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Electron-ion collisional dynamics in dense plasma is important for studying plasma transport properties, nonequilibrium plasma evolution, and electron damage in diffraction imaging applications using bright X-ray free-electron lasers (FELs). We participated in the first direct measurements of ultrafast electron-ion collisional dynamics using resonant core-hole spectroscopy in a solid-density magnesium plasma, created and diagnosed with a focused X-ray FEL beam of the Linac Coherent Light Source operated at SLAC, Menlo Park, California, USA.