Identification of carbon forms in soot materials of astrophysical interest

We determined the carbon structures in condensed soot samples of variable C/H ratio by using both scanning and transmission electronmicroscopy.We identified several types of carbon structures. All the samples are formedmainly by “chainlike aggregates” of amorphous grains. A small amount of more ordered forms of carbon is also detected. A “poorly graphitized carbon” is formed on a pre-existing amorphous carbon substrate due to auto-annealing. “Bucky-carbons” are “proper” products of condensation. Occasionally, rare graphitic carbon ribbons and single-crystal carbon platelets are observed. The “chain-like aggregates” are the only form sensitive to hydrogen content variations and, probably, the main form responsible for the UV spectral response. In agreement with previous analyses on similar soot samples, we conclude that the internal structure of the amorphous grains drives the overall spectroscopic properties of hydrogenated soots. The relations between structural and spectral trends, common to several kinds of soot samples, suggest that carbon ordered on a micro-scale only, rather than graphitic carbons, is more appropriate to interpret the interstellar UV extinction bump position. The carbon forms, “proper” products of soot condensation processes, are similar to those detected in chondritic interplanetary dust particles. This could imply that they are of primary origin, rather than derived from parent body processing.