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Cluster and Nanocrystal Research GroupFree-electron laser

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Interaction of Intense Soft X-Rays with Matter: Experiments at the Free-Electron Laser


Femtosecond laser pulses have developed into a very dynamic field of research in the last decade. However, until recently the experimental efforts were limited to the optical spectral range. Today, super-intense femtosecond light pulses in the vacuum ultraviolett (VUV) and X-ray regime from Free-Electron Lasers open the door for a whole new generation of experiments and scientific visions, ranging from basic questions about photon - matter interaction to time resolved, element specific chemical dynamics and imaging of individual (bio-) molecules.

In our research field "Experiments with FEL radiation" we primarily investigate the interaction of super-intense VUV femtosecond light pulses with matter.  The interaction of intense VUV radiation with matter differs substantially from that in the optical spectral range. One important aspect is that at the shorter wavelengths the radiation field oscillates much faster. Therefore, many of the processes dominating in the optical range (e.g. field ionization, tunnel ionization) will be suppressed in the VUV. The experiments are carried out in the gas phase on atoms, molecules, and clusters. Hereby clusters are of particular interest as they exhibit densities similar to bulk materials while having a limited size far below the dimensions of the focal spot. Thus, they serve as  ideal "nano-lab" in which important processes of VUV radiation - matter interaction can be probed without a interfering surrounding. Additionally we develop new detectors and experimental techniques for imaging as well as pump - probe experiments at FELs.

First experiments were carried out at the DESY TTF-FEL. The FEL beam of hv = 13 eV (approx. 100 nm) photons was focussed on an atom or cluster jet and the resulting fragments were collected in time-of-flight mass detectors. Additionally the fluorescence light from the clusters could be collected and the pulse intensity could be monitored.


The time-of-flight spectra showed only atomic fragments for all investigated samples. Additionally higher charge states up to 8+ were seen for clusters with an average size of  <N> = 30000 atoms. The ion flight times for the detected ions decreased for increasing cluster size, indicating that the fragments were literally ejected from the original cluster. Detailled analysis of the observed ion energies showed that up to 50 photons per atom were absorbed in the largest clusters.







The experiments revealed that an surprisingly large amount of energy was deposited in the clusters. This lead to multiple ionization of the atoms in the cluster and subsequent complete fragmentation through Coulomb explosion. These experimental results contradicted the then existing electro-dynamic models for laser - matter interaction which describe the optical regime very well. They show that for shorter wavelength, i.e., higher energy laser light new tools have to be developed which have to include direct ionisation and multi-photon transitions. These quantum mechanical effects are included in more recent theoretical efforts, which give insight into the observed new processes at short wavelength.

With the availability of the DESY VUV-FEL in Spring 2005 these experiments are going to be pushed to even higher energies into the regime of the giant core level resonances of the rare gases. Additionally we are working on novel imaging techniques for time-resolved measurements at the FEL and new detectors which allow to collect all fragmentation projects in a momentum resolved manner (Coltrims).



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