Two-color, single-shot time-of-flight electron spectroscopy of atomic neon was employed at the Linac Coherent Light Source (LCLS) to measure laser-assisted Auger decay in the x-ray regime. This x-ray-optical cross-correlation technique provides a straightforward, non-invasive and online means of determining the duration of femtosecond (>40 fs) x-ray pulses.In combination with a theoretical model of the process based on the softphoton approximation, we were able to obtain the LCLS pulse duration and to extract a mean value of the temporal jitter between the optical pulses from a synchronized Ti-sapphire laser and x-ray pulses from the LCLS. We find that the experimentally determined values are systematically smaller than the length of the electron bunches. Nominal electron pulse durations of 175 and 75 fs, as provided by the LCLS control system, yield x-ray pulse shapes of 120 ± 20 fs full-width at half-maximum (FWHM) and an upper limit of 40 ± 20 fs FWHM, respectively. Simulations of the free-electron laser agree well with the experimental results. Contents

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...

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 ...

Abstract. We present a systematic study of the photoionization of noble gas atoms exposed simultaneously to ultrashort (20 fs) monochromatic (1-2% spectral width) extreme ultraviolet (XUV) radiation from the Free-electron Laser in Hamburg (FLASH) and to intense synchronized near-infrared (NIR) laser pulses with intensities up to about 10 13 W cm −2 . Already at modest intensities of the NIR dressing field, the XUV-induced photoionization lines are split into a sequence of peaks due to the emission or absorption of several additional infrared photons. We observed a plateau-shaped envelope of the resulting sequence of sidebands that broadens with increasing intensity of the NIR dressing field. All individual lines of the nonlinear two-color ionization process are Stark-shifted, reflecting the effective intensity of the NIR field. The intensity-dependent cutoff energies of the sideband plateau are in good agreement with a classical model. The detailed structure of the two-color spectra, including the formation of individual sidebands, the Stark shifts and the contributions beyond the 6 Author to whom any correspondence should be addressed.

We have observed the simultaneous inner-shell absorption of two extreme-ultraviolet photons by a Xe atom in an experiment performed at the short-wavelength free electron laser facility FLASH. Photoelectron spectroscopy permitted us to unambiguously identify a feature resulting from the ionization of a single electron of the 4d subshell of Xe by two photons each of energy ð93 AE 1Þ eV. The feature's intensity has a quadratic dependence on the pulse energy. The results are discussed and interpreted within the framework of recent results of ion spectroscopy experiments of Xe obtained at ultrahigh irradiance in the extreme-ultraviolet regime. DOI: 10.1103/PhysRevLett.105.013001 PACS numbers: 32.80.Rm, 32.80.Fb, 32.80.Hd, 42.50.Hz The advent of high-intensity extreme-ultraviolet (EUV) and x-ray free electron lasers (FELs) has heralded a new era in the study of nonlinear optical processes. These facilities put this well established area into a new domain where the optical field exhibits a high degree of coherence in addition to an unprecedented combination of high average and peak intensity at high photon energy. As a result, inner-shell electrons and, indeed, highly correlated multielectron excited states can become important mediators of the multiphoton-matter interaction. Since it began operation in 2005, the EUV Free Electron Laser in Hamburg (FLASH) [1] has hosted a wide range of investigations in atomic and molecular physics including experiments on dilute targets such as molecular ions and highly charged ion beams, few-photon few-electron ionization processes, two-color coherent processes, and ultrafast pump-probe experiments [2,3].The simplest nonlinear process one can drive in an intense EUV laser field is two-photon ionization. Consequently, it is a prototypical process and its pursuit has become an important benchmark experiment for the study of the response of the simplest quantum systems, ranging from atoms to clusters, to intense high-frequency electromagnetic fields. Some years ago, high-order harmonic sources developed to the point where they could attain the threshold intensities needed to drive two-photon processes in an atom. The very first results were reported in breakthrough experiments at the very limit of intensities that can be obtained with these high-order harmonic sources. These included two-photon single and double ionization of He [4][5][6] and single ionization of the valence shells of Ar and Xe [7]. However, in focused beams, FLASH yields irradiance levels which can approach 10 16 W cm À2 [8], and so it provides a platform for a dramatic expansion of the range of sequential and simultaneous multiphoton ionization experiments that could be performed at EUV wavelengths. A good example is an experiment where FLASH permitted the full kinematics of two-photon double ionization of the valence shell of Ne to be recorded by using the cold target recoil ion momentum spectroscopy technique [9].Exceptional behavior of the light-matter interaction at high irradiance has been demonstrated in two...