Ultrafast imaging of nanoclusters with intense x-ray laser pulses

Free electron lasers (FEL) open the door for novel applications ranging from single-shot imaging of nanostructures to ultrafast element-specific X-ray spectroscopy. First results have shown that flash imaging, i.e., the reconstruction of a nanostructured object from a single-shot scattering experiment, is feasible . Currently, first imaging experiments of biological nanocrystals and single viruses are underway. For virtually all experiments with FELs it is of fundamental importance to study how the absorption and ionization properties of nanoscale systems develop in the short-wavelength strongfield domain. Experiments on X-ray laser pulse cluster interaction with a combined spectroscopy and imaging approach have been performed. Atomic clusters are ideal for investigating the light matter interaction because their size can be tuned from the molecular to the bulk regime thus allowing to distinguish between intra and interatomic processes. The ionization processes turned out to be strongly wavelength dependent and the data show qualitatively different processes for (soft) X-ray pulses compared to the optical strong field regime. For shorter wavelength, electrons are emitted from the clusters in a direct multistep photoionization process and inverse Bremsstrahlung type absorption is not significant. However, after frustration of direct electron emission the photo and Auger electrons are trapped in the increasingly deep cluster Coulomb potential and a plasma of supraatomic density is created. Energy exchanging collisions in this plasma can lead to creation of fast electrons. The investigation of core shell systems gives evidence for efficient charge redistribution within the cluster.