Journal Article European XFEL FEL Heisenberg RIXS Kirkpatrick–Baez mirror system RIXS Research Support, Non-U.S. Gov't Research Support, U.S. Gov't, Non-P.H.S. SASE Wigner X-ray Raman scattering X-ray resonant diffraction charge order duration free-electron laser free-electron lasers hRIXS microfocus photochemistry plane-grating monochromator beamline resonant X-ray diffraction resonant inelastic X-ray scattering soft X-rays spectrogram spin dynamics spin order time-resolved spectroscopy transient absorption soft X-ray spectroscopy