## 3 Linear Streaking Measurements at Free-Electron Lasers
### Experimental implementation  
For the purpose of the streaking measurement at an XFEL an experiment was conducted (for further experimental details and theoretical background see [96]) at the atomic, molecular and optical science (AMO) instrumental end station at the LCLS [97]. It comprises a vacuum chamber within which an optical laser co-propagates with the XFEL and a magnetic bottle electron spectrometer [98], a high collection efficiency spectrometer that operates over a broad energy range capable of detecting the photoelectron spectra for every single shot. The streaking laser was based on an optical parametric amplifier with the idler tuned to 2.4 um central wavelength, horizontal polarization of the electric field, a pulse energy of \(\sim\)30 uJ and a compressed pulse duration of \(\sim\)50 fs full width at half maximum (FWHM). The output beam  
Figure 7: Photoelectron streaking schematic. A short extreme ultraviolet (XUV) pulse generates a photoelectron bunch by ionizing noble gas atoms in the presence of a strong, few-cycle phase-stable NIR laser pulse. The XUV pulse and a NIR streaking pulse are precisely synchronized to each other. The current profile of the generated photoelectrons is given by the temporal structure of the ionizing XUV pulse. After photoionization, the electrons are subject to the electric field \(E_{\mathrm{L}}(t)\) of the NIR pulse which introduces a change in the final momentum distribution of the photoelectrons, depending on the instant of release \(t_{0}\) into the streaking laser field. The momentum change is detected with a time-of-flight (TOF) detector, which collects the electrons released along the direction of the streaking field polarization. The TOF is oriented such that it is axially aligned with the linear polarization of the laser and the XUV pulse. (Figure from [95]). of the NIR laser was focused to a \(1/e^{2}\) beam diameter of \(\sim\)450 \(\upmu\)m in the interaction region, yielding an optical intensity on target of roughly 4 \(\times\) 10\({}^{11}\) W/cm\({}^{2}\) or a corresponding peak vector potential of the streaking field of \(A_{\text{peak}}\approx\) 0.18 atomic units. As a target for photoexcitation neon gas was streaming from a nozzle at a background pressure of \(\sim\)1 \(\times\) 10\({}^{-7}\) mbar in the experimental chamber, and the photoelectrons were detected with the MBES. The LCLS was working at a repetition rate of 60 Hz in the so-called 'low-charge mode', with an electron bunch charge of 20 pC around the point of maximum compression at \(\sim\)10 kA, producing XFEL pulses expected to be of sub-10 femtosecond duration [11]. In order to further minimize the XFEL pulse duration, the'slotted spoiler' [32] was inserted into the electron beam at the second bunch compressor chicane along the SLAC Linac ahead of X-ray generation in the undulator. With an XFEL photon energy of \(E_{\text{X-ray}}\) = 1790 eV and a binding energy of \(E_{\text{B}}\) = 870 eV for the 1s electron in neon, the central energy of the photoelectrons is distributed around the mean value of 920 eV with a standard deviation of \(\sim\)2 eV rms. The bandwidth of the photoline, corresponding to the energy spread of the X-ray pulse, was measured to be \(\sim\)3.3 eV rms. The various measured parameters of the XFEL are summarized in Figure 8.  
For the streaking experiment one has to carefully overlap the XFEL pulse with the NIR laser spatially and temporally. A fine-scan was performed by integrating the electron detection signal in an energy region close to the unstreaked photoline and changing the length of the path of the optical laser in small steps (compare Figure 9 for the measurement principle). The better the XFEL and the NIR laser are overlapped, the more electrons see the NIR field and the detected electron spectrum  
Figure 8: Measured XFEL parameters.