Quantum-Mechanical Calculation of Ionization-Potential Lowering in Dense Plasmas

Abstract

The charged environment within a dense plasma leads to the phenomenon of ionization potential depression (IPD) for ions embedded in the plasma. Accurate predictions of the IPD effect are of crucial importance for modeling atomic processes occurring within dense plasmas. Several theoretical models have been developed to describe the IPD effect, with frequently discrepant predictions. Only recently, first experiments on IPD in Al plasma have been performed with an x-ray free-electron laser (XFEL), where their results were found to be in disagreement with the widely-used IPD model by Stewart and Pyatt. Another experiment on Al, at the Orion laser, showed disagreement with the model by Ecker and Kröll. This controversy shows a strong need for a rigorous and consistent theoretical approach to calculate the IPD effect. Here we propose such an approach: a two-step Hartree-Fock-Slater model. With this parameter-free model we can accurately and efficiently describe the experimental Al data and validate the accuracy of standard IPD models. Our model can be a useful tool for calculating atomic properties within dense plasmas with wide-ranging applications to studies on warm dense matter, shock experiments, planetary science, inertial confinement fusion and studies of non-equilibrium plasmas created with XFELs.

Molecular-dynamics approach for studying the nonequilibrium behavior of x-ray-heated solid-density matter

“…To validate the XMDYN approach towards a free-electron thermal equilibrium, we use an average-atom (AA) extension of XATOM [31], which is based on concepts of average-atom models used in plasma physics [32][33][34][35][36]. AA gives a statistical description of the behavior of atoms immersed in a plasma environment.…”

Compton spectra of atoms at high x-ray intensity

“…They have also been used for xmolecule to calculate molecular electronic structure [59] and molecular data [60] to investigate molecular x-ray multiphoton ionization dynamics [60] and ultrafast explosion dynamics of small polyatomic molecules induced by intense XFEL pulses [61]. Lastly, xatom has been extended to treat atoms and ions immersed in a plasma environment to investigate ionization potential lowering in warm dense matter [62,63], which has further been employed for studying x-ray resonant magnetic scattering in materials [64]. Also note that the atomic electronic continuum states that are accurately calculated by xatom have been used for modeling high harmonic generation of rare gas atoms [65].…”

Atomistic three-dimensional coherent x-ray imaging of nonbiological systems

“…None of these atomistic MD methods [54,73,74] yet includes the plasma-induced effect of ionization potential depression (IPD) [75], but we note that the magnitude of the changes in ionization potential (∼100 eV) contributes only modest changes to photoionization rates and cross sections for hard x-ray energies well above the ionization potentials of the atoms in the system. Including these effects is a topic for future work, as a previous calculation of IPD [76] employs assumptions of fixed nuclei and thermalized electron distributions, neither of which is valid for our finite nanosystem that is rapidly undergoing electron rearrangement and Coulomb explosion. We note that our MC-MD code was validated by reproducing the experimental kinetic-energy distribution of ionized electrons from 1000-atom Ar clusters exposed to intense 5 keV XFEL pulses [54,77].…”