Optical properties of carbonaceous dust analogues

Structurally different carbon materials with an increasing sp2/sp3 ratio were synthesized by pyrolizing cellulose materials at 400, 600, 800 and 1000 oC . The hybridization ratios and the internal structures in these sampleswere determined by using High Resolution Transmission Electron Microscopy, Electron Energy Loss Spectroscopy, Raman spectroscopy, IR spectroscopy and elemental combustion analysis. The pyrolized carbon samples were embedded in an epoxide resin and the surface was grinded and polished. The reflectance of the samples could be measured in the range between 200 nm and 500 μm, coveringmost of the spectral range relevant for radiative transfer calculations. Using the Lorentz-oszillator fit method we were able to determine the optical constants n and k. A big change in the optical behaviour could be seen between 600 and 800 oC, which is caused by the presence of free charge carriers in the samples pyrolized at 800 and 1000 oC. In these samples the graphitic areas have been grown to about 1.5 nm in size. The absorption behaviours of small spheres in the Rayleigh limit and a continous distribution of ellipsoids (CDE) were calculated. The spectral index β in the FIR region (λ≥ 100 μm) increases from 1.2 for the very disordered still aliphatic-dominated carbon material pyrolized at 400 oC to 2.2 for the more ordered graphitic sample carbonized at 1000 oC in case of spherical particles. In the more graphitic samples pyrolized at 800 and 1000 oC, the spectral index β is strongly dependent on the shape of the particles.