## 4 Experimental Stations
### Experimental Station Alvra  
The layout of the Experimental Station Alvra (ESA) hutch is shown in Figure 13. Alvra is focussed primarily on two techniques: X-ray spectroscopy [18; 130] and Serial Femtosecond Crystallography (SFX) [131; 132]. X-ray absorption spectroscopy (XAS) involves measuring the X-ray transmission or X-ray fluorescence of a sample as a function of incident monochromatic X-ray energy [16]. These measurements can provide information on the local electronic and geometric structure around the absorbing atom [130] and can be applied to ordered or disordered samples in almost any form, including species in solution or solid state samples. SwissFEL will be particularly suited for these experimental techniques due to its variable-gap undulators [133; 134] (see Section 2.4) which can easily scan the X-ray energy of the XFEL over a very wide range, allowing techniques such as X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) to be used [130]. SFX [19] is an XFEL technique that has been developed for protein crystallography where a stream of micrometer-sized crystals is delivered into the focussed X-ray beam using a range of different injector techniques [135; 136; 137], and the diffraction pattern from each crystal is recorded on a large two-dimensional pixel detector. Though the intense XFEL pulse destroys the crystal, because an ultrashort X-ray pulse (\(<\)50 fs) is used the diffraction occurs before the atoms have time to move from their lattice positions [22; 138]. SFX can resolve room-temperature protein structures to better than 2 A resolution on very small crystals [19; 131], which expands the technique to include samples that are difficult to crystallize, such as membrane proteins [139] and 2D protein crystals [140; 141]. A complementary technique with similar technical requirements to SFX that will also be possible at Alvra is wide-angle X-ray scattering (WAXS, also called X-ray diffuse scattering or XDS), where the sample under investigation is a liquid solution [142]. The result of the X-ray scattering measurement is powder-diffraction-like rings, containing information on the pair-distribution function of the sample [143; 144]. This technique has proven particularly useful in measuring large-scale, light-activated functional protein motions [145; 146; 147; 148]. ESA has the additional capability of performing X-ray emission spectroscopy (XES) which uses X-ray diffraction from an analyzer crystal to measure the scattered or fluorescence X-ray photons with high energy resolution [149]. ESA uses short focal length crystals (25 cm) in a dispersive von Hamos geometry [150] to measure a range of XES energies in a single measurement. This spectrometer can also be used for a variety of other scattering measurements include off-resonant techniques [151; 152; 153; 154] and inelastic X-ray scattering (IXS) [155].  
Figure 12: The experimental areas of SwissFEL. The accelerator is to the left of the above area, with the X-rays travelling from left to right.  
The ability to measure both X-ray scattering and spectroscopy simultaneously has proven to be a powerful combination for resolving structural and electronic dynamics in both molecules and proteins [115; 156; 157; 158].