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The concept of cosmic dust as an important component in the universe is relatively recent and nowadays there is a general consent that dust grains play an active role in the evolution of our universe. Generally, we know the rough composition of the cosmic dust but we need to be considerably more precise in the determination of its detailed chemical and spectral properties in different phases of the dust life cycle. New astronomical observations via earthbound or satellite-based telescopes provide us with a wealth of spectroscopic information on different astrophysical environments. Based on such observational results it is well accepted that a significant component of the cosmic dust is carbonaceous but its nature is still contentious. Carbonaceous grains, their possible precursors or intermediates in the condensation pathways, are discussed as possible carriers of the strong interstellar absorption band at 217 nm, the extended red emission, the interstellar absorption feature at 3.4 µm, and partly the aromatic IR emission bands.

In astrophysical environments, polycyclic aromatic hydrocarbons (PAHs) represent either the grain forming building blocks or by-products in carbon nanoparticles condensations or they are products of grain erosion of larger carbon particles. Polycyclic aromatic hydrocarbons (PAHs) and solid nanometer-sized carbonaceous grains are simultaneously formed via gas-phase condensation. In the laboratory, carbon soot, containing PAHs and other soluble components, has been be prepared by laser-induced pyrolysis of suitable gaseous precursors like ethylene, acetylene and benzene, laser ablation of graphite in varying gas atmospheres. Even in terrestrial carbon condensation processes, the grain formation and the role of PAHs (as side product or precursor) are not sufficiently understood. In addition fullerenes are discussed as possible precursors for carbon grain in condensation processes. For each synthesis scheme, the process parameters influence the grain condensation mechanism and thus the composition of the byproducts that represent the soluble soot components. At condensation temperatures below 1500 °C, the formation of PAHs is preferred. The soluble part of the carbonaceous powder has been separated from its insoluble counterpart by solvent extraction. Several techniques have to be applied to obtain information on the composition of the extract including high-performance liquid-chromatography (HPLC), mass spectrometry, UV/VIS, and IR spectroscopy. In combination with the structural analysis of the condensed carbon grains by high-resolution electron microscopy, valuable information on the soot formation process can be obtained.

 

Fig. 1. Laser pyrolysis (left) and laser ablation (right) setup for high- and low-temperature condensations of carbonaceous material via gas-phase synthesis.

 

The setup of the laser pyrolysis experiment for the production of carbonaceous materials is shown in fig. 1. Different temperatures in the condensation zone can be realized by using either a pulsed or a continuous wave laser with different powers. The morphology and internal structure of the condensed carbon grains can be characterized by high-resolution transmission electron microscopy (HRTEM) which is also be able to identify fullerenes or fullerene fragments in the condensate.

Two different formation processes in gas-phase condensation of carbonaceous matter have been detected, a high-temperature (HT) process (T ≥ 3500 K) where very small fullerene-like carbon particles and fullerenes are produced, and a low-temperature (LT) condensation (T ≤ 1700 K) resulting in large carbon particles with rather plane graphene layers can be observed. In this formation regime PAHs are precursors and by-products of the grain condensation process.

 

Fig. 2. The left part of the image shows typical carbon grains produced in LT condensations. The right part of the figure exhibit the structural and formation characteristics of a HT condensate.

 

The extracts containing mixtures of PAHs show special IR characteristics. In addition to completely unsaturated PAHs, partly hydrogenated molecules can be observed, resulting in a simultaneous appearance of 3.3/3.4 µm bands in the IR. Additionally, the LT condensates show strong aromatic and aliphatic IR bands superimposed to broad plateaus at 8 and 12 µm strongly comparable to the IR bands observed in protoplanetary nebulae. Therefore, it is supposed that LT condensation represents a very likely formation process of dust and PAHs in AGB stars and protoplanetary nebulae.

PAHs are considered as not stable in the interstellar radiation field. Small PAHs with 15-20 C atoms will be quickly destroyed by the interstellar UV irradiation as they are heated to temperatures which correspond to energies exceeding their dissociation threshold [1]. Therefore, we have focused our interest on bigger, partly hydrogenated PAHs and their spectroscopic characterization imitating the conditions of the interstellar medium. For this purpose our laboratory is equipped with a HPLC that can be used to chemically separate individual PAHs or interesting mixtures of them. Cavity ring-down spectroscopy (for further information see corresponding project) will be applied to perform gas-phase spectroscopy at the isolated PAH species in free-jet expansions. Matrix isolation spectroscopy of large PAHs will be employed to assist the selection of neutral or ionized PAHs to be measured by cavity ring-down spectroscopy.

Nanometer-sized diamonds are other astrophysically interesting species. The formation of these cosmic dust species is still a matter of debate. Presolar nanodiamonds with a medium size of 1.6 nm could be identified as a major grain component in primitive meteorites. In contrast, astronomical observations point to the presence of diamond grains larger than 25 nm in the dusty envelopes or protoplanetary disks around young stars. Our group is interested in studying the formation of diamonds in different astrophysical environments and their spectral characterization. In the laboratory laser ablation of graphite in a liquid [2, 3] confines the expansion of the plasma and leads to very high temperatures and pressures in the condensation zone mimicking high-pressure formation of cosmic diamonds (see fig. 3).

 

Fig. 3. Left: liquid phase pulsed laser ablation (LPPLA). Right: High-performance liquid chromatogram of the extracted soluble component of a LT condensate.

 

References:

[1] V. Le Page, T.P. Snow, and V.M. Bierbaum, Astrophys. J. 584 (2003) 316
[2] J. Sun et al., Ultrafine diamond synthesized by long-pulse-width laser, APL 89 (2006) 183115.
[3] S. R. J. Pearce et al., Production of nanocrystalline diamond by laser ablation at the solid liquid interface, Diamond and Related Materials 13 (2004) 661-665.

 

For more information contact Dr. Cornelia Jäger.

Related funding(s): DFG HU 474/21-2.

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