Journal Article soft X-rays European XFEL RIXS FEL Heisenberg RIXS Kirkpatrick–Baez mirrors Research Support, Non-U.S. Gov't X-ray Raman scattering X-ray resonant diffraction XAS XES charge order electron-ion-spectrometers free-electron laser free-electron lasers hRIXS imaging detector liquid jet micro-focus photochemistry pump–probe resonant X-ray diffraction resonant inelastic X-ray scattering spin dynamics spin order time-resolved spectroscopy transient absorption soft X-ray spectroscopy two-color X-ray absorption spectroscopy varied line spacing