The hidden life of cosmic carbon Infrared fingerprint spectroscopy and fragmentation chemistry of gas-phase polycyclic aromatic hydrocarbons

This thesis aims to improve our understanding of how polycyclic aromatic hydrocarbons (PAHs) in astronomical environments interact react to harsh photonic conditions, and how these excited molecules express signatures of their shapes and sizes in their (far-)infrared spectra. Using both mass spectrometry and infrared spectroscopy at the Free Electron Laser for Infrared eXperiments (FELIX) in Nijmegen, we are able to measure the infrared signatures and dissociation characteristics of different types of PAHs, resulting in various astronomically relevant conclusions. In Chapter 4, we showed that that PAHs show a much stronger than expected tendency to retain deuterium than hydrogen under the influence of both UV and IR light, which has important implications for the isotopic balance interstellar environments. In Chapter 5, we showed that phenylium, an important precursor molecule for PAHs, dominantly exists in singlet ground state, and that from there, it is subjected to a facile ring-opening mechanism. In Chapter 6, we presented the first far-infrared spectra of three, small PAHs, providing essential benchmarking opportunities for spectra which were only theoretically available. Trends extrapolated from these far-infrared bands can be used for guided searches for molecule-specific PAH signatures in both the interstellar medium and extraterrestrial atmospheres. Finally, in Chapter 7, we present the IR spectrum and fragmentation of a highly asymmetric, non-planar PAH. We believe that it is likely that similar molecules play a role in the formation of molecular hydrogen in an astronomical context, but that finding specific signatures of irregular species requires a wider experimental survey.