Abstract
The European X-ray Free-Electron Laser (EuXFEL) delivers extremely intense (>1012 photons pulse-1 and up to 27000 pulses s-1), ultrashort (<100 fs) and transversely coherent X-ray radiation, at a repetition rate of up to 4.5 MHz. Its unique X-ray beam parameters enable novel and groundbreaking experiments in ultrafast photochemistry and material sciences at the Femtosecond X-ray Experiments (FXE) scientific instrument. This paper provides an overview of the currently implemented experimental baseline instrumentation and its performance during the commissioning phase, and a preview of planned improvements. FXE's versatile instrumentation combines the simultaneous application of forward X-ray scattering and X-ray spectroscopy techniques with femtosecond time resolution. These methods will eventually permit exploitation of wide-angle X-ray scattering studies and X-ray emission spectroscopy, along with X-ray absorption spectroscopy, including resonant inelastic X-ray scattering and X-ray Raman scattering. A suite of ultrafast optical lasers throughout the UV-visible and near-IR ranges (extending up to mid-IR in the near future) with pulse length down to 15 fs, synchronized to the X-ray source, serve to initiate dynamic changes in the sample. Time-delayed hard X-ray pulses in the 5-20 keV range are used to probe the ensuing dynamic processes using the suite of X-ray probe tools. FXE is equipped with a primary monochromator, a primary and secondary single-shot spectrometer, and a timing tool to correct the residual timing jitter between laser and X-ray pulses.
Highlights
More than three decades ago coherent molecular dynamics were observed in ‘real time’ on a picosecond timescale exploiting short-pulse lasers (Lambert et al, 1981)
Similar to other instruments focused on timeresolved studies at existing X-ray free-electron lasers (FELs) (XFEL) sources (Chergui & Collet, 2017; Alonso-Mori et al, 2015; Abela et al, 2017; Chollet et al, 2015), Femtosecond X-ray Experiments (FXE) uses ultrafast optical laser pulses to generate transient states while the X-ray pulses probe their dynamical evolution
In comparison with similar time-resolved end-stations at other XFEL facilities, the main differences in instrumentation concern the materials and concepts used for design of the beamline components, which have to withstand the high heat load induced by the intense X-ray beam
Summary
More than three decades ago coherent molecular dynamics were observed in ‘real time’ on a picosecond timescale exploiting short-pulse lasers (Lambert et al, 1981). Relevant for the scientific scope of the Femtosecond X-ray Experiments (FXE) instrument are studies focused on tracking correlated electronic and nuclear motion in a strongly nonadiabatic regime during the formation and breaking of chemical bonds, which aim at filming the nuclear motions during chemical reactions with atomic spatial and temporal resolution while tracking the excited-state dynamics of reacting molecules (Mara et al, 2017; Miller et al, 2017; Lemke et al, 2017; Zhang et al, 2014, 2017; van Driel et al, 2016; Shelby et al, 2016; Biasin et al, 2016; Pande et al, 2016; Barends et al, 2015; Kern et al, 2015; Zhang & Gaffney, 2015; Kim et al, 2015; Canton et al, 2015; Levantino et al, 2015; Arnlund et al, 2014). The scientific instrument FXE is designed to advance into this uncharted territory, utilizing hard X-rays to make fs-resolved ‘molecular movies’ of ensuing nuclear and electronic dynamics while exploiting the facility’s uniquely high average flux (Bressler et al, 2012)
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