# Recent Advances in Ultrafast Structural Techniques
## 2 Structural Techniques Involving Ultrashort X-ray Pulses
### Table-Top fs Hard X-ray Plasma Diffractometers
They concluded that the PF\({}_{6}\)\({}^{-}\) counterions play a key role in charge relocation with a behaviour in the solid state that is markedly different from that in diluted liquid solutions where the spatial separation among solvated anions and cations is much larger. Similar anionic effects were found in (EDO-TTF)\({}_{2}\)PF\({}_{6}\) by Gao et al. [41] through the implementation of FED. More recent efforts in the development of fs-hard X-ray plasma sources driven by mid-infrared (mid-IR) lasers demonstrated an increase in brightness of about 25 times [42]. The emission efficiency of X-ray photons increases with the kinetic energy of the re-entering electron, which is proportional to \(I_{\text{peak}}\,\lambda^{2}\). Thus, the application of intense fs-mid-IR lasers may bring the effective brightness of table-top fs-hard X-ray diffractometers to the level of \(\sim\)10\({}^{5}\)-10\({}^{6}\) photons/pulse (at sample position). This is about one order of magnitude below the flux achieved at the Sub-Picosecond Photon Source (SPPS) [43]. SPPS was a test facility that operated at the Stanford Linear Accelerator Center (SLAC) before the  
Figure 4: **Results from the above table-top fs-X-ray plasma source. ** (**a**) Radially integrated and normalized diffracted intensity I(20)/I\({}_{0}\)(0 2 2) from [Fe(bpy)\({}_{3}\)]\({}^{2+}\)(PF\({}_{6}\)\({}^{-}\))\({}_{2}\) in powder form. The inset shows a diffraction pattern recorded after an integration time of 8 h. The data have been normalized to the intensity of the (0 2 2)-Bragg reflection without photoexcitation and the asterisks mark the peaks originating from non-equivalent sets of lattice planes. (**b–f**) Temporal evolution of normalized changes of Bragg diffraction intensities as a function of scattering angle (**b**) and selected Bragg reflections (**c–f**). The red lines are continuous B-splines of the data. (**g–h**) Contour plots of 2D projections (\(\Delta\)P\({}_{\text{Pe}_{\text{u}}}\) (u, v, t = +250 fs)) obtained by integrating the changes in electron density (\(\Delta\)e(r, \(t\) = +250 fs)) along the w-direction. The dashed lines indicate the contours of stationary electron density corresponding to (**g**) the six fluorine atoms and the phosphorus atom in the middle, and (**h**) the Fe atom, and one bipyridine unit. Please note that \(\eta\) denotes the fraction of unit cells undergoing photoinduced changes. Figure adapted from Reference [39] with permission for AIP Publishing LLC. Linac Coherent Light Source (LCLS) became available in 2009 (see section below). In addition to the above, fs-mid-IR drivers have been recently applied for the generation of coherent ultrahigh harmonics in gases reaching keV-photon energies [44; 45]. Although high harmonic generation (HHG) is not yet an alternative for the production of fs-hard X-ray pulses for coherent diffraction, HHG sources with photon frequencies between the vacuum UV and soft X-ray regime have found applications in fs-spectroscopies and attosecond (att, 1 att = 10\({}^{-18}\) s) metrologies [44; 45; 46; 47; 48; 49; 50; 51; 52; 53; 54; 55; 56; 57; 58; 59; 60; 61; 62; 63; 64; 65].