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Abstract
A femtosecond pump–probe technique, capable of probing transient changes in intense X-ray-excited matter with monochromatic X-ray pulses and accessing data for both positive and negative delay times, is proposed. The application of this technique to single-crystal silicon revealed ultrafast X-ray-induced electron excitation occurring on a timescale of 10 fs, along with the delayed onset of atomic disordering relative to the electronic excitation. This technique will pave the way for atom-specific multidimensional spectroscopy and diagnostics of exotic states of matter created by X-ray irradiation with sophisticated probing techniques, such as single-crystal diffraction, inelastic scattering, X-ray Thomson scattering, and absorption spectroscopy.