Search

Frontiers in physics 10, 880793 (2022). doi:10.3389/fphy.2022.880793 special issue: "Electronic and Nuclear Quantum Dynamics of Molecules in Intense Laser Fields", We report on the use of extreme ultraviolet (XUV, 30.3 nm) radiation from the Free-electron LASer in Hamburg (FLASH) and visible (Vis, 405 nm) photons from an optical laser to investigate the relaxation and fragmentation dynamics of fluorene ions. The ultrashort laser pulses allow to resolve the molecular processes occurring on the femtosecond timescales. Fluorene is a prototypical small polycyclic aromatic hydrocarbon (PAH). Through their infrared emission signature, PAHs have been shown to be ubiquitous in the universe, and they are assumed to play an important role in the chemistry of the interstellar medium. Our experiments track the ionization and dissociative ionization products of fluorene through time-of-flight mass spectrometry and velocity-map imaging. Multiple processes involved in the formation of each of the fragment ions are disentangled through analysis of the ion images. The relaxation lifetimes of the excited fluorene monocation and dication obtained through the fragment formation channels are reported to be in the range of a few tens of femtoseconds to a few picoseconds., Published by Frontiers Media, Lausanne

Dissertation, Christian-Albrechts-Universität zu Kiel, 2022; 148 pp. (2022). = Dissertation, Christian-Albrechts-Universität zu Kiel, 2022, 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.

Zeitschrift für physikalische Chemie 0(0), 1-27 (2020). doi:10.1515/zpch-2020-0009, A general framework for the simulation of ultrafast pump-probe time resolved experiments based on Born-Oppenheimer molecular dynamics (BOMD) is presented. Interaction of the molecular species with a laser is treated by a simple maximum entropy distribution of the excited state occupancies. The latter decay of the electronic excitation into the vibrations is based on an on-the-fly estimation of the rate of the internal conversion, while the energy is distributed in a thermostat-like fashion. The approach was tested by reproducing the results of previous femtosecond studies on ethylene, naphthalene and new results for phenanthrene., Published by ˜Deœ Gruyter, Berlin