A high-energy Laue X-ray emission spectrometer at the FXE instrument at the European XFEL.

The high-energy-resolution X-ray emission spectroscopy (XES) spectrometers available at the Femtosecond X-ray Experiment (FXE) instrument of the European XFEL operate in Bragg (reflective) geometry, with optimum performance in the range between 5 and 15 keV. However, they quickly lose efficiency above around 15 keV due to the decrease in reflectivity of the crystal analyzers at such high photon energies. This hampers high-energy-resolution spectroscopy experiments on heavy elements (e.g. 4d metals), which thus do not fully profit from the high-photon-energy capabilities of the European XFEL. Here we present the design, implementation and performance of a novel high-resolution XES spectrometer operating in Laue (transmission) geometry optimized for measurements at high photon energies (>15 keV). The High-Energy Laue X-ray emIssiOn Spectrometer (HELIOS) operates mainly in dispersive mode by placing the crystal analyzer inside or outside the Rowland circle. The Laue spectrometer performance in terms of energy resolution and efficiency is presented and discussed. Two Laue analyzers, silicon and quartz, have been tested at SuperXAS of the Swiss Light Source and at FXE of the European XFEL. The quartz analyzer was found to be about 2.7 times more efficient than the silicon one. The Laue spectrometer energy resolution (ΔE/E) reached at the FXE instrument is around 1.2 × 10^-4. Depending on different user requests, the resolution can be further increased by using higher diffraction orders. The new Laue spectrometer increases the existing portfolio of XES spectrometers at FXE, enabling efficient implementation of ultrafast X-ray spectroscopies with high energy resolution at photon energies above 15 keV. This spectrometer will allow the expansion of studies in the field of ultrafast sciences, particularly including investigation of 4d elements using hard X-rays.