# The DREAM Endstation at the Linac Coherent Light Source
## 1 Introduction  
Ion and electron time-of-flight spectroscopy has enabled photoionization physics and the chemical dynamics of quantum systems by evolving to provide capabilities such as high energy resolution, high angular resolution of charged particles, and order-of-magnitude higher collection efficiency in the past few decades. Furthermore, advanced analysis through multi-particle covariance and coincidence methods of experimental data has provided insights into light-matter interactions across a broad excitation wavelength ranging from the infrared to X-ray photoionization regime. One such particular technique for coincidence measurements is the so-called cold-target recoil ion momentum spectroscopy (COLTRIMS) [1, 2]. COLTRIMS is a state-of-the-art technique that is capable of reconstructing the momentum representation of the ions, electrons, atoms and molecules undergoing photon impact reactions. Thus, this is a powerful tool for following photoinduced reaction dynamics in the time domain for various photon-interacting systems. Another experimental technique based on coincidence measurement is Coulomb Explosion Imaging (CEI) [3].  
Initially inspired by high-energy and collision physics, CEI enabled the study of molecular fragmentation pathways through further development of this experimental technique over time [4; 5; 6]. Recently, CEI and coincidence techniques found widespread application in the atomic physics community through the development of reaction microscopy technique enabled by COLTRIMS. One of the most significant advantages of a reaction microscope is the possibility of capturing all constituent ions in a photofragmentation process, including hydrogen atoms/ions. This is crucial for investigations of ultrafast photochemical dynamics involving multiple channels with various structural changes. Many pump-probe spectroscopic methods have investigated ultrafast photochemical reactions in the gas phase. Most of these spectroscopic methods are significantly more sensitive to ultrafast changes in a molecule's electronic structure than its structural dynamics. However, the correlated nature of electronic and nuclear motion during non-adiabatic dynamics requires an equally detailed investigation of the structural dynamics. Novel femtosecond time-resolved imaging techniques enabled by ultrashort X-ray free-electron laser (XFEL) [7; 8] and electron sources [9; 10] have demonstrated the substantial benefit of direct measurement of the nuclear geometry evolution during non-adiabatic processes.  
As mentioned, COLTRIMS and CEI are both equally capable of detecting light and heavy atoms involved in photoionization processes, providing information in momentum space in a molecular frame of reference. This differentiates COLTRIMS from other gas-phase X-ray or electron diffraction techniques in terms of spatial sensitivity to target size. Therefore, sensitive methods such as coincidence measurements can be crucial for investigations of ultrafast photochemical dynamics involving multiple channels with diverse structural changes [11; 12; 13; 14; 15; 16]. To acquire sufficient statistics, coincidence methods require the detection of a large number of events. This requirement is increasingly met by high-repetition-rate optical laser systems and the next generation of XFELs, such as LCLS-II. Here, we present the design of the Dynamic Reaction Microscope (DREAM) endstation at the TMO instrument [17] located at LCLS-II, the 1 MHz repetition rate superconducting FEL at SLAC.