# Recent Advances in Ultrafast Structural Techniques
## 2 Structural Techniques Involving Ultrashort X-ray Pulses  
### Laser Slicing at Third-Generation Synchrotron Facilities  
Synchrotron radiation continues to be one of the most powerful sources of polychromatic light capable of covering a wide spectral range that extends from THz to hard X-rays [12]. Depending on the facility, type of insertion device, beam optics, and required degree of monochromaticity; X-ray fluxes at synchrotrons typically range from about 10\({}^{4}\) to 10\({}^{8}\) photons/pulse (at the sample position). Moreover, photon energies \(>\)6 keV (photon wavelengths \(<\)2.1 A) are often required in order to attain atomic spatial resolution. The temporal resolution of synchrotron sources is dictated by the length (duration) of the stored electron bunch, which commonly ranges from 50 ps to 200 ps (fwhm, full width at half maximum) [13,14]. Laser slicing techniques [15, 16, 17, 18, 19, 20, 21, 22] enabled the production of \(\sim\)100 fs (fwhm) hard X-ray pulses at third-generation synchrotron facilities such as the Advanced Light Source (ALS) and the Swiss Light Source (SLS). Figure 1 illustrates the laser slicing principle originally proposed by Zholents and Zolotorev [16] and demonstrated by Schoenlein et al. [15] at ALS. Here, an intense fs-laser pulse with a field strength of \(\sim\)10\({}^{9}\) V m\({}^{-1}\) co-propagates with the stored electron bunch through a wiggler. Under optimal resonant conditions, the high laser peak intensity introduces an energy modulation in the underlying electrons of several MeV. This energy modulation is much larger than the energy spread of the unperturbed electrons in the bunch and, therefore, energy-modulated electrons can be transversely separated from the main beam through the implementation of a bending magnet, and subsequently used to produce fs-X-ray pulses in another bend. Finally, an aperture is applied to select one of the spatially separated fs-X-ray pulses, which then can be used for probing the ultrafaststructural changes. Note that timing jitter here is negligible since the electron beam modulation (or fs-X-ray pulse) and the photoexcitation pulse originate from the same initial laser pulse via beam splitters. All pulse durations below are given in fwhm if not stated otherwise. Figure 2 shows recent results obtained by Huber et al. [20] at the FEMTO slicing facility of SLS for the prototypical Peierls 1D charge-density wave (CDW) system, K\({}_{0.3}\)MoO\({}_{3}\). The amplitudon in this material has a frequency (period) of 1.7 THz (590 fs) [23] at T \(\sim\) 0 K. Fs-X-ray diffraction experiments were performed in a grazing incidence geometry while the K\({}_{0.3}\)MoO\({}_{3}\) single crystalline sample was maintained well below T\({}_{\rm c}\). The crystal was excited by 100-fs laser pulses centered at \(\lambda\) = 800 nm and probed by 80-fs X-ray pulses with \(\lambda\) = 1.8 A. The relative intensity changes of the \(\left(1\,3.252\,\,\overline{0.5}\right)\)-reflection of the superlattice lattice (SL) were measured as a function of t (delay) at various photoexcitation fluences (Figure 2a). Results were modeled (Figure 2d) by implementing an equation of motion derived from a double-well potential (Figure 2b). Huber et al. [20] were able to observe the coherent lattice dynamics associated with the CDW formation. The authors interpreted the results as a rearrangement (atomic motion) of the SL distortion evolving along a generalized excitation-dependent potential energy surface. Their observed recovery of the SL distortion remains unclear. The results presented in Figure 2 reflect typical measurements that should be performed in order to extract valuable physical information from time-dependent diffraction data; i.e., many time points as well as different photoexcitation levels are usually required. Thus, depending on the probed volume, diffraction efficiency, and X-ray beam brightness, several weeks of continuous data collection may be necessary. In addition, sample degradation may introduce additional challenges during extensive data acquisition.