Termination of bottom-up interstellar aromatic ring formation at C6H5+

Abstract

The aromatic molecule benzene is considered to be the essential building block for larger polycyclic aromatic hydrocarbons (PAHs) in space. Despite the importance of benzene in the formation of PAHs, the formation mechanisms of interstellar benzene are not well understood. A single ion–molecule reaction sequence is considered when modelling the formation of benzene in the interstellar medium, beginning with the protonation of acetylene. Although this process has been used to model the initial steps in the formation of PAHs, it has not been experimentally measured. To explore this reaction mechanism, we have carried out an experimental study of sequential ion–molecule reactions beginning with protonation of acetylene under single-collision conditions. Surprisingly, we found that the reaction sequence does not result in benzene but, instead, terminates at C₆H₅⁺, which is unreactive towards either acetylene or hydrogen. This result disproves the previously proposed mechanism for interstellar benzene formation, thus critically altering our understanding of interstellar PAH formation.