The pulse duration of soft x-ray free-electron laser ͑FEL͒ radiation is directly measured by time-resolved observation of doubly charged helium ions at 51.8 eV. A wave front splitting autocorrelator produces two correlated FEL pulses with a resolution of better than a femtosecond. In the interesting intensity range from 10 13 to 10 16 W / cm 2 direct and sequential double ionization contribute to the ion yield which has significant influence on the correlation width, being a general feature at high photon energies. Here, a duration of L = ͑29Ϯ 5͒ fs is derived for the soft x-ray pulses at FLASH.
We describe a novel experimental facility at the BESSY II synchrotron, aiming at probing element-specific magnetic properties at the nm-length scale. It consists of a novel photoemission electron microscope with photoelectron spin analysis combined with a microfocus beamline and full X-ray polarization control. The facility is capable of magnetic imaging in applied fields allowing the observation of magnetic switching phenomena on a 30-nm-length scale.
For time-resolved extreme ultraviolet (XUV) pump-XUV probe experiments at the Free electron LASer in Hamburg (FLASH), a split-and-delay unit (SDU) has been built. It is implemented in beamline BL2 which provides a focal spot size of about 20 μm diameter in the experiment. The beam is divided geometrically into two paths which can be delayed from −3 to +15 ps with respect to each other. The transmission up to 200 eV photon energy is above 35% in one beam path and 74% in the other. The latter transmits the XUV beam again from 305 to 570 eV (>1% transmission). Thus almost the whole spectral range at FLASH is covered by the SDU with reasonable transmission, including the option to transport high-energy third harmonic radiation in one of the beam paths. Both beam paths are realigned into the original direction of the radiation at the end of the SDU. Thus the utilization of the divided as well as the original beam is enabled by simply moving the optical elements of the SDU into or out of the beam. Using the SDU, the coherence length and the average pulse duration at FLASH was determined to be 0.9-1.8 μm, depending on the wavelength, and about 30 fs, respectively, for the specific electron bunch parameters.
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