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Ionization dynamics of helium clusters and droplets in intense short-wavelength light pulses

Subject Area Optics, Quantum Optics and Physics of Atoms, Molecules and Plasmas
Term from 2014 to 2023
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 260415380
 
Final Report Year 2023

Final Report Abstract

Based on several methodical developments, the project has provided a wealth of novel results. The project has benefitted a lot from the engagement of the project leaders and their groups in the development of experimental end stations at short wavelength Free Electron Lasers, for the work on He droplets especially at FERMI (Trieste) and EuXFEL in Hamburg. Since the access to these unique large scale facilities is very limited, demonstrating the feasibility of single shot imaging of gas phase nanoparticles with a lab-based HHG source was an important achievement. It opened also the road to extend studies to the attosecond regime. On the other hand, it also turned out that some aspects of our program, in particular imaging resonant processes with two-color pump probe at FERMI, were very ambitious and experimentally challenging, such that several campaigns turned out to be needed for solving the various experimental problems. The basic idea is very simple and extremely promising: To image the dynamics of individual particles in their initial state and, fs to ps later, in an evolved state, thus unraveling their light-induced changes. The different colors of the pulses make it possible to separate the images. We are convinced that, while technically challenging, a new field of research is emerging here, and patience is needed to overcome the difficulties. Our efforts have laid the groundwork for further development. A publication on the two-color method developed at FERMI is in preparation, and the recent two-color beamtime at EuXFEL with the Mudrich group exemplifies the ongoing efforts which will lead to the realization of this new method. With the development of various analysis tools for the automatic classification, analysis and reconstruction of scattering patterns the 3D shape of He nanodroplets could be determined. Depending on the experimental condition they spin very fast and surprisingly, although being superfluid, they take the shape of droplets of a classical fluid and can deform into highly elongated prolate shape. Then, we succeed in generating droplets, with up to several micrometer, almost spherical, which spin much less and are especially suited both for studying electronic structure and light induced, ICD type processes and ultrafast dynamics as well as growth of embedded ultracold matter. In our project, we could investigate ultrafast excitation and relaxation processes in great detail, especially the resonant channels. They are extremely fast due to a quantum fluid bubble effect, much faster than initially predicted by theory. Especially the structural studies have triggered a lot of theoretical work. In the course of our work which was started in 2014, the study of electronic processes and dynamics in He droplets has developed into a very active, fast growing field, with several new groups having entered to it.

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