Synthesis, processing, and spectroscopic characterization of nanometer-sized polycyclic aromatic hydrocarbons with astrophysical impact
Final Report Abstract
The present project was devoted to the spectroscopic characterization of large polycyclic aromatic hydrogen (PAH) molecules, including their synthesis and their processing by UV light. This study was motivated by essentially three astrophysical phenomena, for which PAHs can possibly be made responsible. These are the UV bump at 217.5 nm, the diffuse interstellar bands (DIBs) between 440 and >800 nm, and the aromatic infrared bands (AIBs). To simulate the astrophysical synthesis conditions, the PAHs were produced by gas-phase condensation in a flow reactor employing laser pyrolysis of a light hydrocarbon gas (C2H4). The PAHs were extracted from the soot (by washing the filter in solvents) and further processed by liquid chromatography to obtain fractions with narrow size distributions. Various methods have been applied to characterize the structural properties of the samples. Mass spectrometric analysis revealed a PAH size distribution with molecules containing up to 44 C atoms. Employing nuclear magnetic resonance and infrared spectroscopy, we found that a surprisingly large fraction of the larger PAHs were hydrogenated, giving rise to a functionalization with aliphatic >CH2 and –CH3 groups. Irradiation with extreme UV light was used to produce and study PAH cations. The main objective of the proposal, however, was the spectroscopic characterization of the PAH samples in the entire spectral range from the UV (~ 190 nm) to the IR (~ 20 µm) and the comparison of the laboratory data with astronomical observations, under particular consideration of the afore-mentioned phenomena. Matrix spectroscopy in Ne at 6 K was used to study the absorption spectroscopy of neutral and ionized PAHs in the UV and visible spectral ranges. In the IR regime, small PAH grains pressed into CsI pellets were investigated at room temperature, in addition to the low-temperature Ne matrices. The essential results of our studies are: • It is found that neutral and ionized PAHs containing about 50 C atoms may very well contribute to the UV extinction bump at 217.5 nm. PAHs condensed to clusters or small grains are particularly well suited to comply with the astronomical observations. • The electronic spectroscopy of the mixtures of PAHs obtained by laser pyrolysis and processed by liquid chromatography revealed smooth and featureless absorption spectra. This observation can be rationalized by the large variety of PAH molecules, each giving rise to a different spectrum so that the overall absorption becomes very smooth. As a similar large variety of PAH molecules is expected to be present in astronomical objects, it should be extremely difficult to observe individual PAH molecules by astronomical spectroscopy. On the basis of this result, we are tempted to conclude that regular PAH molecules (neutral, completely passivated by hydrogen, no side groups, and no substitution of carbon by nitrogen) are not responsible for any of the strong DIBs. • The structural characterization of our PAH samples revealed the existence of aliphatic side groups, which give rise to characteristic vibrational bands in their IR absorption (and emission) spectra. The same fingerprints can also be identified in astronomical spectra. Comparison with laboratory data enables the researchers to draw conclusions on the average structure of the PAH molecules and on the physical properties of the astronomical object under investigation.
Publications
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Electronic spectroscopy of medium-sized polycyclic aromatic hydrocarbons: Implications for the carriers of the 2175 Å UV bump, Astrophys. J. Lett. 712, L16 – L20 (2010)
M. Steglich, C. Jäger, G. Rouillé, F. Huisken, H. Mutschke, and Th. Henning
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A- bundances of PAHs in the ISM: Confronting observations with experimental results, Astron. Astrophys 530, A26-1-15 (2011)
R. Gredel, Y. Carpentier, G. Rouillé, M. Steglich, F. Huisken, and Th. Henning
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Can neutral and ionized PAHs be carriers of the UV extinction bump and the diffuse interstellar bands?, Astrophys. J. 742, 2/1 – 2/12 (2011)
M. Steglich, J. Bouwman, F. Huisken, and Th. Henning
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Electronic spectroscopy of FUV-irradiated diamondoids: A combined experimental and theoretical study, Astrophys. J. 729, 91/1 – 91/10 (2011)
M. Steglich, F. Huisken, J. E. Dahl, R. M. K. Carlson, and Th. Henning
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The smoothness of the interstellar extinction curve in the UV: Comparison with recent laboratory measurements of PAH mixtures, Astron. Astrophys. 540, A110/1 – A110/6 (2012)
M. Steglich, Y. Carpentier, C. Jäger, F. Huisken, H.-J. Räder, and Th. Henning
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The abundances of hydrocarbon functional groups in the interstellar medium inferred from laboratory spectra of hydrogenated and methylated polycyclic aromatic hydrocarbons, Astrophys. J. Suppl. Ser.
M. Steglich, C. Jäger, F. Huisken, M. Friedrich, W. Plass, H.-J. Räder, K. Müllen, and Th. Henning
