## 3 Linear Streaking Measurements at Free-Electron Lasers
### Experimental implementation
(**a**) Single-shot distribution of central XFEL photon energy, measured with the magnetic bottle electron spectrometer (MBES); (**b**) Single-shot distribution of XFEL photon energy spread (rms); (**c**) Distribution of relative arrival time between NIR laser and XFEL, measured for a nominal delay set to zero (at full temporal overlap); (**d**) Cross-correlation intensity scan, by moving the NIR pulse in small steps through temporal overlap with the X-rays pulse and incorporating the arrival time jitter (Blue bars and points are the experimental data, and red full curves are fits to the data). deviates from a pure photoline. Thus, one gets a time-dependent measure of the strength of the overlap. This signal can be used to find the maximum temporal overlap (see Figure 8d). **Direct time-domain measurement of ultrashort X-ray pulses**  
In this experiment [69], the attosecond streaking technique was applied to X-ray FEL pulses for the first time. The XFEL and the streaking laser typically are two separate laser sources and need to be externally synchronized to keep them temporally overlapped. Nevertheless, even with synchronization a residual shot-to-shot arrival time jitter between the two lasers remains. Since the arrival time jitter between XFEL and NIR is of the same order as the NIR pulse duration, it is not possible to assign a definite delay and corresponding value of the vector potential with respect to the streaking field to each single XFEL pulse. With the presented scheme and following analysis one can overcome the synchronization issue and derive an upper limit of the X-ray pulse duration for each single shot in a manner that is decoupled from all machine parameters and can be used simultaneously with any ongoing measurement. Before presenting the details of the evaluation, it is an important prerequisite for the following analysis to make an estimate of the X-ray pulse duration compared to the streaking laser period. Following the previous reasoning, for longer X-ray pulses (enclosing one or more cycles of the streaking field) the energy shift of the streaked photoelectron spectrum is negligible, thereby resulting  
Figure 9: Determination of XFEL–NIR arrival time jitter. Panels (**A**,**B**) show the simulation of X-ray pulses (blue, black, purple and yellow) overlapping with the NIR streaking field (red) at different relative delays and the corresponding photoelectron spectra. Panel (**C**) depicts the measured cross-correlation obtained by integrating the signal over a small region of the spectrum at a point 4\(\sigma\) away from the unstreaked photoline for different delay settings. In the case of very poor overlap (the situation depicted as the yellow XFEL pulse in panel (**A**) there is nearly no streaking effect: the measured photoline stays narrow and the signal in the region of interest is small. As the X-ray pulse overlaps more and more with a vector potential of higher amplitude (the cases of the blue, purple and black pulse, in ascending order of streaking potential amplitude), the generated PE spectra become ever more broadened and the integrated signal rises accordingly. Therefore, the detected width of this cross-correlation can be interpreted as the convolution of the NIR pulse duration and the average arrival time jitter between the XFEL and the streaking field (panel (**C**)), which amounts to ~93 fs rms in our case (Figure from [96]). in uniformly larger streaked spectral widths.