Free-electron x-ray laser measurements of collisional-damped plasmons in isochorically heated warm dense matter

Collisional-damped plasmons were obtained in highly-spectrally resolved measurements of isochorically heated solid aluminum providing for the first time the determination of electron-electron and strong electron-ion collisions. $8$~keV x-rays from the Linac Coherent Light Source have been focused to micrometer diameter focal spots heating solid aluminum foils to temperatures up to $7$~eV. The forward scattering spectra show plasmons that directly determine the density and temperature from the plasmon frequency shift and from the detailed balance relation indicating a warm dense matter state with large ion coupling parameters of up to $40$. These experiments show a non-quadratic plasmon dispersion relation shifted to lower plasmon energy shifts at wave numbers of $0.5$~{\AA} $ < k < 2$~{\AA} not predictable by dispersion relations like the Gross-Bohm relation and the Born-Mermin approximation. Therefore, the measured collisional-damped plasmons indicate collisions beyond the Born approximation and have been described successfully by taking into to account electron-electron collisions as well as strong electron-ion collisions. In this talk, we will discuss the first measurement of strong collisions affecting strong collisions in the electron-ion collision model and conductivity.