# Recent Advances in Ultrafast Structural Techniques
## 3 Structural Techniques Involving Ultrashort Electron Pulses
### Ultrabright Relativistic FED Setups: RF-Photonijectors  
RF-photoinjectors are capable of producing ultrabright and ultrashort electron bursts, which are currently exploited as probes in relativistic FEDs. The use of radiofrequency allows for routine extraction fields \(\approx\) 100 MVm\({}^{-1}\) to be attained. Additionally, RF-photoinjectors are capable of producing ultrabright and ultrashort pulses of electrons with kinetic energies that are usually within the range of 1-5 MeV for FED measurements. At these energies, the space-charge effects that leads to pulse broadening are largely suppressed due to relativistic effects that diminish Coulombic repulsion in by a factor of \(\gamma^{3}\) in both the longitudinal and transverse directions [246]. The progress in the development of relativistic FED setups over the last 10 years has been tremendous. Photoinjectors were originally optimized for higher bunch  
Figure 20: **All-optical control and metrology of electron pulses. ** Experimental setup: a 1-ps laser pulse from a Yb:YAG regenerative amplifier is frequency doubled and generates an electron pulse from a thin-film gold photocathode. The laser also drives two optical-rectification stages, each generating single-cycle THz pulses with energy of up to 40 nJ. (Inset) THz resonator structures are laser-machined in a 30-mm-thick aluminum foil. The first element, used for compression, is oriented at 45\({}^{\circ}\) to the electron beam, providing time-dependent longitudinal forces on the electrons. The second THz resonator, used for streaking, is oriented normal to the beam, resulting in time-dependent transverse deflection. Figure adapted from Reference [245] with permission from Science AAAS. charges \(\#\text{e}^{-}>10^{9}\) (instead of \(\#\text{e}^{-}\approx 10^{6-7}\)) as well as for higher kinetic energies to be used as sources in XFELs. Relativistic FED was found to be of great importance to XFELs since the generation of high-quality low emittance electron beams are beneficial to both technologies. University-based pioneering MeV FED setups have been developed at UCLA [184; 207] and Osaka, [187; 247] and later launched at BNL, [192] SLAC [171; 172; 173; 174; 175; 176; 177; 178; 179; 180; 193; 194; 195; 196] and DESY [248; 249]. The amount of existing knowhow in accelerator centers has been critical to obtain stable RF-photoinjectors with minimal laser-RF timing noise to reach \(\approx 100\) fs (rms) temporal resolution [250]. Recent results obtained at the MeV-UED line of SLAC are shown in Figure 21 for the study of CH\({}_{3}\)I in the gas phase. Relativistic FED is the ideal tool for monitoring structural dynamics in molecular beams. Electron pulses moving at near the speed of light are necessary to avoid temporal broadening caused by velocity mismatch between the optical pump pulse and the electron probe pulse [4; 251]. Yang et al. [173] conducted the first direct observation of a nuclear wave-packet with atomic spatiotemporal resolution during nonadiabatic processes involving conical intersections. More experiments with similar level of details are expected for the investigation of ultrafast structural dynamics in the gas phase. Figure 21: **Relativistic ultrafast gas-phase electron diffraction. ****(a)** Schematic of relativistic FED experiment. **(b)** A model for a CF\({}_{3}\)I molecule oriented along the laser polarization axis. **(c)** Simulated pair distribution function (PDF) for a molecular ensemble with a \(\cos^{2}\theta\) angular distribution. **(d)** A model for a CF3I molecule oriented perpendicular to the laser polarization. **(e)** Simulated PDF for a molecular ensemble with a \(\sin^{4}\theta\) angular distribution. The laser polarization is indicated by the double-headed arrow. Gray, light green, and purple represent carbon, fluorine, and iodine, respectively. **(f)** Potential energy surfaces depicting the two-channel excitation in CF\({}_{3}\)I along the C–I bond length coordinate, with major states labeled and reaction pathways marked by arrows.