# Recent Advances in Ultrafast Structural Techniques
## 3 Structural Techniques Involving Ultrashort Electron Pulses  
### Introduction to Femtosecond Electron Diffraction (FED)  
Since the very first stroboscopic experiments utilizing electron pulses as structural probes in time-resolved diffraction experiments in the 1980s, [164; 165], there has been a continuous progress in the development of brighter and faster electron diffractometers. This advancement has occurred alongside several breakthrough ultrafast electron diffraction (UED) discoveries in the gas phase, [166; 167; 168; 169; 170; 171; 172; 173] and in solid crystalline materials measured in reflection [174; 175; 176; 177; 178] and transmission [179; 41; 179; 180; 181; 182; 183; 184; 185; 186; 187; 188; 189; 42; 43; 44; 45; 46; 47; 48; 49; 50; 51; 52; 53; 54; 55; 56; 57; 58; 59; 60; 61; 62; 63; 64; 65; 66; 67; 68; 69; 70; 71; 72; 73] geometries. Great progress has been achieved in terms of FED (or UED) technology, with three main types of approaches being widely implemented in our research community (ordered in increasing design complexity and cost): (i) Compact (direct current, DC) FED designs, (ii) Hybrid (DC-RF) FED setups (i.e., with electron pulse rebunching) and (iii) Relativistic (RF) FED instruments. Figure 13 presents the layout of a compact FED setup with a conventional DC fs-electron source. The main difference among FED instruments is configuration/design of the electron gun, which is highlighted in Figure 13 by a dashed box. In a DC fs-electron source, electrons are generated via photoemission and accelerated by a DC electric field. A magnetic lens (ML) is used to focus the diffraction pattern on the detector screen. Note that the ML could be placed after the sample (akin to an objective lens in a transmission electron microscope (TEM)). This configuration greatly reduces the electron pulse propagation distance to the sample and, therefore, improves the temporal resolution (vide infra).  
A 300 kV DC-FED setup currently under development at the Ultrafast electron Imaging Lab (UeIL) of the University of Waterloo is shown in Figure 14. As can be observed, FED systems are laboratory scale tools, which depending on their design may even fit on top of optical tables. Care should be taken when operating FED instruments at acceleration voltages >30 kV; energetic electrons produce via corona discharge may lead to the generation of X-rays when impacting the walls  
Figure 13: **Femtosecond electron diffraction (FED) layout for a compact electron gun design.** In the present compact FED setup, a fs-optical pulse (green) generates, via the photoelectric effect, a fs-electron bunch by back-illuminating an ultrathin -20-nm thick gold film. Electrons are accelerated by a 100 kV DC potential drop. A magnetic lens can be implemented before or after the sample to focus the diffraction pattern on the detector. Typical electron beam diameters at the sample position range from 50 to 500 \(\upmu\)m. Figure adapted from Reference [181] with permission from Science AAAS.  
of the vacuum chamber. Therefore, proper radiation shielding in addition to dosage monitors are a must in FED laboratories.