Hydrogenated amorphous carbon

Amorphous carbonaceous materials can show a great diversity of optical properties due to the variability in their microstructure. Especially in the infrared range, the optical constants can differ by orders of magnitude according to the conducting or insulating electrical behavior of the material. The amorphous-carbon data contained in the database cover a wide range of these properties as is illustrated by Fig. . The differently pyrolized celluloses are representative for a suit of carbonaceous material ranging from strongly disordered (insulating) to graphitized (conducting) material.

Figure: Complex refractive index of hydrogenated amorphous carbon prepared by pyrolysis (annealing) of cellulose at different temperatures.

Figure: Absorption coefficient calculated for Spheres and a CDE in vacuum from the optical data of Fig. for at 400 and 1000$^o$C pyrolized cellulose

Especially interesting for astronomy is the calculations of the absorption and scattering cross sections of small particles in vacuum. In Fig. , the strongly disordered material pyrolized at a temperature of 400$^o$C shows an absorption efficiency in the wavelength region between 0.6 and 100 $\mu$m which is smaller by 3 orders of magnitude compared to the other carbon materials. The absorption efficiency normalized by the particle radius of carbonaceous particles in the far infrared follows a power law (Q $_{abs}/a \sim \lambda^{-\beta}$). The spectral index $\beta$ depends strongly on the internal structure of the carbon materials. The spectral index $\beta$ in the long wavelength tail is considerably lower for the highly disordered material than the exponents of the carbon material pyrolized at higher temperature ([14]). There is a gradual increase of $\beta$ for spherical grains with increasing graphitization due to higher pyrolysis temperature. Our calculations for different particle shapes show that there is no morphological effect on the spectral index for the low-temperature samples in contrast to the more graphitic materials. For the latter materials we find a significantly lower index in the case of broad shape distributions (CDE) compared to spherical grain shapes. This is caused by percolation effects, present in the more graphitized samples which contain free charge carriers. We should note that the results of the CDE calculations serve as an illustrative example. For a more realistic calculation, one has to assume a special aggregate structure and/or shape distribution of the individual particles ([15]). For extreme values of the refractive indices, computational methods for the calculation of the absorption by aggregates or elongated particles meet their limits.