# A Reaction Microscope for AMO Science at Shanghai Soft X-ray Free-Electron Laser Facility
## 1 Introduction  
A reaction microscope (REMI), or cold-target recoil-ion momentum spectroscopy (COLTRIMS), is/uses a device designed to measure the momenta of charged reaction fragments in coincidence [1; 2; 3; 4]. In 1987, the first system of spectrometers to detect recoil ions using static gas as a target, at room temperature, was built by [PERSON] et al. [5]. A uniform magnetic field generated by a pair of Helmholtz coils, having been introduced a few years later, measures electrons with a 4\(\pi\) solid angle [6; 7]. Since then, kinematically complete experiments have become possible and the continuous development of supersonic gas jet technology [8; 9; 10] and detector technology, based on a micro-channel plate and delay-line anode, has greatly improved momentum resolution [11; 12; 13; 14]. After development of more than three decades, this technology has become mature and is presently widespread. By measuring the energies and emission angles of charged products with high precision, detailed information can be obtained for deeper investigation of atomic and molecular reactions with ions, electrons or photons. In the past 10 years, REMI/COLTRIMS has been employed for a tremendous amount of studies in synchrotron radiation. On the other hand, with the rapid innovation of laser technology, especially chirped pulse amplification technology [15], ultra-short laser pulses at high intensities have provided extraordinary experimental conditions for the study of atomic, molecular and optical (AMO) physics. Through the combination of reactionmicroscopes and laser devices [16; 17; 18], many physical phenomena of the reactions of gas targets with photons have been explored, such as multi-photon ionization, above-threshold ionization, tunneling ionization, high harmonic generation (HHG) and so on. As a new generation of coherent light source, X-ray free-electron lasers (FEL) have the characteristics of high power, a wide and continuously tunable wavelength, short pulses and good coherence, serving a number of applications in various areas of science [19; 20]. In AMO science, the most far-reaching applications are manifested in the topics of nonlinearity induced by single- or many-photon absorption, and the ultrafast molecular dynamics and structural imaging of a single molecule in the regime of UV-to-hard-X-ray photons [21; 22; 23; 24; 25]. Few-photon induced atomic and molecular dynamics [26; 27; 28; 29; 30] and isomerization [31; 32] have been studied in detail. Some exotic physical phenomena, such as the \"hollow atoms\" [33] and the \"molecular black hole\" [34] have been observed recently. The former is formed by quickly ejecting a few inner-shell electrons, resulting in the phenomenon of \"transparency\" to X-rays [35]. In the latter case, the X-ray pulse first strips the number of electrons from a heavy atom, due to its huge cross section, and then a highly positive resulting charge traps electrons from neighboring atoms within the molecule. X-ray lasers can also ionize a number of electrons from one heavy atom within a large biomolecule, which leads to the rapid redistribution of electrical charge in a tiny time window and provides rich information for analyzing biomolecules using X-ray lasers. Further, some basic physical processes of atoms and molecules have been reached using X-ray lasers with the control of wavelength, phase and intensity [36; 37]. Moreover, the circularly polarized light generated in free-electron lasers is used to study the circular dichroism of atoms, linear molecules and chiral molecules [38; 39; 40]. More recently, experimental studies were extended to the photoelectron diffraction imaging in a molecular breakup frame using an X-ray FEL [41]. The construction of the Shanghai Soft X-ray Free-Electron Laser Facility (SXFEL) began in 2014. Two beamlines, including the seeded line and SASE line, together with five end stations, including cell imaging, AMO, ultrafast physics and surface chemistry, are expected to be put into user operation in 2022 [42].