Crossed beam investigation of elementary reactions relevant to the formation of polycyclic aromatic hydrocarbon (PAH)-like molecules in extraterrestrial environments

Abstract

The reactions of ground state carbon atoms, C( 3P j), with benzene, C 6H 6, and phenyl radicals, C 6H 5, with methylacetylene, CH 3CCH, were investigated in crossed beam experiments at collision energies of 21.8 and 140 kJ mol −1 to investigate elementary reactions relevant to the formation and chemistry of polycyclic aromatic hydrocarbons (PAHs) in extraterrestrial environments. The C( 3P j) reaction proceeds via complex formation and gives a cyclic, seven-membered C 7H 5 doublet radical plus atomic hydrogen. This pathway has neither an entrance nor exit barrier, and is exothermic. Together with the experimental verification of the carbon versus hydrogen exchange under single collision conditions, the findings have an important impact on the chemistry of aromatic molecules in interstellar clouds and outflow of carbon stars. Even in the coldest molecular clouds ( T=10 K), the benzene molecule can be destroyed upon reaction with carbon atoms, whereas they are resistant toward an attack of oxygen and nitrogen atoms. Since the aromatic benzene unit is ubiquitous in extraterrestrial, PAH-like material, our results suggest that PAHs might react with carbon atoms as well. On the other side, the reaction of C 6H 5 radicals with methylacetylene to form phenylmethylacetylene is direct. Since an entrance barrier inhibits the reaction in cold molecular clouds and in the atmospheres of hydrocarbon rich planets like Jupiter and Saturn and satellites such as Titan, this reaction is expected to play a role in PAH synthesis only in high temperature interstellar environments, such as circumstellar outflows of carbon stars.