# Recent Advances in Ultrafast Structural Techniques
## 2 Structural Techniques Involving Ultrashort X-ray Pulses
### Laser Slicing at Third-Generation Synchrotron Facilities
Laser slicing enabled the generation of fs-X-ray pulses in synchrotrons at the expense of lowering the X-ray beam flux to about 10\({}^{2}\)-10\({}^{3}\) photons/pulse, which is comparable to the effective brightness of a fs-hard X-ray plasma source (see below). Figure 1: **Schematic of the fs-laser slicing method for generating fs-hard X-ray pulses at ALS. ** Laser interaction with the electron bunch in a resonantly tuned wiggler introduces an energy modulation. The modulated electrons are spatially separated by a dispersive bend. Thus, subsequently produced fs-X-ray pulses will follow a slightly different path and can be selected for probing. Reproduced from Reference [15] with permission from Science AAAS. In addition to laser slicing, the operation of synchrotrons in the low alpha mode has also allowed the production of low-intensity X-ray pulses as short as 0.6 ps (rms, root mean square) [24]. There is a legitimate need for attaining ultrashort as well as ultrashright hard X-ray and electron pulses. Generally speaking, excitation fluences must be sufficiently high to transform at least 1% of the molecules (or unit cells) in order to discern structural changes from background signals and a very limited number of materials are able to survive multiple excitation cycles; metals such (Al, Au,  
Figure 2: **Results from FEMTO line (laser slicing) at the Swiss Light Source (SLS). ****(a)** Time evolution of the normalized SL diffraction peak intensity at different excitation fluences (F). The horizontal dashed line in the higher fluence scan represents saturation background. Solid lines in red are fits obtained by implementing an equation of motion derived from a double-well potential that includes a phenomenological damping term. They grey vertical line denotes the time at which the SL diffraction intensity recovers and peaks, \(t\) = 0.35 ps. **(b)** Double-well potential; the grey potential corresponds to the unperturbed charge-density wave (CDW) state (\(\eta\) = 0). Note that \(\eta\) is a scaled excitation parameter proportional to F so that \(\eta\) = 1 corresponds to the threshold value for overshooting, i.e., where the double-well shape of the potential vanishes (red curve). The green circle represents the collective atomic displacement of the SL distortion or dimmers along the structural coordinate of the Peierls distortion. The transient quasi-equilibrium position after excitation is labeled Xeq. **(c)** Schematic of the inhomogeneously excited crystal in the case of a high fluence excitation that leads to \(\eta_{0}>1\), the \(z\) axis corresponds to the axis perpendicular to the surface plane; \(\delta_{L}\) corresponds to the laser penetration depth. **(d)** Simulated diffraction signals of a thin layer for certain values of \(\eta\). Reproduced from Reference [20] with permission from the American Physical Society (APS). etc. ), semiconductors (Si, GaAs, InSb), ceramics and ceramic-like materials (Al\({}_{2}\)O\({}_{3}\), ZnO, YBCO, etc.) are some typical examples. However, radiation labile specimens such as organic compounds and biomolecular systems only allow for a very limited number of pump shots before showing clear signs of degradation. At such excitation intensities, thermal effects, long-lived photochemical intermediates and photoproducts alter the specimen structure.