## 1 Introduction--Laser-Assisted Time-Domain Characterization at Free-Electron Lasers
### Ultrafast temporal characterization for novel attosecond X-ray experiments  
From the perspective of the scientific researcher, the special opportunities opened up by free-electron laser sources lie in their extreme spatial and temporal coherence, combined with the achievable ultrahigh intensity, broadly adjustable photon energy range and ultrashort pulse duration. Using these sources will ultimately enable novel ultrafast experiments with attosecond temporal resolution, like site-specific X-ray pump/X-ray probe measurements [39] and serial crystallography for studies of structural dynamics [40], or atomic-scale diffractive imaging as proposed by [PERSON] et al. [41] on energy-selected target states of molecules, surfaces and nanoparticles. Recent experiments withhigh-energy electrons [42; 43] and X-rays [44] have already demonstrated the first steps towards this so-called molecular movie on the femtosecond scale. For an extensive overview of the experimental capabilities and scientific achievements during the last years at LCLS see Ref. [45]. To realize the goal of time-resolved X-ray measurement with attosecond resolution, one first has to consider the special complexities arising from the work with attosecond XFEL pulses, foremost the characterization of their temporal structure. As with any signal based on the self-amplified spontaneous emission (SAE) process [46], the temporal shape of each subsequent single XFEL pulse is implicitly different from the previous one, demanding a single-shot measurement technique for its investigation. However, until recently no direct experimental determination of the structure of these X-ray pulses in the time domain was accomplished. Therefore, in this paper we want to concentrate on the unique aspects and concomitant challenges that arise from the availability of X-ray free-electron laser pulses of femtosecond and even sub-femtosecond durations and their characterization on a suitable time scale. **Indirect measurement methods**  
SAE XFEL pulse durations have been indirectly inferred (see also the review in Ref. [47]) from measurements on the electron bunch length before the undulator [48] or from statistical analysis of the spectral coherence properties of the ultrashort X-ray pulses [49]. However, these characterization techniques cannot provide single-shot information on the temporal structure of the X-ray pulses and only statistically averaged quantities for possible pulse length predictions can be derived from these measurements. Furthermore, studies of fundamental ionization processes [50] and sideband measurements, also reported on in this review [51], have revealed a clear deviation of the actual X-ray pulse duration from the one inferred from electron bunch length measurements. A substantial improvement of the temporal resolution and the reliability for X-ray pulse characterization has been  
Figure 1: Sketch of the working principles for few-femtosecond and sub-femtosecond X-ray free-electron lasers (XFEL) pulse generation. (**a**) The upper part shows the spatially chirped electron bunch at the center of the compressor chicane with a marked tilt with respect to the direction of propagation. (**b**) The slotted foil is depicted, leaving only a narrow central region of the bunch pass unspoiled ((**a,b**) are reproduced with permission from [32], Copyright American Physical Society, 2004). (**c**) The upper panel shows a schematic of the components involved in attosecond X-ray pulse production. W1 and W2 are two wiggler sections that generate the modulation of the electron bunch by interaction with high-power, few-cycle optical lasers. (**d**) In panel, the calculated energy modulation of the electrons along the electron bunch produced in the interaction with a few-cycle, 1200 nm laser pulse in a wiggler magnet is shown. (**e**) The resulting attosecond X-ray power profile after 50 m of propagation in the undulator is plotted in panel ((**c-e**) are reprinted under the terms of the Creative Commons Attribution 3.0 License from [38], 2005).