We present the temporal characteristics of the split-and-delay unit at FLASH2 via visibility measurements which characterize the temporal resolution of the combined system of the FEL and the split-and-delay unit. The use of the split-and-delay unit at FLASH2 allows the users at the beamlines FL23 and FL24 at DESY to perform such pump-probe experiments. By using wavefront beam splitting, grazing incidence mirrors, and two different coatings the whole spectral region of FLASH2 is covered, and even harmonics up to 1800 eV are transmitted with a transmission of T>0.06. It is concluded that user experiments with a pump-probe scheme from the picosecond regime down into the sub-femtosecond region can be carried out.
We report on ion emission from plasma produced on thick targets irradiated with nanosecond and femtosecond pulses delivered by mid-ultraviolet and soft x-ray lasers, respectively. To distinguish between different ion acceleration mechanisms, the maximum kinetic energy of ions produced under different interaction conditions is plotted versus laser fluence. The transformation of the time-of-flight detector signal into ion charge density distance-of-flight spectra makes it possible to determine the mean kinetic energy of the fastest ion groups based on the influence of the acoustic velocity of ion expansion. This allows obtaining additional characteristics of the ion production. The final energy of the group of fast ions determined using the ion sound velocity model is an order of magnitude larger in the fs-XFEL interaction than in the ns-UV one. On the contrary, the ablation yield of ions in our experiment is seven orders of magnitude greater when applying ns-UV laser pulses, not only due to higher energies of UV laser pulses, but also due to a significant difference in interaction and ion formation mechanisms.
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