Formation of Polycyclic Aromatic Hydrocarbons and Carbonaceous Solids in Gas-Phase Condensation Experiments

Abstract

Carbonaceous grains represent a major component of cosmic dust. In order to understand their formation pathways, they have been prepared in the laboratory by gas-phase condensation reactions such as laser pyrolysis and laser ablation. Our studies demonstrate that the temperature in the condensation zone determines the formation pathway of carbonaceous particles. At temperatures lower than 1700 K, the condensation by-products are mainly polycyclic aromatic hydrocarbons (PAHs), that are also the precursors or building blocks for the condensing soot grains. The low-temperature condensates contain PAH mixtures that are mainly composed of volatile 3-5 ring systems. At condensation temperatures higher than 3500 K, fullerene-like carbon grains and fullerene compounds are formed. Fullerene fragments or complete fullerenes equip the nucleating particles. Fullerenes can be identified as soluble components. Consequently, condensation products in cool and hot astrophysical environments such as cool and hot AGB stars or Wolf Rayet stars should be different and should have distinct spectral properties.

The smoothness of the interstellar extinction curve in the UV

“…These species, if structurally comparable to the other PAHs, must contain at least one non-aromatic CH 2 group to have a closed electronic shell structure. The aliphatic character of some constituents of the laser pyrolysis condensate was also verified via IR absorption spectroscopy, which will be shown in an upcoming publication (for comparison, see also Jäger et al 2006Jäger et al , 2009). The MALDI-TOF studies have yet another interesting result.…”

“…2 and Jäger et al 2009). Unfortunately, it is not possible at the moment to enrich mixtures with such large PAHs in sufficient quantities for spectroscopic studies, mainly because of their poor solubility.…”

“…The applied CO 2 laser pyrolysis method is described elsewhere (Jäger et al 2009;Steglich et al 2010). The molecular components of the condensate were extracted with the solvents methanol (CH 3 OH) or dichloromethane (CH 2 Cl 2 ).…”

“…The constituents of these extracts, which could unambiguously be identified, are known PAHs. However, PAH-like molecules with multiple hydrogen saturation, i.e., containing CH 2 groups and, thus, displaying aliphatic character, must be present as well (see below and Jäger et al 2009). For simplicity, we will refer to all molecular components as "PAHs".…”

Catalytic conversion of acetylene to polycyclic aromatic hydrocarbons over particles of pyroxene and alumina

“…Only a few studies have been conducted to date, most of which concern possible routes for PAH formation in the outflows of carbon-rich AGB and post-AGB objects [5][6][7]. Frenklach & Feigelson [8] proposed a hydrogen abstraction and acetylene addition (HACA) radical-based mechanism, in which C 2 H 2 can add to radical sites on an aromatic structure and form a new aromatic ring in a temperature window of 900-1100 K [9]. Woods et al [10,11] and Carelli et al [12] have discussed ion-molecule chemistry involving C 2 H 2 to form benzene.…”