# Recent Advances in Ultrafast Structural Techniques
## 2 Structural Techniques Involving Ultrashort X-ray Pulses
### Timing Techniques at XFEL Facilities
[149] demonstrated the use of single-cycle THz pulses for the characterization of extreme ultraviolet (XUV) FEL pulses. Appropriate target atoms can be chosen to extend THz streaking into the hard X-ray regime where the main disadvantage is the large reduction in the photoionization cross-section when compared to soft X-rays [72; 161]. Figure 11: **Single-shot, single-cycle THz streaking. ****(a)** Experimental setup: A fs-laser pulse from a Ti:Sapphire, appropriately delayed with respect to the free electron laser (FEL) pulse, is split into two parts. Most of the pulse energy is used for tilted-wavefront THz generation in LiNbO\({}_{3}\). The remaining part can be used for characterizing the THz pulse via electro-optic sampling (EOS). This could be done in situ (prior streaking measurements) in a zinc telluride (ZnTe) nonlinear crystal positioned at the gas target location. In the streaking measurement, the collinear FEL photon pulse ejects a burst of photoelectrons from the gas, with a temporal profile identical to the incident soft XUV/X-ray FEL pulse. The THz pulse is used to streak the photoelectron burst and consequently characterize the FEL pulse. (**b-d**) Schematic of the operational principle: Blue and red curves represent the electric field and corresponding vector potential of the single-cycle THz pulse. The vertical axis corresponds to the kinetic energy of the photoelectron emission, which is determined by a time of flight (TOF) spectrometer. (**a**) The FEL pulse does not overlap with the THz pulse, and therefore the kinetic energy distribution of photoelectrons ejected by the FEL pulse is unaffected. (**c**) The FEL pulse overlaps with an extreme of the THz vector potential, leading to a maximally downshifted photoelectron spectrum with minimized spectral broadening. (**c**) Temporal overlap occurs near the zero crossing within the effective streaking window (\(\Delta t\)) of the vector potential where the time of arrival as well as the temporal profile can be determined. Figure adapted from Reference [149] with permission from Springer Nature. The aforementioned THz streaking method is probably one of the most elegant characterization tools, which is sensitive to fluctuations in SASE amplification and in combination with a dispersive X-ray spectrometer [147; 148] could provide a quasi-complete description of XFEL pulses. However, the most popular timing methods at XFELs, owing to simplicity, are spatial [150; 154; 155; 156; 157; 162; 163] and spectral [150; 152] encoding. These are cross-correlation techniques based on X-ray induced material changes and confer a means to determine the relative timing between the fs-optical pump pulse and the X-ray probe pulse. Figure 12 illustrates a spectrogram approach recently introduced by Hartmann et al. [158] that combines the best features (or avoids the disadvantages) of spatial and spectral encoding to provide an outstanding timestamping error of sub-1 fs (rms). With such a timing method in hand, and routine 5-fs optical and 5-fs SASE X-ray pulses, the effective time-resolution of XFELs is now down to the 5-fs level providing the possibility to monitor high frequency vibrational modes and electronic coherences. Figure 12: **Schematic of the single-shot geometry for measurement of the spectrogram. (a)** The X-ray and optical beams are crossed in a silicon nitride membrane and their relative delay is encoded in the spatial beam profile of the optical probe. The crossing angle \(\alpha\) and beam diameters define the time window in which the X-ray-induced absorption is probed. (b) Measured spectrogram using an unpumped normalization spectrogram to calculate the change in transmission. The result of the two-step edge-finding algorithm to determine the X-ray arrival time is overlaid as a black dashed line. Figure adapted from Reference [158] with permission from Springer Nature.