Low-temperature nitrogen-bearing polycyclic aromatic hydrocarbon formation routes validated by infrared spectroscopy

Abstract

Polycyclic aromatic hydrocarbons (PAHs) are abundant in many regions of the Universe, representing a major reservoir for cosmic carbon. However, their formation pathways in cold regions of space remain elusive. Recent astronomical detections show that current astrochemical models drastically underestimate the abundance of aromatic molecules and suggest that additional formation pathways such as ion–molecule reactions need to be considered. Here we reveal efficient low-temperature formation pathways towards nitrogen-containing PAHs via exothermic pyridine+ and acetylene ion–molecule reactions. The experimental approach combines kinetics with spectroscopic probing and unambiguously identifies key reaction intermediates and the final nitrogen-containing PAH product quinolizinium+, a structure that is thought to contribute to the 6.2 μm interstellar emission feature. This study not only provides competing formation pathways relevant in the chemistry of the interstellar medium and Titan’s atmosphere, but also delivers information to verify in-silico potential energy surfaces, astrochemical models and infrared observations.