New opportunities for studying a matter under extreme conditions using synchrotron radiation and XFEL

The study of matter under extreme conditions is of fundamental importance for condensed matter physics, materials science, and planetary science. At high pressure, temperature, or irradiation, the interplay of structural and electronic properties governs phenomena that are directly relevant to understanding planetary interiors, inertial confinement fusion, and the behavior of functional materials. Large-scale facilities, such as synchrotron radiation sources X-ray Free Electron Lasers (XFELs), provide unique tools to probe these conditions on the atomic scale. However, such experiments remain challenging both in terms of diagnostics and data interpretation, requiring advanced theoretical models and reconstruction techniques. The first part of this thesis focuses on the local structural analysis of solid and liquid gold[1,2]. The study of short-range order in liquid metals has attracted considerable interest since the seminal works of Turnbull[3] and Frank[4] in the 1950s, who first hypothesized the importance of icosahedral short-range order in undercooled liquids. X-ray Absorption Fine Structure (XAFS), combined with Reverse Monte Carlo (RMC) modeling, has since been established as a powerful approach to reconstruct three-dimensional atomic structures in disordered systems such as liquids and glasses. Recent developments of the RMC-GnXAS code [5,6], based on accurate multiplescattering theory with relativistic corrections, now allow for a consistent refinement of experimental Extended x-ray absorption fine structure (EXAFS) and X-ray Diffraction (XRD) data. In this work, I applied RMC-GnXAS to Au at the L3-edge across solid and liquid phases. For the solid phase, both two-body (pair) and three-body (triplet) contributions were included, allowing for a more accurate description of the linear arrangements present in crystalline and quasi-crystalline environments. The combined refinement enabled the reconstruction of pair and triplet distribution functions, as well as bond-angle distributions, confirming the proper functioning of RMC-GnXAS in capturing many-body correlations. In contrast, for the liquid phase, only two-body correlations were considered, since the lack of long-range linear order strongly suppresses the contribution of three-body terms. Here, the analysis of the pair distribution function combined with a commonneighbour analysis [7] revealed that nearly 46% of the local environments correspond to icosahedral or nearly-icosahedral motifs, consistent with the structural behavior observed in other close-packed liquid metals. The second part of this thesis addresses electronic excitations in matter driven by XFEL irradiation. Inelastic X-ray scattering is a key diagnostic of plasmons and collective excitations, but highresolution measurements are hampered by the broad bandwidth of Self-Amplified Spontaneous Emission (SASE) pulses. While seeded or monochromatized XFEL beams have traditionally been used to overcome this limitation, they come at the cost of reduced photon flux. In this study, we applied a reconstruction approach based on the inverse matrix method with Tikhonov regularization [8] to exploit the intrinsic stochastic structure of full SASE spectra. Applying this technique to scattering data from Al and Fe thin films at the HED beamline of the EuXFEL, we improved the energy resolution compared to conventional averaging, leading to a reduction in the apparent FWHM of the elastic peaks. The results demonstrate that SASE-based spectroscopy, when coupled with robust reconstruction algorithms, can provide high-resolution access to collective excitations while retaining the high photon flux necessary to drive matter into extreme states. Together, these two complementary investigations, local structural analysis of Au and plasmon reconstruction in transiently heated metals, establish new methodologies for probing matter under extreme conditions. By combining advanced theoretical modeling, computational refinement, and novel data-reconstruction techniques, this work contributes to a deeper atomistic understanding of both structural and electronic properties in regimes relevant to planetary interiors, astrophysical plasmas, and functional materials design.