Non-linear phenomena in atoms and clusters induced by intense VUV radiation from a free electron laser

We report on first experiments of rare gas cluster dynamics and spectroscopy using light from the DESY free electron laser at 13eV. The energy absorption is found to be very efficient and the clusters fragment completely. Non-linear effects appear at a peak power density threshold much lower than for visible light. The data are consistent with a three-step ionization process, but the nature of the first step is still controversial.