We report recent results on the performance of FLASH (Free Electron Laser in Hamburg) operating at a wavelength of 13.7 nm where unprecedented peak and average powers for a coherent EUV radiation source have been measured. In the saturation regime the peak energy approached 170 µJ for individual pulses while the average energy per pulse reached 70 µJ. The pulse duration was in the region of 10 femtoseconds and peak

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Two-color above threshold ionization of helium and xenon has been used to analyze the synchronization between individual pulses of the femtosecond extreme ultraviolet ͑XUV͒ free electron laser in Hamburg and an independent intense 120 fs mode-locked Ti:sapphire laser. Characteristic sidebands appear in the photoelectron spectra when the two pulses overlap spatially and temporally. The cross-correlation curve points to a 250 fs rms jitter between the two sources at the experiment. A more precise determination of the temporal fluctuation between the XUV and infrared pulses is obtained through the analysis of the single-shot sideband intensities.

Two-photon ionization of atomic helium has been measured by combining femtosecond extreme-ultraviolet pulses from the free-electron laser in Hamburg ͑FLASH at DESY͒ with intense light pulses from a synchronized neodymium-doped yttrium lithium fluoride laser. Sidebands appear in the photoelectron spectra when the two laser pulses overlap in both space and time. Their intensity exhibits a characteristic dependence on the relative time delay between the ionizing and the dressing pulses and provides an inherent time marker for time-resolved pump-probe experiments. The measurements of the sidebands are in good agreement with theoretical predictions and allow for a direct analysis of two-photon ionization, free from processes related to interference between multiple quantum paths. DOI: 10.1103/PhysRevA.74.011401 PACS number͑s͒: 32.80.Rm, 32.80.Fb., 42.50.Hz The achievement of short-wavelength free-electron-laser ͑FEL͒ action at DESY in the year 2000 ͓1͔, based on the process of self-amplified spontaneous emission ͑SASE͒, represented a synergistic tour de force as optical and accelerator technologies were combined to produce ultrashort laser pulses at high fundamental photon energies with high peak and average power. The first experiments on rare-gas clusters revealed new insights into intense laser-matter interactions ͓2͔. In contrast to visible and infrared laser-matter interactions, where valence electrons are the primary participants, the fundamental FEL photon energy lies far above the ionization potential of all stable matter. Under these conditions, inner-shell electrons can be excited into resonant and nonresonant continuum states ͑e.g., ͓3,4͔͒ and will be the predominant mediators of the underlying photoprocesses ͑linear and nonlinear͒.Starting in mid-2005 the free-electron laser in Hamburg ͑FLASH͒ covers a much larger wavelength range compared to the first lasing in 2000. In view of its unprecedented characteristics ͓5͔, an associated time-synchronized optical laser facility opens up new and particularly exciting research opportunities allowing the investigation of fundamental photoionization processes in intense bichromatic laser fields where one field can directly ionize valence and/or inner-shell electrons in a single step. The dynamics of various processes can be investigated, including ultrafast electronic relaxation of autoionization states ͓6͔, coupling between two autoionization states ͓7,8͔, wave-packet formation of high-lying Rydberg states ͓9͔, fast dissociation of molecules upon inner-and outer-shell photoexcitation, etc. Such a pump-probe setup has recently been implemented at the FEL facility at DESY, which provides either femtosecond or picosecond infraredvisible pulses, synchronized with a rms jitter of less than 1 ps to the femtosecond xuv pulses from the FEL ͓10͔. Here, we present experimental results obtained with this system, combined with a corresponding theoretical analysis, on the photoionization of the most prototypical of atoms for such studies-helium-in the presence of a strong o...

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