Ultrafast Chemical Dynamics

The formation and breaking of chemical bonds are the most fundamental steps in chemical reactions. Studying the quantum dynamics that underlie these processes in real time is thus of great interest for physics, chemistry and many other fields of research. The aim of the proposed experiments is to gain a deeper understanding of dynamic processes in molecules from a novel perspective. The project will push the method of table-top femtosecond transient absorption spectroscopy into new territories, utilizing probe pulses in the soft X-ray regime with photon energies beyond 100 eV. Femtosecond X-ray pulses will be generated by high-harmonic generation. The novel method combines the advantages of X-ray absorption spectroscopy and femtosecond pump-probe spectroscopy. Transitions involving core-level electrons enable the study of electronic and nuclear dynamics with element-specificity and sensitivity to local valence electronic structures. In the proposed experiments high-order harmonic probe pulses with photon energies up to 180 eV will provide access to the sulfur 2p edge, facilitating the study of the chemistry of sulfur- containing compounds. The temporal evolution of the molecular valence electronic structures will be monitored in real-time and from the perspective of distinct reporter atoms. Experiments in Berkeley and Graz will probe spin-orbit and vibrational wave packets in neutral and strong-field ionized carbon disulfide as well as the photoinduced breaking of the S-C bond in cyclic thiophene molecules (ring- opening reaction), which is of particular relevance for energy related technologies. Especially, the introduction of well-controlled ultraviolet (UV) excitations is an important next step in the field of femtosecond X-ray transient absorption spectroscopy, which will facilitate the investigation of many naturally occurring photochemical processes triggered by UV sunlight under controlled conditions. Exploring the most fundamental mechanisms in nature is the basis for understanding complex systems and their time-dependent behavior. The development of novel methods for the study of ultrafast photo- initiated molecular dynamics will create new opportunities to unravel key processes in physics, chemistry, life sciences, material science as well as environment and energy related technologies.

The project focused on the investigation of photo-induced dynamics in organic molecules employing femtosecond time-resolved XUV transient absorption (TXA) spectroscopy. The major part of the project was dedicated to the investigation of a photo-induced ring-opening reaction in heterocyclic molecules, a type of reaction that plays an important role in organic chemistry. Thereby, the TXA method provided a novel tool for the study of such dynamics. In particular, the element sensitivity inherent to the method due to the excitation of core-level electrons allowed for an exciting new view onto the evolving dynamics, triggered by either UV or strong IR laser pump pulses, with femtosecond time resolution.The experiments revealed that strong-field ionization of the five-membered heterocylic selenophene molecule initiates a multi-step process, which has been studied from the perspective of the Se reporter atom employing TXA spectroscopy. Upon the initial production of highly excited molecules, the heterocyclic ring opens and, eventually, stable ring-opened molecules as well as bare selenium ions are produced on a time scale faster than 200 fs. Furthermore, the modification of the experimental setup in the course of the project enabled the investigation of large organic molecules, which where hitherto not accessible to TXA experiments: Ferrocene is a classic organometallic sandwich compound composed of a central iron atom located between two aromatic hydrocarbon rings. Upon strong-field ionization, the molecule dissociates and free Fe ions as well as both carbon rings are liberated within 250 fs. A further molecule that has been investigated was 1,2- dibromoethane, for which the complex multi-step dissociation dynamics triggered by strong laser pulses has been explored. Further experiments in the course of the project where dedicated to the study of UV induced dynamics in selenophene and bromoform. In particular, for selenophene it was found that, in contrast to strong-field ionization, in this case the dominant dissociation channel leads to the liberation of neutral atoms.In the course of the project, the TXA setup has been optimized for the study of large organic compounds, which opened up new possibilities for the method. This resulted in the first TXA work on complex ring-opening dynamics in organic molecules and the investigation of dynamics in molecules as large as ferrocene, which attracted great interest by the community and revealed new insights into photo-induced molecular dynamics.