Periodic Reporting for period 3 - c-TSD-p (Time-Resolved Structural Imaging of Chemical Transition State Dynamics)
Berichtszeitraum: 2023-11-01 bis 2025-04-30
For bonds to be broken and new bonds to be formed, initially far apart chemical reactants have to come close to each other and evolve through transient intermediate configurations known as the transition state. Transition state dynamics is closely related to reaction mechanisms and of fundamental importance in chemistry. Much work has been done to unravel these dynamics, which often involve major structural rearrangement of atoms. However, there are a lot of open questions since to date none of the applied spectroscopic techniques has directly delivered the time-dependent transition state structure of bimolecular reactions. Femtochemistry has been mostly focused on intramolecular interactions. In contrast, we aim at time-resolved investigations of intermolecular interactions. These are much harder to tackle with laser methods, but much more relevant for chemistry. Our novel probe is capable of imaging, one molecule at a time, the full three-dimensional atomic configuration of individual transition states as they evolve: Reaction precursors are prepared using neutral and ionic complexes as well as small clusters with defined initial structure and tunable internal temperature. Starting from these well-defined initial configurations, chemical dynamics is initiated by a femtosecond laser pulse. Timed Coulomb Explosion Imaging, induced by extremely short intense laser or X-ray pulses together with coincidence momentum imaging of the fragments is then applied as a probe. The latter yields the evolving transition state structure by mapping the position of the atoms as a function of time-delay between the two pulses. The structural information obtained gives also insight into how mechanisms of bimolecular ground state chemical reactions change in the presence of solvent molecules and under the influence of a control pulse. It is anticipated that light will be shed on some of the long-standing open questions surrounding transition state dynamics - with the goal in mind to find routes of making chemistry greener.
In this first phase, the progress of the project was significantly impacted by COVID as well as by my acquisition of a prestigious Heisenberg professorship from the German Research Foundation (DFG) and ultimately my appointment as professor at the University of Kassel. This consolidation entailed a change of host institution. Lab space was initially not available and had to be constructed. The University undertook on a considerable effort to build up-to-date labs, which took time. Moreover, we obtained additional funding for a high-repetition rate laser system as infrastructure for the group, essential for the ERC project. The labs are now operational, and the laser system was recently installed. A reaction microscope was bought from the previous host institution and installed in the new lab, to carry out part of the objectives. As a result, at this point, we have a great infrastructure but are just starting with the lab-based work. A couple of additional team members have recently joined the group. To mitigate these foreseeable challenges, we have carried out scientific studies at Free Electron Lasers, the second pillar of the proposal, within collaborations. In total, my team and I contributed to six beamtimes since the start of the ERC funding. First results on the time-resolved dynamics of small molecules, probed with X-ray-driven Coulomb Explosion, have been published.
For the technique of Coulomb Explosion Imaging, it is important that the charging process occurs in such a short time that the transition state remains frozen by inertia. It is fruitful to explore both intense laser fields of the duration of only a few optical cycles, a lab-based source, as well as extremely short, intense X-ray pulses from an X-ray Free Electron Laser (XFEL), an advanced light source operated as a user facility. These drivers might be sensitive to different aspects of the transition state dynamics. Strong laser fields remove valence electrons directly by strong-field ionization, while X-rays remove inner-shell electrons and subsequently valence and bonding electrons by ultrafast Auger processes. In this reporting period, we have carried out Coulomb explosion experiments at XFELs within collaborations, including the recently commissioned European XFEL near Hamburg.
In the remainder of the project, we will focus in particular on lab-based experiments with dedicated experimental setups in the labs that have now been commissioned at the University of Kassel to continue to tackle the scientific goals outlined above.
In the remainder of the project, we will focus in particular on lab-based experiments with dedicated experimental setups in the labs that have now been commissioned at the University of Kassel to continue to tackle the scientific goals outlined above.