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 has been a subject of great interest to researchers for decades. A significant contender in this chemical make-up, constituting ∼10% of the total carbon in the universe, is a set of molecules called polycyclic aromatic hydrocarbons (PAHs). These molecules, widely-considered as key contributors to the Unidentified Infrared Bands, may be integral in elucidating the physical environments surrounding them, thereby adding to our understanding of the interstellar medium. Several important photochemical and photophysical reactions involving PAHs form the basis of this work. Under the powerful radiation of the interstellar environment, PAHs undergo ionisation relatively easily. They can also fragment with the loss of H, H₂ and small hydrocarbons in different charged states, and are even known to isomerise. The fundamental aspects of the dominant chemical processes related to PAHs can be studied and quantified within a laboratory framework. Further, incorporation of the requisite environmental conditions of interstellar space in the experimental set-ups is essential in producing useful results. This work integrates three spectroscopic techniques in order to shed light on the relevant photochemical processes involving PAHs. Using ultrafast pump-probe spectroscopy, we studied the relaxation and fragmentation dynamics of three PAHs, namely, fluorene, phenanthrene, and pyrene. The molecules were pumped using ∼90 fs long XUV pulses with 30.3 nm photon energy from the Free-electron Laser, FLASH and they were probed using IR pulses at 810 nm with ∼60 fs pulse durations. We were able to determine the lifetimes of the excited states of these PAHs in different charged states. We could also elucidate the fragmentation patterns of these molecules having multiple competing fragmentation channels. Another set of pump-probe experiments with a 810 nm pump and 405 nm probe pulse allowed us to study the relaxation and fragmentation dynamics of fluorene. It was seen that both the XUV and IR as pump pulses result in qualitatively similar fragmentation patterns. Slightly longer relaxation times of the electronically excited, singly charged fluorene were seen in the latter set of experiments due to the higher energy of the 405 nm probe pulse. The electronic structure of doubly charged phenanthrene and doubly-dehydrogenated phenanthrene cations was studied using near-edge X-ray absorption fine structure (NEX-AFS) spectroscopy. 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. All the different possible doubly-dehydrogenated structures of phenanthrene were considered for this analysis. Finally, to account for the high temperatures of the experimental source used, the X-ray spectra were averaged for the vibrationally displaced ensembles. This work also describes the set-up of a nearly completed IR-UV ion dip spectrometer to study higher-order clusters of polycyclic aromatic hydrocarbons. The resonance-enhanced multiphoton ionisation scheme, which allows the ion dip spectra to be measured, was tested for two previously studied molecules, indole and acenaphthene. The spectra are presented in this work. Through this multi-spectroscopic approach, we investigate fundamental properties and processes involving PAHs, which can be relevant in the context of the interstellar medium.