Electronic structure and ultrafast fragmentation dynamics of polycyclic aromatic hydrocarbons

Abstract

The interstellar space is filled with numerous molecules. The interaction of the interstellar radiation with these molecules plays an active role in maintaining the charge balance of the interstellar medium and governs the photochemical and photophysical processes. Since the polycyclic aromatic hydrocarbon (PAH) molecules contain 10% of the total galactic carbon content, these molecules are of astrochemical interest. In this dissertation, the interaction of such class of molecules with harsh radiation (extreme ultraviolet, infrared, visible, and soft X-rays) is investigated, and the electronic structural and dynamical information is extracted. The electronic structures of monocationic and dehydrogenated monocationic phenanthrene, a small PAH, are explored using near edge X-ray absorption fine structure spectroscopy at carbon K-edge. Furthermore, we performed time-resolved pump-probe experiments to explore the average relaxation lifetimes of vibronically excited fluorene, another small PAH, in its monocationic and dicationic form are also reported after thorough investigation of their fragmentation patterns and pathways. To study the fragmentation dynamics of fluorene, we used ultrafast time-resolved XUV-Vis and IR-Vis pump-probe spectroscopy. This thesis provides the detailed description of the experimental, theoretical, and analytical methods used to obtained the understanding of electronic structure and fragmentation dynamics of small PAHs.

The large-scale facilities at Deutsches Elektronen Synchrotron (DESY) in Hamburg, namely, Positron-Elektron-Tandem-Ring-Anlage (PETRA) III and the Free-electron LASer in Hamburg (FLASH), were used to provide adequate tools to perform the experiments. The results obtained thus give a detailed insight into the photophysics of the PAHs, which is crucial to understand the vast interstellar medium.