Ab initiocalculation of the ion feature in x-ray Thomson scattering

Abstract

The spectrum of x-ray Thomson scattering is proportional to the dynamic structure factor. An important contribution is the ion feature which describes elastic scattering of x rays off electrons. We apply an ab initio method for the calculation of the form factor of bound electrons, the slope of the screening cloud of free electrons, and the ion-ion structure factor in warm dense beryllium. With the presented method we can calculate the ion feature from first principles. These results will facilitate a better understanding of x-ray scattering in warm dense matter and an accurate measurement of ion temperatures which would allow determining nonequilibrium conditions, e.g., along shock propagation.

Ab initiosimulations of the dynamic ion structure factor of warm dense lithium

“…In particular, the q(k) from the DFT-MD simulations is lower than calculations in Debye-Hückel theory and shows antiscreening for k = (2.5 − 5) Å −1 , as already found in other works [29,38,67]. The result of q(k) in [27] is consistent with Debye-Hückel using Z f = 1.35.…”

“…On the other hand, ions in our DFT-MD simulation are treated classically. The total form factor N ( k) is calculated by averaging over the time steps of the simulation [29],…”

“…The plasma parameters density and temperature are usually extracted from a best fit of the scattering spectrum using both the ion as well as the electron feature. Potential discrepancies could point to inhomogeneities or two-temperature states [29] that might be generated in such pump-probe experiments. Here, we compare our DFT-MD results with an XRTS experiment on warm dense Li [27].…”

Warm Dense Matter Demonstrating Non-Drude Conductivity from Observations of Nonlinear Plasmon Damping

“…The elastic scattering contribution arises from weakly and tightly bound electrons and is expressed in |N (k)| 2 S ii (k); it is sensitive to the ionic structure factor S ii (k) and the total electronic form factor N (k). The total form factor can be derived from our DFT-MD simulated ion number density and single-electron wavefunctions [33]. The autocorrelation function of the ion number density provides the ionic structure factor [36].…”

“…The dynamic electrical conductivity σ(ω) and the DSF S ee (k, ω) are calculated from the ion number density and the single electron wave-functions [28,33,36,52]. Both quantities are derived from DFT-MD, implemented within the Vienna Ab initio Simulation Package (VASP) [53][54][55] which is well-suited for the study of WDM.…”