# Recent Advances in Ultrafast Structural Techniques
## 3 Structural Techniques Involving Ultrashort Electron Pulses
### Bright Compact Direct Current (DC) FED Setups  
It was not until the work of Siwick et al. [179] demonstrating the first fully resolved structural transition using 600 fs electron pulses that it was realized that space-charge effects could be diminished by reducing the electron propagation distance and maximizing the extraction field, \(|\mathrm{E}|\). Detailed N-body simulations and mean-field analytic models provided a greater understanding of pulse propagation dynamics of sub-relativistic electron bunches [216; 217; 218; 219]. Nowadays, the use of charge-tracking programs, [220; 221] that are required for the proper design of FED instrumentation, is routine. Historically, charge-tracking algorithms and powerful software packages were implemented by accelerator physicists, yet for a while UED scientists were largely unaware of this development due to the lack of interaction between communities. Despite having a much simpler design than that of MeV relativistic machines, compact FEDs have provided unprecedented results in the study of novel crystalline materials. Several representative experiments performed in transmission and reflection geometries with compact FED instruments are shown in Figures 15 and 16, respectively. FED has been found to be particularly advantageous for the investigation of 2D materials [183; 185; 186; 222]. Figure 15a,b presents results obtained by Eichberger et al. [183] for the study of 1T-TaS\({}_{2}\), a prototypical CDW system. FED has enabled the observation of atomic motions on timescales short enough to follow even the effect of non-equilibrium electronic distribution in this strongly correlated crystal. Since FED provides diffraction information over the entire 2D Brillouin zone at once, it is capable of unveiling different processes that give rise to changes in diffracted intensities. Figure 15c,d illustrates FED measurements gathered by Waldecker et al. [190] in the investigation of Ge\({}_{2}\)Sb\({}_{2}\)Te\({}_{5}\) (GST), a phase-change compound commonly used for optical data storage. In this study, the combination of transient absorption/reflectivity and FED data allowed the authors obtain a microscopic picture of the initial steps in the photoinduced amorphization process in GST. Figure 15e-m presents FED results in Me\({}_{4}\)P[Pt(dmit)\({}_{2}\)]\({}_{2}\) (Me\({}_{4}\)P = tetramethylphosphonium, dmit = 1,3-dithiol-2-thione-4,5-dithiolate), a crystalline system with a relatively large unit cell (a = 2.89 A, b = 12.6 A, c = 37.4 A). Ishikawa et al. [223] applied transient reflectivity and FED to elucidate the key molecular motions that couple to an orbital-driven charge transfer reaction, i.e., a dimer expansion and a librational mode. These results illustrate the potential of advanced FED instruments to address fundamental structural phenomena occurring in increasingly complex molecular systems. On the other hand, FED experiments in reflection geometry (reflection high-energy electron diffraction, RHEED) with 30 keV electrons enabled the investigation of an indium (In) monolayer adsorbed on a (1 1 1)-Si surface (see Figure 16). At room temperature, the In atoms form regular arrays of quasi-one-dimensional (4 \(\times\) 1) zigzag wires, separated by rows of silicon atoms as shown in Figure 16c. Around 125 K, a first-order Peierls-like transition to the (8 \(\times\) 2) CDW phase occurs that is accompanied by a metal-to-insulator transition, see Figure 16a. With a temporal resolution of about 330 fs achieved via the implementation of pump pulse front tilting, [176; 177]. Frigge et al. [178] enabled the observation of In-In bond breaking and formation that drive the structural transition through the non-thermal excitation of critically damped soft phonon modes. Figure 15: **Compact FED setups, measurements in transmission geometry. ****(a–b)** Time evolution of the CDW state in 1T-TaS2 upon photoexcitation. (**a**) The circles correspond to FED and the solid traces to transient reflectivity data, respectively.