25 Octobre – Thesis defense - Noémie Jourdain

14 h Amphi Bécquerel - Ecole Polytechnique (Palaiseau)

Properties of copper under extreme conditions and at thermal out-of-equilibrium.

Ultrashort laser sources development enables nowadays the possibility for matter to reach both extreme pressure and temperature (~ 10 000 K)  onditions, or what we call "Warm Dense Matter". Working with femtosecond lasers leads to out-of-equilibrium phenomena during which a large amount of energy is deposited in the electrons while the lattice remains cold. We used XANES spectroscopy to follow both the ultrafast evolution of the electronic structure and the local atomic order after the irradiation of such a laser.  Moreover, we can nowadays employ Quantum Molecular Dynamics to simulate Warm Dense Matter. We computed XANES spectra for thermal out-of-equilibrium situations (Te ≠ Ti) and in thermodynamic con gurations similar to the experimental ones. The confrontation of experimental data and these calculations brings a deep understanding of the phenomena involved and their evolutions. The present study deals with the X-ray absorption near copper L3 and L2 edges (respectively 932 and 952 eV). Calculations show a pre-edge structure in the spectra which evolution gives the electronic temperature dynamics. These simulations also suggest that the loss of the crystalline order should give rise to the disappearance of the post-edge structures. Several experiments have been realized using Eclipse laser and a table-top station dedicated to time-resolved XANES measurements at CELIA laboratory. At first, some XANES spectra have been acquired using a X-ray source produced by the irradiation of a CsI solid target. This source duration of ~ 10 ps rms (approximately the thermal equilibration timescale for copper) restrained our temporal resolution. We then used a xenon clusters gas jet to produce a X-ray source of comparable emissivity but a signi cantly shorter duration (~ 1.5 ps rms). Numerous out-of-equilibrium XANES spectra have been acquired and for di erent excitation degrees. The excellent signal-to-noise ratio allows us to follow the evolution of the post-edge structure and deduce the associated dynamics of the loss of crystalline structure. Finally, we performed the same experiment with a X-ray source coming from the betatron radiation at LOA laboratory. With this source, a temporal resolution of ~ 100 fs was achieved and allows us to fully characterize the instantaneous heating of the electrons by the laser and far-from-equilibrium states.

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