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

Two-color multiphoton ionization of atomic helium was investigated by combining extreme ultraviolet (XUV) radiation from the Free Electron Laser in Hamburg with an intense synchronized optical laser. In the photoelectron spectrum, lines associated with direct ionization and above-threshold ionization show strong variations of their amplitudes as a function of both the intensity of the optical dressing field and the relative orientation of the linear polarization vectors of the two fields. The polarization dependence provides direct insight into the symmetry of the outgoing electrons in above-threshold ionization. In the high field regime, the monochromaticity of the XUV radiation enables the unperturbed observation of nonlinear processes in the optical field. DOI: 10.1103/PhysRevLett.101.193002 PACS numbers: 32.80.Fb, 32.80.Rm Multiphoton single-color ionization in intense optical or infrared laser fields has been the subject of multiple experimental and theoretical studies for more than two decades and is by now a very well understood process (e.g., [1]). The extension of these studies to multiphoton absorption in the photoionization continuum was followed by the discovery that high order harmonics of the fundamental laser frequency are emitted in the extreme ultraviolet (XUV) when a strong femtosecond optical laser pulse interacts with a gas jet (e.g., [2,3]). The combination of different wavelengths, one in the XUV and the other in the visible or near infrared, opens new opportunities. It has recently permitted the investigation of above-threshold ionization (ATI) as the result of the combined interaction of both fields [4][5][6]. In this case the dominant contribution comes from processes in the course of which the emitted electron exchanges photons with the dressing laser field via stimulated emission (or absorption) resulting in a comb of sidebands disposed on both sides of the main photoelectron line.Theoretical studies have established that the sideband intensity depends on the electron kinetic energy as well as on the strength and polarization state of the optical laser field [7]. Fitting theoretical profiles to the measured sideband signals should yield the main parameters which govern the photon-atom interaction in this regime. For example, changing the polarization of either of the radiation beams gives rise to ''dichroic effects'' in the photoelectron spectrum. It therefore opens the possibility to control the relative contributions of photoionization channels with different angular momenta.This approach has been extensively used in studies of atomic ionization by weak monochromatic radiation from synchrotrons and continuous lasers, where at least one resonant intermediate state is involved, and the basic photon-electron interaction is completely dominated by this resonant excitation [8]. The use of high harmonic XUV sources to generate similar processes in the nonresonant continuum is complicated by very difficult analysis, since contributions from several harmonics and their mutual interferenc...

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.

Using a noninvasive, electro-optically based electron bunch arrival time measurement at FLASH ͑free electron laser in Hamburg͒ the temporal resolution of two-color pump-probe experiments has been significantly improved. The system determines the relative arrival time of the extended ultraviolet pulse of FLASH and an amplified Ti:sapphire femtosecond-laser pulse at the interaction region better than 90 fs rms. In a benchmarking pump-probe experiment using two-color above threshold ionization of noble gases, an enhancement in the timing resolution by a factor of 4 compared to the uncorrected data is obtained. © 2009 American Institute of Physics. ͓DOI: 10.1063/1.3111789͔Progress in generation and application of ultrashort laser pulses over the past two decades has been tremendous. Efforts toward decreasing the pulse duration and the concomitant widening of the spectral range are steadily increasing, thereby enabling the investigation of dynamical phenomena on faster and faster timescales. At FLASH ͑free electron laser in Hamburg͒, 1 in particular, investigations on the phenomena in the extended ultraviolet ͑XUV͒ regime with pulses of only few tens of femtoseconds are now possible, with a worldwide unrivalled high power such that a whole new class of experiments is accessible. The pump-probe excitation scheme is the most promising concept to study dynamical processes on the femtosecond level. Two ultrashort pulses are required for this kind of experiment, first, initiating a reaction and the second permitting the observation of the induced changes. The temporal resolution of this scheme is determined by precise knowledge of the delay between both pulses and, ultimately, by their duration. At FLASH the XUV-free electron laser ͑XUV-FEL͒ pulse can be combined with an optical laser pulse. This scheme has already been utilized from the beginning of its operation as a user facility in 2005 and has led to several exciting results, e.g., 2-6 two complementary laser systems are available. First, a system delivering a high energy pulse ͓Ͼ10 mJ, 120 fs full width at half maximum ͑FWHM͔͒, albeit the repetition rate is limited to 10 pulses per second. The second system provides trains of 20 J pulses ͑120 fs FWHM͒ that map exactly onto the complex timing pattern of the FEL, delivering up to 4000 pulses per second. Since the FEL and the optical laser are independent sources of femtosecond pulses the synchronization between both is of vital importance to perform well defined pump-probe experiments. However, due to technical limitations the synchronization is compromised by the inherent timing jitter reducing the effective temporal resolution from the pulse duration limit of 120 fs ͑FWHMӍ 50 fs͒ root mean square ͑rms͒ width ͑since the FEL pulse has a pulse duration of only few tens of femtoseconds͒ to about 250 fs rms. This jitter is dominated by the accelerator itself. Our approach to increase the temporal resolution is to measure the arrival time of the pulses during a pump-probe experiment independently and shot by shot without ...

We describe an experimental system designed for single-shot photoelectron spec-