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) spectrometersavailable at the Femtosecond X-ray Experiment (FXE) instrument of theEuropean XFEL operate in Bragg (reflective) geometry, with optimumperformance in the range between 5 and 15 keV. However, they quickly loseefficiency above around 15 keV due to the decrease in reflectivity of the crystalanalyzers at such high photon energies. This hampers high-energy-resolutionspectroscopy experiments on heavy elements (e.g. 4d metals), which thus do notfully 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 opti-mized for measurements at high photon energies (> 15 keV). The High-EnergyLaue X-ray emIssiOn Spectrometer (HELIOS) operates mainly in dispersivemode by placing the crystal analyzer inside or outside the Rowland circle. TheLaue spectrometer performance in terms of energy resolution and efficiency ispresented and discussed. Two Laue analyzers, silicon and quartz, have beentested at SuperXAS of the Swiss Light Source and at FXE of the EuropeanXFEL. The quartz analyzer was found to be about 2.7 times more efficient thanthe silicon one. The Laue spectrometer energy resolution (�E/E) reached at theFXE instrument is around 1.2 � 10 4. Depending on different user requests, theresolution can be further increased by using higher diffraction orders. The newLaue spectrometer increases the existing portfolio of XES spectrometers atFXE, enabling efficient implementation of ultrafast X-ray spectroscopies withhigh energy resolution at photon energies above 15 keV. This spectrometer willallow the expansion of studies in the field of ultrafast sciences, particularlyincluding investigation of 4d elements using hard X-rays.