# Recent Advances in Ultrafast Structural Techniques
## 2 Structural Techniques Involving Ultrashort X-ray Pulses
### Table-Top fs Hard X-ray Plasma Diffractometers  
Table-top fs-X-ray plasma diffractometers have been successfully implemented to unveil ultrafast structural dynamics in different materials [25; 26; 27; 28; 29; 30; 31; 32; 33; 34; 35; 36; 37; 38; 39]. These time-resolved instruments are based on the use of chirped-pulse amplification [40] with pulse energies in the range of 1-100 mJ depending on the repetition rate of the laser and the average power. Fs-hard X-ray pulses are produced when a solid target is irradiated by fs-laser pulses carrying a peak intensity \(I_{\text{peak}}>10^{16}\) W cm\({}^{-2}\). The generation of fs-X-ray pulses from a plasma source involves the extraction of electrons from the metal target by strong field ionization with a small laser spot. This step is followed by electron acceleration in vacuum and electron re-entrance into the target material owing to the strong electric field. Then, X-ray photons are generated via collisional inner-shell ionization followed by radiative transitions of outer-shell electrons into inner-core holes. Such processes occur every cycle of the electric field of the fs-laser driving pulse. The source is the main challenge in the design of table-top fs-X-ray plasma diffractometers. The surface of the target material is ablated after each laser shot and, therefore, it must be translated to preserve surface uniformity. Cu tape of about 20 \(\upmu\)m in thickness is often utilized as target material producing the characteristic Cu K\({}_{\alpha}\) and K\({}_{\beta}\) X-ray emission lines. The emission of X-ray photons is highly isotropic (all directions). Hence, the implementation of X-ray optics becomes crucial to obtain a decent photon flux at the sample position. The schematic of a table-top fs-X-ray plasma diffractometer is shown in Figure 3. The X-ray source is driven by 50-fs laser pulses with a central wavelength, \(\lambda=800\) nm. After losses associated with X-ray collection, this instrument delivers at the sample position an effective X-ray photon flux of 5000 Cu K\({}_{\alpha}\) photons/pulse with a repetition rate of 1 kHz [38]. Figure 3: **Layout of the table-top fs-X-ray plasma diffractometer at the Max Born Institute. ** In this example, a (1 1 1)-oriented single crystalline (SC) Bi sample is photoexcited by 400-nm laser pulses generated by frequency doubling in a \(\beta\)-barium borate (BBO) crystal. The pump arm is mechanically chopped. A photodiode (PD) is used to determine the state of the sample (i.e., excited or unexcited) on shot to shot basis. Diffracted X-ray photons are measured by an energy-resolving CdTe diode (Amptek). The inset shows a spectrum with a very low X-ray flux on the detector. Figure adapted from Reference [38] with permission from the authors. Figure 4 illustrates some representative measurements obtained by Freyer et al. [39] with the table-top fs-X-ray plasma diffractometer shown in Figure 3 but in a different configuration to perform experiments in transmission geometry (i.e., these measurements required the implementation of a large area high-resolution detector). Freyer et al. [39] studied a polycrystalline sample of [Fe(bpy)\({}_{3}\)]\({}^{2+}\)(PF\({}_{6}\)\({}^{-}\))\({}_{2}\). The authors were able to determine the electron density changes that follow a charge transfer reaction (Figure 4g-h). Recall that X-ray photons scatter off the electron density, i.e., they interact with the electron cloud and, therefore, variations in diffracted intensities (Figure 4b-f) correspond to changes in electron density arising from charge redistribution and/or nuclear rearrangements. Freyer et al. [39] carried out a careful analysis of the electron density maps that applies to centrosymmetric crystals under relatively low excitation fluence. The authors found that each photoexcited complex largely affects its surroundings, highlighting the many-body character of this charge transfer.