Photochemical processes play a central role in nature. Understanding these processes at the quantum level remains a major challenge for both experiment and theory. Even in a single molecule, a large number of degrees of freedom come into play through the complex interplay of electrons and nuclei, while the resulting dynamics unfold on ultrafast timescales — typically faster than a trillionth of a second (1 picosecond = 10⁻¹² s). Respectively, the complexity increases by orders of magnitude when more than one molecule is involved. To address this challenge, we must reduce the problem to its essential building blocks, that are isolated functional units comprised of one or a few molecules and study these systems with advanced spectroscopic methods capable of mapping their photochemical reactions in full detail. Such experiments, however, remain very difficult.
This project aims to close this gap by establishing a new experimental approach. We combine cutting-edge femtosecond laser technology with photoelectron spectroscopy of molecular systems prepared under ultrahigh vacuum conditions. In particular, we generate ultrashort extreme ultraviolet (XUV) pulses to ionize the target molecules and precisely track their reactions in real time. A key innovation is a new interferometric measurement concept that significantly enhances both temporal and energy resolution. Together, these techniques enable a gap-less mapping of molecular dynamics along the entire reaction coordinate, thereby resolving the coupled electronic-nuclear dynamics with high resolution.
The spectroscopic probes are prepared using molecular and cluster beam techniques, which enables the synthesis of isolated single molecules up to small molecular complexes. This approach minimizes the complexity of the target systems to the core functional units. In addition, controlled environmental effects can be introduced ranging from simple solvents to more complex molecular solvent networks. In this way, we adopt a bottom-up strategy to gradually advance our understanding of molecular photochemistry from the level of single, isolated molecules towards complex many-body systems found in nature.
Our experiments focus on fundamental photochemical processes, such as photoisomerization, charge transfer, and intramolecular energy redistribution. The advanced spectroscopic methods provide a uniquely detailed characterization of these processes. At the same time, the reduced complexity of our model systems ensures that the results can be directly compared to high-level quantum chemical calculations. In doing so, the project will deliver benchmark data that drive the development and validation of theoretical models, advancing step by step our understanding of the fundamental molecular processes that drive nature.
