Journal Article Research Support, Non-U.S. Gov't XUV Raman spectrometer free-electron laser CDA CuA site DFT Kirkpatrick–Baez mirror system NOM Research Support, U.S. Gov't, Non-P.H.S. TD-DFT X-ray optics alignment copper thiolate complexes core effect density functional theory interferometry ligand-free Au nanoclusters metrology for synchrotron optics microfocus origin of Au nanocluster fluorescence plane-grating monochromator beamline ray tracing reflectometry surface charge surface effect ultrasmall gold nanoparticles wavefront sensor