Direct evidence for a single-step radiation-induced assembling of benzene ring from acetylene trimer at cryogenic temperatures

Abstract

The discovery of carbon-rich species, including polycyclic aromatic hydrocarbons (PAH) and fullerenes, in the space environment is among the most intriguing findings in molecular astrophysics and astrochemistry of the recent decades. The mechanism of their formation remains unclear and the one of the key issues is related to making “first ring”, i.e., formation of benzene from simple small molecules abundant in the outer space. Different routes involving C2, C3 and C4 hydrocarbons are considered in literature. Here we report the first direct evidence for a single-step radiation-induced formation of benzene from pre-existing isolated acetylene trimer occurring in frozen inert environment at very low temperature (6 K). The finding is based on the correlation between trimer decay and build-up of benzene as monitored by FTIR spectroscopy. This concerted pathway may be applicable to acetylene aggregates trapped in the bulky ices or in the grain pores and it emphasizes the importance of concept of molecular organization in the chemical history of the Universe.