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K. Kassem:
"Laser Pulse induced Molecular Fragmentation in a Van der Waals-System";
Supervisor: M. Kitzler, M. Dorner-Kirchner; Institut für Photonik, 2020; final examination: 2020-05-15.

A key aspect in the field of photonics is the interaction of light with matter. Particularly interesting questions in laser-matter interaction are connected to the reaction of matter, such as atoms, molecules or solids, to a stimulation on ultrashort time-scales concomitant with atomic or even electronic motion. Research on this topic has been enabled by the availability of ultrashort laser pulses with pulse durations that nowadays reach the fundamental limit of a single light oscillation period. Over the last few decades, triggered by the invention of chirped pulse amplification that led to the availability of ultrashort laser pulses with extreme peak intensities, the field was extended to the regime of non-perturbative non-linear light-matter interaction and the study of processes on the time-scale of attoseconds. When an intense laser pulse impinges on a system such as a molecule, a plethora of processes on different time-scales may be initiated, with ionization taking a key role. The interaction with the strong laser field may lead to a change of the molecule´s electronic state, potentially accompanied by a rearrangement of the structure of the molecule. Driving intramolecular electronic dynamics with the strong field of a high-intensity laser pulse therefore allows studying experimentally the ultrafast dynamics of electrons in atoms, molecules, and, since very recently, also in solids. Even though control over the laser electric field has now reached a high level of perfection, a major difficulty is still reading out the induced molecular dynamics. These dynamics both of the electrons and nuclei can be extremely complex even for relatively small molecules given their many degrees of freedom. There have been numerous scientific studies and achievements in the research of light-matter interaction for atoms and smaller molecules. However, as the complexity of the experiments and their analyses strongly increases with the size and number of molecules involved, the laser-induced dynamics of polyatomic molecules is still widely uncharted territory, let alone the dynamics of molecules in compounds such as dimers or clusters. The purpose of this work was to take first steps towards the investigation of the strong field-induced dynamics of polyatomic molecules bound to a neighbouring molecule in a dimer formed by a van der Waals (vdW) bond. Of particular interest were laser-ionization and fragmentation processes and how they are altered by the presence of the neighbouring molecule from the case of an isolated molecule. In the experiments vdW-dimers were formed by supersonic expansion of gas-mixtures into an ultrahigh vacuum chamber. The such created molecular jet contains a small fraction of dimers. The heterodimer C2H2-CO2 and the homodimer C2H2-C2H2 were chosen as target systems. The interaction of these dimers with ultrashort intense near-infrared laser pulses with durations of 25 fs and peak intensities of a few 1014 W/cm2 was investigated. Structural dynamics induced in the molecular compound by the interaction with the strong laser field was monitored using the widely applied technique of Coulomb explosion imaging that allows measuring atomic distances based on the kinetic energies or momenta of the ionic fragments generated upon laser interaction. For measuring the fragments´ momenta we used photo-ion photo-ion coincidence spectroscopy (PIPICO) using a reaction microscope. To study the dynamics of bond-breaking process in the vdW dimer, pulse pairs in a pump-probe arrangement were applied. The most important conclusion that can be drawn from the conducted experiments is that the ionization-fragmentation process takes place differently for molecules in a dimer environment as compared to isolated molecules, despite the very small energetic distortion introduced by the vdW-bond. The measured data show that the fragments of a vdW-bound molecule show lower kinetic energy which indicates fragmentation from stretched molecular bonds. Interestingly, a striking difference between the homo- and heterodimer was found: Only the heterodimer showed significant alteration of the molecular bonds. This can be reasoned by a larger dipole induced by the neighbouring molecule in comparison with the homo-dimer. Furthermore, it was possible to trace vibrational wave packet dynamics in the dimer induced by the laser interaction. Finally, by comparison of the different fragmentation reactions occurring in the experiment, we were able to identify the electronic states that were populated during the laser interaction and from which the fragmentation reaction originated.