The recent progress in the development of X-ray free electron lasers (XFELs) allows for\nthe delivery of over 10^11 high-energy photons to solid-density samples in a femtosecond time\nscale. The corresponding peak brightness of XFEL induces a nonlinear response of matter in a\nshort-wavelength regime. The absorption of an XFEL pulse in a solid also results in the creation\nof high energy density (HED) matter. The electronic structure and related fundamental properties\nof such HED matter can be investigated with the control of XFEL and various X-ray spectroscopic\ntechniques. These experimental data provide unique opportunities to benchmark theories and\nmodels for extreme conditions and to guide further advances. In this article, the current progress in\nspectroscopic studies on intense XFEL-matter interactions and HED matter are reviewed, and future\nresearch opportunities are discussed.
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