IDENTIFICATION OF CARBON FORMS IN SOOT MATERIALS OF ASTROPHYSICAL INTEREST

We determined the carbon structures in condensed soot samples of varia ble C/H ratio by using both scanning and transmission electron microsc opy. We identified several types of carbon structures. All the samples are formed mainly by ''chain-like aggregates'' of amorphous grains. A small amount of more ordered forms of carbon is also detected. A ''po orly graphitized carbon'' is formed on a pre-existing amorphous carbon substrate due to auto-annealing. ''Bucky-carbons'' are ''proper'' pro ducts of condensation. Occasionally, rare graphitic carbon ribbons and single-crystal carbon platelets are observed. The ''chain-like aggreg ates'' 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 sampl es, suggest that carbon ordered on a micro-scale only, rather than gra phitic carbons, is more appropriate to interpret the interstellar UV e xtinction bump position. The carbon forms, ''proper'' products of soot condensation processes, are similar to those detected in chondritic i nterplanetary dust particles. This could imply that they are of primar y origin, rather than derived from parent body processing.