We report on the design and capabilities of a reaction microscope (REMI) end-station at the Shanghai Soft X-ray Free-Electron Laser Facility (SXFEL). This apparatus allows high-resolution and 4π solid-angle coincidence detection of ions and electrons. The components of REMI, including a supersonic gas injection system, spectrometer, detectors and data acquisition system, are described in detail. By measuring the time of flight and the impact positions of ions and electrons on the corresponding detectors, three-dimensional momentum vectors can be reconstructed to study specific reaction processes. Momentum resolutions of ions and electrons with 0.11 a.u. are achieved, which have been measured from a single ionization experiment of oxygen molecules in an infrared (IR), femtosecond laser field, under vacuum at 1.2×10−10 torr, in a reaction chamber. As a demonstration, a Coulomb explosion experiment of oxygen molecules in the IR field is presented. These results demonstrate the performance of this setup, which provides a basic tool for the study of atomic and molecular reactions at SXFEL.

The photoion–photoion coincidence (PIPICO) is a simple and effective approach for the selection of correlated fragments in a specific dissociating channel in molecules. We propose here a charge-encoded multi-photoion coincidence (cMUPICO) method, in analogy to traditional PIPICO, however in which the charge of individual fragments is taken into account. The cMUPICO method allows for clearly displaying coincident channels for dissociation channels containing three more fragments with unequal charge states, invisible in the traditional PIPICO. As a demonstration, three-body fragmentation dynamics of CO2 in strong IR laser fields is analyzed, and 11 dissociation channels are effectively identified, five of which are first found with cMUPICO. The present results show that cMUPICO is a powerful and practical tool for distinguishing various dissociation channels with multiply charged multi-photoions.

Multi-ionization and subsequent Coulomb explosion (CE) of the N2O molecule irradiated by linearly polarized 800 nm laser field is investigated by a reaction microscope, where a number of CE channels of N2Oq+ with q{less than or equal to}5 for two-body fragmentation and q{less than or equal to}8 for three-body fragmentation were observed. For two-body CE, by analyzing the internuclear separations extracted from kinetic energy releases (KERs), dissociation branching fractions, and laser intensity dependence, interestingly we found that fragmentation N2O5+→N3++NO2+ is produced directly from dissociating N2O3+ via non-sequential stairstep ionization whereas most of others result from the sequential stairstep ionization. For three-body CE, 25 fragmentation channels of N2Oq+ (q = 3-8) are distinguished in present charge-encoded multi-photoion coincidence plot and the concerted fragmentation mechanism is nominated in a typical Dalitz plot. With the help of the numerical computation with the measured KERs and momentum correlation angles, the geometric structures of molecular ions prior to fragmentation are reconstructed, which display the bending motion and simultaneous two-bond stretching before the CE. Increasing of bond length for high charged N2Oq+ indicates the dominating stairstep ionization in three-body fragmentation.