Translucent interstellar clouds produce three kinds of absorptions: continuous extinction, believed to be caused by interstellar dust particles, identified bands of simple radicals – most of them carbon bearing ones - and more than 400 unidentified diffuse interstellar bands (DIBs). The latter are commonly believed to be carried by complex molecular species likely carbon bearing ones (chain species based on a carbon scheleton, polycyclic aromatic hydrocarbons, fullerenes); however, none of them was until now identified beyond a doubt. High S/N spectra allow to determine precisely the corresponding column densities of the identified molecules and their rotational temperatures which differ seriously from object to object in cases of centrosymmetric species. Despite many laboratory based studies of possible DIB carriers, it has not been possible to unambiguously link these bands to specific species. This is unfortunate, as an identification of DIBs would substantially contribute to our understanding of chemical processes in the diffuse interstellar medium. The presence of substructures inside DIB profiles, discovered by Sarre et al. (1995) and Kerr et al. (1998), supports the idea that DIBs are very likely molecular features of gas phase species.  Extinction is believed to be caused by interstellar dust particles  of various sizes and shapes. The recent surveys of the extinction  curves (extinction law) demonstrate a great variety of the observed  curves which proves that grains are different if observed in different  clouds. It is to be emphasized that physical conditions inside individual clouds can be drastically different which is being revealed by changes of the extinction law which is accompanied by changes in molecular and diffuse spectra. This demonstrates complex chemistry of translucent interstellar clouds. The observed molecular species are likely building bricks for prebiotic molecules.

Carbon bearing molecules in interstellar clouds

249th National Meeting (2015)

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