Journal Article Research Support, Non-U.S. Gov't free-electron laser Elettra synchrotron FEL diagnostics FERMI free-electron laser X-ray absorption spectroscopy XUV pulse duration diagnostic beam diagnosis coherent diffraction imaging cross correlation detector diffraction limit extreme ultra-violet four-wave mixing four-wave-mixing free-electron lasers high-vacuum environments holography laser slicing liquid flat jets non-linear optics phosphors pixelated phosphor detector pump–probe radiation background scattering storage ring synchrotron light source time-resolved experiments