carbonaceous molecules and particles in the transition phase between the gas phase towards clusters and “young” par¬ti¬cles. Information about these properties has a three-fold relevance: (i) For diagnostics purposes, knowing the optical properties of the related species is crucial to solve pressing issues related to measuring soot formation and soot formation mechanisms in lab-scale and practical devices. (ii) Understanding the interaction of the early cluster/particles in the transition phase with radiation can shed light on binding forces and thus the formation mechanism of the related species. (iii) There is a greater fundamental interest in understanding the optical properties of carbonaceous particles at all stages of their formation process because of their relevance in atmospheric radiative balancing (green house contribution of soot has the largest overall uncertainty), absorption and re-emission of deposited particles (e.g., on snow) influencing the local temperature balance, and even interstellar optical transmission.Unlike conventional soot research in flames, where the transition from gas-phase precursors towards particles occurs in a very narrow zone that is characterized by extreme gradients in temperature and species concentration, the experiments in this project will be carried out in shock tubes. Here, the various stages in particle formation are well separated in time (and thus in space because of the rapidly moving shock wave that initiates the reaction). This will enable a selective analysis of the various intermediates involved in carbonaceous particle formation. The project takes advantage of a unique combination of experimental capabilities. The shock tube operated at around atmospheric pressure is equipped with optical diagnostics for temporally- and spatially-resolved absorption measurements (kinetics spectrometer) and spectrally- and reaction-time-resolved laser-induced fluorescence (RTR-LIF) and laser-induced incandescence (RTR-LII) measurements. These spectroscopic methods are coupled with highly sensitive time-resolved extinction measurements that record what is conventionally interpreted as “the onset of soot formation” as well as with an ex situ characterization of the formed species. These combined methods provide comprehensive information about the absorption and emission properties as well as the smallest structures that allow for laser heating towards incandescence.