# Recent Advances in Ultrafast Structural Techniques
## 2 Structural Techniques Involving Ultrashort X-ray Pulses
### Introduction to Fourth-Generation Fs-Hard X-ray Sources: X-ray Free Electron Lasers
#### 2.4.3 Fs-X-ray Absorption Spectroscopy  
Fs-X-ray absorption spectroscopy (fs-XAS), often in combination with fs-X-ray emission and other relevant spectroscopies, [131] has emerged as a powerful technique to monitor structural dynamics in solution, [132; 133; 134; 135; 136; 137; 138; 139; 140] crystalline phases, [141; 142; 143] and even at surfaces. [144] XAS is element-specific and depending on the element and core-shell involved in the process, X-ray photon energies usually range from 100 eV to 100 keV. There are two main regions in XAS: X-ray photoexcitation below the ionization threshold (IE) gives rise to pre-edge bands that arise from transitions to unoccupied or partially filled orbitals. For photon energies slightly above IE, there is a jump in the absorption cross-section and the quasi-bound photoelectron with low kinetic energy, and therefore high cross-section, undergoes multiple-scattering events with the surrounding atoms. This leads to a strong oscillatory behavior in the X-ray absorption spectrum. This region is denoted as XANES (X-ray absorption near-edge structure) or NEXAFS (near-edge X-ray absorption fine structure). As the kinetic energy of the photoelectron increases, the cross-section decreases and weaker modulations, in the so-called EXAFS (extended X-ray absorption fine structure) region appear as a consequence of single scattering contributions. The surrounding molecular arrangement is encoded in the observed absorption features, and therefore the structural information can be recovered through the application of well-established data analysis protocols. [145; 146]  
Initial pioneering fs-XAS experiments were performed at the femtosecond lines of SLS [133; 134; 143] and ALS [135; 136] via laser slicing (see Section 2.1 above). With the advent of XFELs, the enhancement in signal to noise ratio (SNR) in fs-XAS data is remarkable. The main challenge, however, in the use of XFEL radiation for fs-XAS experiments is the intrinsic spectral instability arising from the stochastic nature of SASE. The rather broad SASE bandwidth (for spectroscopy) requires the use of a monochromator while large variations in the central wavelength results in large shot-to-shot intensity fluctuations of the order of 100%. There are normalization protocols that implement non-invasive shot intensity monitors to account for such large fluctuations. The application of dispersive spectrometers seems to be another suitable detection approach. [147; 148] Figure 10a (red symbols) illustrate results obtained by Gawelda et al. [140] at the micro-XAS synchrotron beam line of SLS. Here, intense 100-fs pump pulses (\(\lambda\) = 400 nm) excite an aqueous solution of [Fe(\(\mathrm{ppy}\))\({}_{3}\)]\({}^{2+}\) while 100-ps tunable monochromatic hard X-ray pulses probe the photoinduced changes by XANES at \(t\) = +50 ps. The differential XANES spectrum is shown in Figure 10b together with fs-XANES measurements (blue stars) performed by Bressler et al. [133] at the FEMTO line of SLS (laser slicing, see Section 2.1) for the same system at \(t\) = + 300 fs utilizing 100-fs hard X-ray pulses. The lack of brightness caused by laser slicing is evident. Recent results obtained by Mara et al. [138] at LCLS (see Figure 10, bottom panel) illustrate the near 10\({}^{10}\) increase in ray flux. They implemented Fe K-edge fs-XANES and fs-XES to directly probe the ligand environment of the iron and its spin state, respectively. Mara et al. [138] were able to unambiguously revealed the geometric and electronic structural dynamics of photoexcited ferrous cytochrome c (cyt c), providing a direct correlation between axial ligand rebinding to the Fe and the temperature decay. The latter enabled experimental quantification of a protein entatic contribution towards the stabilization and control of an active site metal-ligand bond.