The complex heterogeneous nature of chars has confounded the complete analysis of the Raman spectra of these materials. The additional shoulders observed on the defect (D)-band and high intensity valley between the D and graphitic (G)-bands represent the primary regions of uncertainty. In this paper the effects of various vacancy and substitution defects in a coronene parent molecule have been systematically analyzed using density functional theory (DFT). The impacts of these defects are best understood in terms of a reduced symmetry as compared to a "parent" coronene molecule. Based on simulation results, a total of ten potential bands have been assigned between 1000 cm-1 and 1800 cm-1. These bands have been used to deconvolute a thermoseries of cellulose chars produced by pyrolysis at 300-700 °C. The shoulder observed in chars near 1200 cm-1 has been assigned to the symmetric breathing mode of various small polyaromatic hydrocarbons (PAH) as well as rings containing seven or more carbons. Intensity between 1400 cm-1 and 1550 cm-1 results from a range of coupled vibrational modes from various defect structures. The deconvolution of cellulose derived chars shows consistent growth of PAH clusters, loss of oxygen, and development of non-hexoganal ring systems as pyrolysis temperature increased.

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We have recently developed vetted methods for obtaining quantitative infrared directional-hemispherical reflectance spectra using a commercial integrating sphere. In this paper, the effects of particle size on the spectral properties are analyzed for several samples such as ammonium sulfate, calcium carbonate, and sodium sulfate as well as one organic compound, lactose. We prepared multiple size fractions for each sample and confirmed the mean sizes using optical microscopy. Most species displayed a wide range of spectral behavior depending on the mean particle size. General trends of reflectance versus particle size are observed such as increased albedo for smaller particles: for most wavelengths, the reflectivity drops with increased size, sometimes displaying a factor of 4 or more drop in reflectivity along with a loss of spectral contrast. In the longwave infrared, several species with symmetric anions or cations exhibited reststrahlen features whose amplitude was nearly invariant with particle size, at least for intermediate and large size sample fractions: that is, ≳150  μm. Trends of other types of bands (Christiansen minima, transparency features) are also investigated as well as quantitative analysis of the observed relationship between reflectance versus particle diameter.

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