# Recent Advances in Ultrafast Structural Techniques
## 3 Structural Techniques Involving Ultrashort Electron Pulses
### Ultrabright Hybrid DC-Radio Frequency (RF) FED Setups: Compression of Electron Pulses  
Electron pulse compression methods are based on either the static [230; 231; 232; 233; 234] or active [235; 236; 237; 238; 239] application of electric or magnetic fields that can reverse the longitudinal momentum-position distribution of the electrons in the bunch. These electron beam components effectively act as a temporal lens and the sample is to be positioned at the temporal focal point. Among all proposed and/or demonstrated approaches for electron pulse rebunching, the implementation of a cylindrical TM\({}_{010}\)-RF cavity [235; 236; 237; 238; 239] became the most widely accepted and successful method. Figure 18 illustrates the electron pulse compression principle in a hybrid 100-kV DC-RF FED instrument. The RF pill lens provides an axial electric field which is uniform along the cavity axis and varies in time according to \({\rm E_{z}}({\rm z,t})=\ {\rm E_{0}}({\rm z})\ \cos(2\pi{\rm rf}\ +\ \varphi)\); f \(\approx\) 3 GHz is the resonant frequency of the cavity and \(\varphi\) a phase that assures that the electron pulse does not experiences any net gain or loss of energy. The choice of a frequency of 3 GHz was made to benefit from the existing S-band technology used by the RF accelerator community, in which this frequency is a standard. N-particle simulations [236] and an experimental demonstration [237] showed that such an approach can indeed provide ultrashort electron bursts containing \(\hbar\)e\({}^{-}\)\(>10^{6}\). However, laser-RF synchronization noise has limited (until very recently, see below) the instrument response of hybrid DF-RF FED setups to about 300-400 fs [238; 239]. Ultrabright fs-electron bunches provided unique  
Figure 18: **Layout of a hybrid DF-RF FED setup. ** The electron bunch is represented by a colored gradient where faster electrons are in violet and slower electrons in red. The electron pulse is generated via the photoelectric effect. This part is essentially the same as in a compact DC electron gun. The longitudinal momentum-space distributions within the electron bunch are illustrated below. The effect of the axial RF electric field is to invert the longitudinal momentum–space distribution causing the electron bunch to recompress during its propagation towards the sample, which is located at the temporal focal point. Magnetic lenses (MLs) are used to control the transverse dimension of the electron beam. Figure adapted from Reference [4] with permission from IOP Publishing. insights into photoinduced phenomena in strongly correlated materials including labile and poorly conductive organic crystals. Figure 19 shows some characteristic results obtained by Gao et al. [41] and Morrison et al. [189] in the FED study of (EDO-TTF)\({}_{2}\)PF\({}_{6}\) and VO\({}_{2}\), respectively. Gao et al. [41] studied the formation of a light-induced TIS in (EDO-TTF)\({}_{2}\)PF\({}_{6}\) that is distinct from its LT and HT phases but resembles optical properties of the HT metallic phase [240; 241]. Morrison et al., [189] on the other hand, discovered a metal-like M1 phase of VO\({}_{2}\); see hatched  
Figure 19: **Movics recorded with 100-kV hybrid DC-RF FED setups. ** Key molecular motions leading to charge delocalization in (EDO-TTF)\({}_{2}\)PF\({}_{6}\). **(a–c)** Photoinduced time-dependent structural changes in (EDO-TTF)\({}_{2}\)PF\({}_{6}\). **(a)** Differences between the diffraction patterns of the photoinduced phase and the initial low temperature (LT) phase as a function of \(t\). **(b)** Difference between the diffraction patterns of the high-temperature (HT) and LT phases. Formation of a photo-induced transient intermediate structure (TIS). **(c)** Unit cells illustrating the arrangement of EDO-TTF molecules and PF\({}_{6}\) counter-ions in the LT-unit cell.