Clusters in intense FLASH pulses: ultrafast ionization dynamics and electron emission studied with spectroscopic and scattering techniques

FLASH, the first FEL operating at short wavelength, has paved the way for novel types of experiments in many different scientific disciplines. Key questions for the first experiments with this new type of light source are linked to light--matter interaction and ionization processes. This paper gives an overview of the ultrafast ionization dynamics and electron emission of pure and doped rare gas clusters illuminated with intense short-wavelength pulses by summarizing the findings of recent years' work at FLASH. 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 us to distinguish between intra and interatomic processes. The ionization processes turned out to be strongly wavelength dependent. Plasma absorption, while dominant at 13 eV, becomes insignificant at photon energies above 40 eV. The cluster ionization and disintegration proceed in several steps on a time scale from fs to ps. Insight into the involved processes can be obtained with ion and electron spectroscopy. The high intensity of FLASH pulses opens the door for a new imaging approach to study nanoparticles. Scattering patterns of single and few clusters can be recorded in a single shot. Initial results of scattering experiments and their comparison to Mie calculations show that two- and three-dimensional structural information of gas phase particles can be obtained this way.