## 7 The SQS Instrument
### SQS Instrumentation and Capabilities  
The optical layout of the beam transport system enables experiments using the direct beam from the variable gap SASE3 undulator or the reduced bandwidth radiation (\(\Delta\mathrm{E/E}\leq 10^{-4}\)) from the soft X-ray monochromator. A Kirkpatrick-Baez adaptive mirror system assures a tight focusing of the beam down to spot sizes as small as about 1 micron (see Figure 6). The bendable high-polished mirrors allow adjustments of the focal spot size and displacement of the focus to three different interaction regions separated by 39 and 200 cm, respectively. The FEL radiation properties, such as pulse energy, pulse duration, arrival time, spectral distribution and focal spot size, are monitored with the help of several diagnostic devices installed downstream and upstream of the interaction regions inside the dedicated and enclosed experiment area. Figure 6: Schematic outline of the SQS scientific instrument at the SASE3 FEL. Shown are major instrumentation items installed in the photon beam transport tunnel and experiment hutch comprising the beam transport, focusing and diagnostic devices as well as the three interchangeable experimental vacuum chambers AQS, NQS and SQS-REMI. The general concept of the instrument is based on a two-chamber system thus separating applications on \"Atomic-like Quantum Systems\" (AQS), such as free atoms, atomic ions, and small molecules, and on \"Nano-size Quantum Systems\" (NQS), such as clusters, nanoparticles and large biomolecules, all typically larger objects [79]. The AQS chamber will be equipped with a set of spectrometers enabling the analysis of electrons, ions and photons with high-energy resolution and the determination of the angular distribution of the particles. Six electron time-of-flight (TOF) analyzers can be used for angle-resolved high kinetic energy resolution experiments at distinct angles in the dipole and in the non-dipole planes. A velocity-map-imaging (VMI) spectrometer provides the full information about the angular distribution of the emitted electrons and ions, and is designed for electrons up to about 1000 eV kinetic energy. The single pulse analysis of very dilute samples or of processes characterized by extremely low cross sections is possible by means of a magnetic bottle electron spectrometer, which collects electrons over the full solid angle. Finally, a specially designed 1D-imaging XUV spectrometer is dedicated to the analysis of fluorescence emission at high spectral resolution. The use of Wolter optics and a 2D-imaging detector is enabling a spatial resolution of about 10 \(\upmu\)m along the beam propagation direction and thereby a temporal resolution of about 30 fs in crossed beam experiments. The NQS chamber will have as particular feature the option to use the DSSC detector [33] in forward diffraction geometry. Due to the high scattering cross sections in the soft X-ray regime, single pulse imaging of larger molecules and particles becomes possible and will be applied to structural analysis at reduced spatial resolution. The DSSC detector is also used in combination with various particle spectrometers (TOF, VMI) to characterize size and shape of clusters and nanoparticles in parallel to the determination of kinetic energies, fragmentation patterns and emission angles of ions and electrons produced in the interaction volume. In addition, a third, specially designed ultra-high vacuum chamber will host a reaction microscope (SQS-REMI) for the complete characterization of molecular fragmentation processes by the application of electron-ion coincidence techniques [80], taking full advantage of the high repetition rate (until 27,000 pulses per second) available at the European XFEL. Using large area position sensitive delay-line detectors and a well-defined arrangement of magnetic and electric fields to extract and guide the electrons and ions, the kinetic energies of all fragments as well as their relative emission angles can be determined in a single molecule ionization event.