New experimental constraints on the composition and structure of tholins
Résumé
Powdered samples of carbon–nitrogen-hydrogen “tholins” that mimic Titan's atmosphere aerosols were produced under levitation conditions in the laboratory with a dusty plasma (PAMPRE experiment) using different initial N2:CH4 gas mixtures and studied using UV Raman and infrared spectroscopy, X-ray diffraction and high resolution transmission electron microscopy (HRTEM). Comparison between the tholins produced in the PAMPRE experiments and samples prepared by other techniques reveal that they form a fairly homogeneous family of hydrogenated carbon nitride materials. Wall effects during the PAMPRE deposition experiments and other were found to have little effect on the chemical structure of tholins. The first-order UV Raman bands of the disordered carbonaceous materials point to a large contribution of sp2 clusters present compared with olefinic Cdouble bond; length as m-dashN or Cdouble bond; length as m-dashC groupings, whereas features at 690 and 980 cm−1 suggest C3N3 rings are present as a species inserted in the macromolecular network. Diffraction techniques do not indicate the presence of large polyaromatic species in any of the tholins studied, whatever their nitrogen concentration, in disagreement with certain previous observations. This precludes the idea that the nature and degree of absorption in the visible range is controlled by the size of polyaromatic species, as has been observed in series of carbon-based materials obtained via thermal processing. Infrared spectroscopy analysis of the tholins has confirmed earlier identifications of chemical groups present including primary amines, nitriles, and alkyl moieties such as CH2/CH3, but has ruled out CH2/CH3 branches appearing on secondary or tertiary amines. Similar analyses were also performed on a polymeric (HCN)x material, which was found to present several similarities with the tholins, hence suggesting similar polymerization processes.