Polycyclic aromatic hydrocarbons (PAHs) play an important role not only in combustion processes on Earth, but also in the chemistry of the interstellar medium (ISM) and other astronomical environments. Some of the key questions in this context are related to their stability and possible fragmentation pathways upon enegetic processing in the interstellar radiation field (top-down processes), and their formation pathways from smaller molecular units (bottom-up processes), in particular in dilute and cold regions of space. In this talk I will outline how high-resolution vibrational spectroscopy can help to answer these questions through the unique combination of a cryogenic ion trap instrument coupled to the free-electron-lasers at the FELIX Laboratory.
Infrared action spectroscopy is used to structurally characterize, e.g., C2H2-, H- and H2-loss fragments formed upon dissociative ionization of small PAHs ranging in size from naphthalene to pyrene. Fragmentation pathways can be elucidated in combination with quantum-chemical calculations, and reveal an often surprising isomerization behaviour upon ionization. Furthermore, we use a novel combination of ion-molecule reactivity studies and infrared spectroscopy to study formation routes from mono- to polycyclic nitrogen-containing aromatic hydrocarbons, allowing us to elucidate low-temperature reaction pathways at unprecedented detail. Almost as a side product, these different studies provide infrared reference data on a large class of PAH cations that can be used to benchmark quantum-chemical calculations, and, more importantly, that can be compared to astronomical observations such as those from the newly launched JWST.
