Journal Article Research Support, Non-U.S. Gov't Elettra synchrotron FEL diagnostics FERMI free-electron laser X-ray absorption spectroscopy coherent diffraction imaging cross correlation extreme ultra-violet four-wave mixing four-wave-mixing free-electron laser free-electron lasers high-vacuum environments holography liquid flat jets non-linear optics pump–probe scattering time-resolved experiments timing transient grating ultrafast dynamics warm dense matter