Spektroskopische Untersuchungen an polyzyklischen aromatischen Kohlenwasserstoffen mit aliphatischen Seitengruppen
Zusammenfassung der Projektergebnisse
In an attempt to elucidate the role of polycyclic aromatic hydrocarbons (PAHs) with aliphatic side groups as a component of interstellar molecular clouds, we have studied the UV/vis and IR spectroscopy of various polyynyl-substituted PAHs experimentally and theoretically. In this study, we concentrated on the PAHs naphthalene (C10H8), anthracene (C14H10), phenanthrene (C14H10), and pyrene (C16H10) with ethynyl (–C2H) and butadiynyl (–C4H) side groups (8 different molecules) and compared them with their parent molecules without side group. While only 3 polyynyl-substituted PAHs were commercially available, the other 5 polyynylsubstituted species were synthesized by our cooperation partner at the Technical University Dresden (TUD). The synthesis appeared more time-consuming and more costly than expected. Therefore, the amount of sample material available for spectroscopic measurements was rather modest. Moreover, it turned out that most samples were sensitive to air and temperature exposure. Due to the latter property, it was difficult to bring the species with sufficiently high concentration into the gas phase. As a result, for most species, we were not able to perform the rather time-consuming jet experiments that were originally planned. At first, we investigated the mobility of the various new species in our high-performance liquid chromatography (HPLC) apparatus. The experimental values could be fitted very well by the predictions of a simple model taking into account the (calculated) polarizabilities and electric dipole moments of the species under consideration. This data will be extremely useful if one wants to determine the concentrations of polyynyl-substituted PAHs in samples synthesized in the laboratory employing astrophysically relevant high-temperature gas-phase condensation processes like laser pyrolysis. Regarding the UV/vis absorption spectroscopy of polyynyl-substituted PAHs, only one commercially available species could be measured in the gas phase (supersonic jet), namely 9-ethynylphenanthrene. All other species were studied in neon ice matrices in the spectral range between 190 and 450 nm. Upon substitution with polyynyl chains, we observed the following effects, which depend on the presence and length of the chain: (1) The origins of the two lowest-energy transitions experience a red-shift, which scales with the length of the chain. (2) At the same time, the band intensities increase. (3) On the other hand, bands associated with transitions to higher electronic states are getting broader and lower in peak intensity. Hence, lower-energy transitions become more pronounced at the expense of the transitions involving higher electronic states. As far as the astronomical implication is concerned, we can conclude that PAHs with polyynyl side groups are better carrier candidates for the diffuse interstellar bands (DIBs) than PAHs without side chains. However, the presently studied species cannot be made responsible for any of the DIBs known so far. Infrared absorption spectra were measured for grains of polyynyl-substituted PAHs embedded in CsI salt pellets at room temperature. In the spectral range between 2.6 and 30 µm, we observed the typical aromatic modes of the PAH molecules as well as several bands attributed to the C≡C-H bending, C≡C stretching, and acetylenic C-H stretching modes of the ethynyl and butadiynyl side groups. The assignments were possible on the basis of our accompanying theoretical calculations. Comparing the IR spectra of ethynyl-substituted PAHs with older spectra recorded in our group from PAH samples produced by laser pyrolysis, we could identify vibrational bands in the laser pyrolysis soot indicating the existence of ethynyl side groups. As the conditions, under which the laser pyrolysis was performed, resemble those encountered in carbon-rich envelopes of late-type stars, we conclude that PAHs with ethynyl side groups are likely produced in such environments. Comparison of our laboratory IR data with astronomical observations further reveals that polyynyl-substituted PAHs were not observed in regions where PAHs are generally found. Still, these species may be present in other regions, where different conditions prevail, such as the diffuse interstellar medium.
Projektbezogene Publikationen (Auswahl)
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Abundances of PAHs in the ISM: Confronting observations with experimental results, Astron. Astrophys 530, A26 (2011)
Gredel, R., Carpentier, Y., Rouillé, G., Steglich, M., Huisken, F., and Henning, Th.
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On the relevance of polyynylsubstituted polycyclic aromatic hydrocarbons to astrophysics, Astrophys. J. 752, 25 (2012)
Rouillé, G., Steglich, M., Carpentier, Y., Jäger, C., Huisken, F., Henning, Th., Czerwonka, R., Theumer, G., Börger, C., Bauer, I., and Knölker, H.-J.
