Fragmentation of clusters and recombination induced by intense and ultrashort X-ray laser pulses

ABSTRACT Understanding the ultrafast dynamics of matter under extreme conditions is relevant for structural studies andplasma physics with X-ray lasers. We used the pulses from free-electron lasers (FLASH in Hamburg and LCLSin Stanford) to trigger X-ray induced explosions in atomic atoms (Xe) and molecular clusters (CH 4 and CD 4 ).The explosion dynamics depends on cluster size and the intensity of the X-ray pulse, and a transition fromCoulomb explosion to hydrodynamic expansion is expected with increasing size and increasing pulse intensity.In methane clusters experiments at FLASH, the time-of-ight spectrometry shows the appearance of molecularadducts which are the result of molecular recombination between ions and molecules. The recombination dependson the cluster size and the expansion mechanism and becomes signi cant in larger clusters. In Xenon clusterexperiments at the LCLS, measurements of the ion charge states in clusters suggest a formation of Xe nanoplasmawhich expands hydrodynamically. The dominance of low charge states of Xe is due to three-body recombinationprocesses involving electron and Xe ions, and it depends on the X-ray intensity and nanoplasma formation.Keywords: X-ray free electron lasers, atomic and molecular clusters, nanoplasma formation, non{local thermo-dynamics equilibrium