Astrochemically Relevant Polycyclic Aromatic Hydrocarbons Investigated using Ultrafast Pump-probe Spectroscopy and Near-edge X-ray Absorption Fine Structure Spectroscopy
The chemical complexity of the interstellar medium (ISM) has long interested researchers, with a major focus being a set of molecules called polycyclic aromatic hydrocarbons (PAHs). These molecules play a significant role in our understanding of the ISM by elucidating the physical environments that surround them. This work delves into the various photochemical and photophysical processes involving PAHs including ionisation and fragmentation. These processes are studied using three spectroscopic techniques. The first, ultrafast pump-probe spectroscopy, is used in order to study the relaxation and fragmentation dynamics of three PAHs, namely, fluorene, phenanthrene, and pyrene. The pump and probe pulses were chosen from a combination of FEL (from the free-electron laser, FLASH) and optical laser pulses. We could determine the lifetimes of the excited states of these PAHs in different charged states and also discern the fragmentation patterns of these molecules. The second technique used was NEXAFS (near-edge X-ray absorption fine structure) spectroscopy, with which the electronic structure of phenanthrene dications and doubly-dehydrogenated phenanthrene cations was studied. The electronic transitions from the C K-edge to unoccupied molecular orbitals were measured using photons in the region of 275-305 eV at PETRA III. The NEXAFS spectra thus obtained were compared with theoretical X-ray spectra in order to determine the structures of the systems in question. Thirdly, a nearly completed set-up of an IR-UV ion dip spectrometer is described, which will be used to study higher-order clusters of polycyclic aromatic hydrocarbons. The resonance-enhanced multiphoton ionisation spectra of two previously studied molecules, indole and acenaphthene, are presented in this work. Through this multi-spectroscopic approach, we look to investigate fundamental properties and processes involving PAHs, which could be relevant in the context of the ISM.