Spectroscopic Studies of Polycyclic Aromatic Hydrocarbons: Interstellar Aromatic Chemistry Revealed

Apart from benzene, aromatic compounds, crucial in biological and chemical processes, were conspicuously absent from the interstellar inventory before 2017, despite extensive searches. Since then, high-resolution laboratory rotational spectroscopic studies in combination with extremely high-sensitivity spectral line surveys have led to the discovery of numerous cyclic and aromatic molecules in the starless dark cloud TMC-1, a source previously thought unsuitable for such chemical complexity. Detections include polycyclic aromatic hydrocarbons (PAHs) and their cyano derivatives with as many as seven fused rings. Discrepancies of more than four orders of magnitude between observed and predicted abundances challenge established astrochemical models. The detection of benzonitrile in other molecular clouds further suggests that aromatic chemistry is common in space. New spectroscopic studies and analysis methods hold promise to refine models of PAH formation and better constrain PAH stabilities in the diffuse gas, thereby aiding in the identification of the carriers of the diffuse interstellar and unidentified infrared emission bands, and potentially reshaping our understanding of the chemical pathways that link interstellar organic molecules to the origins of terrestrial carbon.