Jean Rouzaud

Nanostructuration of carbonaceous dust as seen through the positions of the 6.2 and 7.7 μm AIBs

Context. Carbonaceous cosmic dust is observed through infrared spectroscopy either in absorption or in emission. The details of the spectral features are believed to shed some light on its structure and finally enable the study of its life cycle. Aims. The goal is to combine several analytical tools in order to decipher the intimate nanostructure of some soot samples. Such materials provide interesting laboratory analogues of cosmic dust. In particular, spectroscopic and structural characteristics that help to describe the polyaromatic units embedded into the soot, including their size, morphology, and organisation are explored. Methods. Laboratory analogues of the carbonaceous interstellar and circumstellar dust were produced in fuel-rich low-pressure, premixed and flat flames. The soot particles were investigated by infrared absorption spectroscopy in the 2−15 μm spectral region. Raman spectroscopic measurements and high-resolution transmission electron microscopy were performed, which offered complementary information to better delineate the intimate structure of the analogues. Results. These laboratory analogues appeared to be mainly composed of sp 2 carbon, with a low sp 3 carbon content. A cross relation between the positions of the aromatic C=C bands at about 6.2 micron and the band at about 8 micron is shown to trace differences in shapes and structures of the polyaromatic units in the soot. Such effects are due to the defects of the polyaromatic structures in the form of non-hexagonal rings and/or aliphatic bridges. The role of these defects is thus observed through the 6.2 and 7.7 μm aromatic infrared band positions, and a distinction between carriers composed of curved aromatic sheets and more planar ones can be inferred. Based on these nanostructural differences, a scenario of nanograin growth and evolution is proposed.

On a reliable structural characterization of polished carbons in meteorites by Raman microspectroscopy

ABSTRACT Polished section of Acapulco meteorite was used for the structural examination of graphitic matter by Raman microspectroscopy. The polishing process was shown to be necessary for the samples, where carbon matter is embedded in metallic phases. However, this process is known to drastically change the Raman spectrum, inducing unacceptable errors in the intrinsic structural characterization of these carbon materials. The deconvolution of spectra shows that the G-band width, related to the only bond stretching of sp2 atoms, gives real structural information, even on polished carbon, whereas the common intensity ratio ID/IG, which is too sensitive to polishing process, has to be avoided.