Interaction of intense VUV radiation pulses with rare gas atoms and clusters

The production of gigawatt radiation pulses of 50-100 fs duration below 100 nm by the free-electron laser (FEL) at DESY in Hamburg has opened the way to studies of matter with intense, short wavelength radiation. First experiments on the interaction with rare gas atom and cluster beams have been conducted. At 98 nm wavelength the ionisation of rare gas atoms is attributed to a stepwise multiphoton ionisation. Compared to the atomic beam the absorption in the cluster beam is strongly enhanced. At a power density of about 3×1013 W/cm2 each atom in a Xe2500 cluster absorbs on average 600 eV, corresponding to about 50 VUV photons. As a result of the strong absorption multiply charged atomic ions up to Xe8+ are produced. Finally, the cluster completely disintegrates by Coulomb explosion, causing high kinetic energies of the ejected ions up to 3 keV. Coulomb explosion begins at a power density approximately two orders of magnitude lower than the threshold for such processes in optical laser experiments. Molecular dynamics simulations indicate that standard collisional heating cannot fully account for the strong energy absorption.