Fig. 1 |. X-ray free-electron laser scheme and experimental design for spectroscopy and diffraction and/or scattering experiments of metalloenzymes and molecular catalysts.

a | Coherent X-rays are generated using relativistic electrons from a linear accelerator propagating through a periodic array of magnets (undulator). The transverse undulating motion of the electrons in the magnetic field gives rise to (spontaneous) X-ray emission. Over a long propagation distance (~100 m), the X-ray field causes microbunching of the electrons at the X-ray wavelength, which, in turn, leads to stronger coherent emission, further microbunching and exponential growth of the coherent X-ray emission. At saturation, the X-ray pulses emitted from this self-amplified spontaneous emission (SASE) process have a relative bandwidth ΔE/E₀ ≈ 0.2%, with a pulse duration of a few to several tens of femtoseconds (TABLE 1). Diagnostics and beam manipulation provide ways to characterize and change properties of the X-ray free-electron laser (XFEL) beam, such as intensity or photon flux, beam position or pointing, X-ray spectrum, polarization, repetition rate, pulse duration and arrival time. (Not all these properties can be easily changed at every beamline at all XFELs.) X-rays from the undulator are conditioned by beamline optics, which typically include a monochromator (based on ruled gratings or Bragg crystals) and focusing mirrors. Narrower bandwidth (and higher spectral brightness) is achievable using self-seeding, whereby monochromatization is done upstream (between undulator segments), with further amplification in subsequent undulator segments. Samples are introduced at the focus of the X-ray beam, for instance, by using a liquid injector to replace the sample volume at the repetition rate of the X-ray pulses. Numerous spectroscopy techniques are based on the detection of fluorescent X-rays, including fluorescence-detected X-ray absorption spectroscopy (XAS), X-ray emission spectroscopy (XES) and resonant inelastic X-ray scattering (RIXS). X-rays are collected (in the direction orthogonal to the beam propagation) and analysed by a spectrometer consisting of imaging optics and energy-selective elements (Bragg crystals for hard X-rays or dispersive ruled gratings for soft X-rays). b | Schematic of the XES and XAS spectral region (for a Mn compound¹⁷⁰) showing the complementarity of the methods (left panel) and schematic of the RIXS spectrum (right panel). VtC, valence to core. Part a adapted with permission from REF.²⁰. Part b adapted with permission from REF.¹⁷⁰.