Continuum Lowering and Fermi-Surface Rising in Strongly Coupled and Degenerate Plasmas

Abstract

Continuum lowering is a well-known and important physics concept that describes the ionization potential depression (IPD) in plasmas caused by thermal-/pressure-induced ionization of outershell electrons. The existing IPD models are often used to characterize plasma conditions and to gauge opacity calculations. Recent precision measurements have revealed deficits in our understanding of continuum lowering in dense hot plasmas. However, these investigations have so far been limited to IPD in strongly coupled but nondegenerate plasmas. Here, we report a first-principles study of the K-edge shifting in both strongly coupled and fully degenerate carbon plasmas, with quantum molecular dynamics (QMD) calculations based on the all-electron density-functional theory (DFT). The resulted K-edge shifting versus plasma density, as a probe to the continuum lowering and the Fermi-surface rising, is found to be significantly different from predictions of existing IPD models. In contrast, a simple model of "single-atom-in-box" (SAIB), developed in this work, accurately predicts K-edge locations as what ab-initio calculations provide.2 PACS: 52.27. Gr, 52.25.Os, 52.70.La, For an isolated neutral atom or atomic ion, the ionization potentials (IP's) of electrons represent the energies required to free these electrons from their bound states. If photons are used to ionize the 1s-core electron of atoms or atomic ions, the photoabsorption spectrum exhibits a sharp edge (the so-called "K edge") above which the ionization probability is increased by orders of magnitude. For an isolated atom or ion, the K edge generally characterizes the ionization potential of 1s-core electron. Namely, the K-edge location is determined by E K-edge = IP = E C -E 1s , with the continuum level E C (E C = 0 for an isolated atom or ion) and the binding energy E 1s of the 1s-core electron. If atoms are immersed into a plasma, the thermal-/pressureinduced ionization of outer-shell electrons can cause the "continuum" to lower. Once the atomic continuum is lowered in a plasma, the ionization potential of bound electrons seems to depreciate. The analytical models 1-4 of ionization potential depreciation (IPD) are often used to infer plasma density/temperature conditions by measuring atomic K-edges in plasmas. They have also been extensively applied to alter the detailed opacity and equation-of-state (EOS) calculations of plasmas. 5-8 Consequently, this well-known physics concept of continuum lowering 1-4 is very important not only to plasma physics but also to planetary science, astrophysics, and high-energy-density physics.For its crucial importance to many fields, the IPD of atomic ions in plasmas has gained considerable attention over the past several years. These revisits have been motivated by recent experiments using both free-electron lasers to monitor K α emission spectra at Linac Coherent Light Source (LCLS) 9-11 and the hot dense plasma experiment at ORION. 12 These precision experiments have revealed the lack of a consistent picture about cont...

Electronic-structure calculations for nonisothermal warm dense matter

“…Over the years, various theoretical models and approaches have been developed to describe WDM and its properties. Among them are the Ecker-Kröll (EK) model [23], the Stewart-Pyatt (SP) model [24], the average-atom (AA) model [25][26][27][28] and its variation [29], finite-temperature density functional theory (DFT) [30][31][32], frequently in combination with ab-initio molecular dynamics (QMD) [33][34][35][36][37][38][39][40][41], timedependent DFT [42], Monte Carlo molecular dynamics [43], quantum kinetic theory [44,45], and quantum Monte Carlo simulations [46,47]. A recent review highlighting and analyzing the last two topics can be found in Ref.…”

Transient ionization potential depression in nonthermal dense plasmas at high x-ray intensity

“…To describe this IPD effect, two distinct models have been available, Ecker-Kröll (EK) [23] and Stewart-Pyatt (SP) [24], both of which are based on thermal equilibrium. Recent experiments using an XFEL [12,25] and a high-power optical laser [26,27] have triggered a controversial debate on the validity of these models, which has been followed by extensive studies on the IPD measurements [28][29][30][31][32][33] and the theoretical treatments of the IPD effect [34][35][36][37][38][39][40][41][42][43], including ab initio electronic structure calculations [44][45][46][47][48][49]. It is worthwhile to note that all of the employed methods are based on the LTE condition for the electronic subsystem.…”

Reconciling ionization energies and band gaps of warm dense matter derived with ab initio simulations and average atom models

“…Many variations of the AA approach exist [25,[31][32][33]. However, when these methods are applied to guide or interpret experiments at extreme densities or highly degenerate plasma conditions, the results appear to deviate from predictions of ab initio methods [26,[34][35][36], which provided the initial motivation for this study.…”