# Recent Advances in Ultrafast Structural Techniques
## 2 Structural Techniques Involving Ultrashort X-ray Pulses
### Timing Techniques at XFEL Facilities  
The intrinsic time resolution of XFELs is limited by synchronization noise (or timing jitter in rms) to about 100-200 fs [72; 149; 150] with 100 fs reflecting short-term pulse-to-pulse variations. Figure 10: **Time-resolved X-ray absorption spectroscopy (XAS). ** Top panels (**a–c**): time-resolved X-ray absorption near-edge structure (XANES) for aqueous [Fe(bpy)\({}_{3}\)]\({}^{2+}\). (**a**) Fe K-edge XANES spectrum of the low spin (LS) ground state (black trace) and of the high spin (HS) quintet state (red dots). The latter is determined from the LS spectrum and the transient spectrum measured at \(t=+50\) ps after fs-laser excitation (Gawelda et al. [140]) synchrotron microXAS line at SLS). (**b**) Transient XANES spectrum (difference between red symbols and black trace shown in panel (**a**)). The blue stars represent the transient spectrum recorded at \(t=+300\) fs via laser slicing at SLS. (**c**) Time scan of the signal (blue points) at the B-feature (arrow in panel (**b**)) as a function of \(t\). The inset shows a long time scan up to a 10-ps time delay. The red trace is the simulated signal assuming a simple kinetic model to describe the spin conversion process. The vertical arrow displays the expected effect of an elongation of 0.2 Å for the Fe-N bond distance difference between the LS and HS states. Error bars are \(\pm 1\) standard deviation. Figure adapted from Reference [133] with permission from Science AAAS. Bottom panels (**d–g**): Ultrafast x-ray absorption spectra of ferrous cyt c. (**d**) Superimposed spectra of ground-state (GS, black) and excited-state (ES, green) cyt c, showing the edge, white line, and shape resonance regions. (**e**) Cyt c heme structural model used in simulations. Fits (blue) to the GS (**f**) and ES (**g**) spectra (red) and associated structural parameters, derived from simulations. The fit residuals are in yellow at the bottom with the obtained structures above. Figure adapted from Reference [138] with permission from Science AAAS. In order to fully exploit the potential of ultrashort hard X-ray pulses for monitoring faster dynamic phenomena different schemes have been proposed and implemented for the characterization of X-ray pulses. [149; 150; 151; 152; 153; 154; 155; 156; 157; 158] Note that any useful characterization tool should provide single-shot sensitivity since a large component of the XFEL timing noise comes from shot-to-shot fluctuations. Ideally, one would like to access the temporal and spectral profiles of each X-ray pulse as well as its relative timing with respect to the fs-optical driving pulse; being the latter the bare-minimum. Among all current options, single-shot, single-cycle terahertz (THz) pulse streaking presents the advantage of characterizing both the temporal pulse shape of the free electron laser (FEL) pulse and its relative arrival time. The experimental arrangement implemented by Grguras et al. [149] at FLASH (DESY, Hamburg) is shown in Figure 11a. A-3 mJ, 50-fs laser pulse (\(\lambda\) = 800 nm) is split into two parts; most of the pulse energy is used for THz generation via pulse front tilting for phase matched optical rectification in a LiNbO\({}_{3}\) non-linear crystal [159; 160]. The remainder (tens of \(\upmu\)J) is used for in situ THz-field characterization by electro-optic sampling (EOS) in a ZnTe crystal. Optical rectification produced about 4 \(\upmu\)J, 2-ps, single-cycle THz pulses centered at 0.6 THz. Figure 11b-d illustrates the THz streaking principle. In order to determine both the temporal shape and the relative arrival time, the FEL pulse should be within the streaking window of the THz vector potential denoted in Figure 11d by \(\Delta t\), which in this case \(\Delta t\)\(\sim\)600 fs. Note that the streaking window should be larger than the intrinsic timing jitter to guarantee that most (if not all) FEL pulses become potentially useful (i.e., they are timed). Grguras et al.