# Atomic, Molecular and Cluster Science with the Reaction Microscope Endstation at FLASH2
## 1 Introduction  
The invention of intense femtosecond infrared (IR) lasers [1] led to a new era in atomic and molecular physics, namely the devolopment of femtochemistry [2]. Today it is often mentioned as one part of a much larger multi-disciplinary field called ultra-fast physics [3]; the observation of light-induced dynamics in small quantum systems on their natural time scale in the range of femto- or even attoseconds. In many cases a first \"pump\" pulse triggers certain dynamics, while a delayed \"probe\" pulse is used to interrogate the system at a later point in time. This way, the temporal evolution of any specific process under investigation like molecular break-up [4], charge migration [5, 6] or isomerization [7] within a molecule, to just name a few, can be observed on their natural time scale by using lasers with correspondingly short pulse lengths in the range of a few femtoseconds. A small number of different technologies exists to produce such short laser-like or coherent light pulses. Depending on the specific application each of them comes with characteristic advantages and drawbacks.  
Free-electron lasers (FELs) are large-scale electron-accelerator-based machines. They cover a huge range of photon energies, from the microwave [8] up to the hard X-ray regime [9], with unsurpassed light intensities for photon energies above 10 eV [3, 10]. However, most FELs are operated at comparablylow repetition rates (typically below 1 kHz) and are providing light pulses that are usually only partly coherent. In contrast, conventional femtosecond lasers in the visible or IR-range provide pulses with high repetition rates, high intensities and excellent coherence properties [11; 12; 1]. However, their spectral range is limited to photon energies of a few eV only. This limitation is resolved with high harmonic-generation (HHG) sources [13; 14], where high-intensity femtosecond IR laser pulses are sent into a gaseous target to drive a non-linear light-atom interaction. This leads to the emission of frequency up-converted (higher harmonic) radiation [15] with unique temporal and spectral properties. HHG sources are ideally suited to produce extremely short pulse durations of only few tens of attoseconds [16] with photon energies of up to several hundred eV [17]. However, these sources are limited, due to the low conversion efficiencies, to light intensities that are orders of magnitude lower than those delivered by modern XUV or X-ray free-electron lasers [18].  
For studies of time-dependent processes in small quantum systems it is of great advantage if an experimental station allows high flexibility with respect to the usage of one or a combination of the above mentioned light sources, which are ideally adapted to the given reaction dynamics. With the reaction microscope (REMI) endstation at FLASH2, we implemented this to a large extent. In its present stage, it routinely allows FEL-based pump-probe experiments with atomic, molecular or cluster targets in the gas phase. The experimental station consists of a multi-particle imaging spectrometer for electrons and ions (REMI), the FEL beamline with integrated split-and-delay and focussing optics for FEL pulse-pair creation [19], an IR laser for femtosecond FEL-pump laser-probe experiments, and a laser-driven HHG source for two-color XUV pump-probe measurements. A manuscript about technical details and benchmarks of the REMI endstation can be found in [19]. In contrast, this review provides with information about physical quantities and experimental schemes which are accessible by the REMI endstation. The following manuscript describes in Section 2 the experimental capabilities of the REMI endstation at FLASH2 and a selection of published results (Sections 3.2 and 3.3) as well as unpublished results (Section 3.1) is briefly discussed. Concluding statements are given in Section 4.