Experimental and computational determinations of optical band gaps for PAH and soot in a N2-diluted, ethylene/air non-premixed flame

Visible light extinction was measured at a height of 20 mm above the fuel tube exit in a nitrogen-diluted, ethylene/air, non-premixed flame and this data was used to determine the optical band gap, Egopt, as a function of radial position in the flame. This height was chosen as previous measurements in our laboratory have shown substantial Raman scattering from thermophoretically-sampled, carbonaceous material at this flame location. Further, this height is at the onset of large signal intensity in annular flame regions from laser-induced incandescence measurements. In our previous work, analysis of the Raman spectrum suggested the source of the scattering was PAH species with sp² conjugation lengths of 1.0–1.2 nm, consistent with a molecular mass range of 500–1000 Da. In the current study, light from a light emitting diode, with center emission wavelength of 445 nm, was collimated, spatially filtered, and then focused into the flame. Transmitted light was recollimated and then directed into a spectrometer. After tomographic reconstruction of the radial extinction field, the optical band gap was derived from the near edge absorption spectrum using a Tauc analysis. The optical band gap, ≈2.4 eV, was then compared with calculations of the electronic structure of a series of D_2h polynuclear aromatic hydrocarbons using time-dependent density functional theory. HOMO–LUMO gaps for these PAH were correlated with the number of aromatic rings in the molecules. From this correlation, the measured band gap suggested that the source of the extinction could be a PAH with as few as 10 aromatic rings.