Density effect on electronic structure of warm dense matter based on x-ray fluorescence spectroscopy

Warm dense matter (WDM), a state of matter which lies at the frontiers between condensed matter and plasma, is one of the main research objects of high energy density physics (HEDP). Compared to the isolated atom, the electron structure of WDM will change because of the influence of density and temperature effect. Since WDM is always strongly coupled and partially degenerated, the precise theoretical description is very complex and accurate experimental studies are also very challenging. In this paper, a study of the density effect on the warm dense matter electron structure based on the x-ray fluorescence spectroscopy is presented. In the experiment, warm dense titanium with density larger than solid density is created based on a special designed hohlraum. Then, using the characteristic line spectrum emitted by the laser irradiation on pump material (Vanadium) as pump source, the titanium will emit fluorescence. The x-ray fluorescence spectroscopy of titanium with different states is diagnosed by changing the delay time between the pump laser and drive laser. The experimental fluorescence spectrum indicates that the energy difference between Kβ and Kα (Kβ-Kα) of the compressed titanium (7.2~9.2 g/cm³, 1.6~2.4 eV) is about 2 eV smaller than that of cold titanium. Two theoretical methods, finite-temperature relativistic density functional theory (FTRDFT) and two-step Hartree-Fock-Slater (TSHFS), are used to calculate the fluorescence spectrum of warm dense titanium. The calculated results indicate that the energy difference (Kβ-Kα) will decrease with density but change slowly with temperature during the calculated state (4.5~13.5 g/cm³、 0.03~5 eV). FTRDFT overestimates the density effect on the line shift, while TSHFS underestimates the density effect. The future work will focus on optimizing the experimental method of x-ray fluorescence spectroscopy, obtaining x-ray fluorescence spectrum of titanium with more state, and then testing the theoretical method for warm dense matter.