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

Abstract

The advent of x-ray free-electron lasers (XFELs), which provide intense ultrashort x-ray pulses, has brought a new way of creating and analyzing hot and warm dense plasmas in the laboratory. Because of the ultrashort pulse duration, the XFEL-produced plasma will be out of equilibrium at the beginning, and even the electronic subsystem may not reach thermal equilibrium while interacting with a femtosecond timescale pulse. In the dense plasma, the ionization potential depression (IPD) induced by the plasma environment plays a crucial role for understanding and modeling microscopic dynamical processes. However, all theoretical approaches for IPD have been based on local thermal equilibrium (LTE), and it has been controversial to use LTE IPD models for the nonthermal situation. In this work, we propose a non-LTE (NLTE) approach to calculate the IPD effect by combining a quantum-mechanical electronic-structure calculation and a classical molecular dynamics simulation. This hybrid approach enables us to investigate the time evolution of ionization potentials and IPDs during and after the interaction with XFEL pulses, without the limitation of the LTE assumption. In our NLTE approach, the transient IPD values are presented as distributions evolving with time, which cannot be captured by conventional LTE-based models. The time-integrated ionization potential values are in good agreement with benchmark experimental data on solid-density aluminum plasma and other theoretical predictions based on LTE. The present work is promising to provide critical insights into nonequilibrium dynamics of dense plasma formation and thermalization induced by XFEL pulses.

Plasma environmental effects in the atomic structure for simulating x-ray free-electron-laser-heated solid-density matter

“…This hybrid XMDYN-XATOM approach has been demonstrated to reproduce electron kinetic energy spectra and ion distributions from rare-gas nanoplasmas [64][65][66][67], the fragmentation dynamics of C 60 [27,68], the Coulomb explosion imaging of iodopyridine [69], and the dynamics of disulfide expansion in thaumatin [70]. It has been recently extended to simulate bulk systems using the supercell approach and periodic boundary conditions [61,[71][72][73].…”

“…Therefore, it is desirable to implement an IPD treatment that is unrestricted by the thermal equilibrium condition in the simulation. To address such inconsistent usage of LTE IPD in NLTE simulations, a treatment of transient IPD without assuming thermal equilibrium has recently been proposed [61] and tested in comparison with LTE IPD and available experimental data [12,33]. The proposed NLTE IPD treatment is based on an electronicstructure calculation that employs real-time information from a classical MD simulation; it is not based on an analytical IPD expression as in the SP and EK models.…”

Nonsequential two-photon absorption in solid Ge irradiated by an intense x-ray free-electron-laser pulse

“…Thus, ionized electrons are trapped and form a dense (solid-density) plasma [12][13][14]. To illustrate the creation and evolution of such a plasma in solid Ge we use the Monte Carlo moleculardynamics simulation tool, XMDYN [18,19], which has been extended through the implementation of periodic boundary conditions to study warm dense matter [15,40,41]. XMDYN handles atomic processes (photoionization, Auger-Meitner decay, and fluorescence) quantum mechanically, and environmental phenomena (collisional ionization, recombination, and Coulomb interaction between charged particles) using a classical treatment [19].…”

“…For better statistical results, we run ten parallel realizations. The plasma environmental effect, namely ionization potential depression (IPD) [12,13,41,43], is not considered for simplicity. Note that for the given x-ray parameters the IPD values are estimated to lie in the range from 100 to 260 eV for charge states between +6 and +14 by employing a hybrid quantum-classical model [41].…”

“…The plasma environmental effect, namely ionization potential depression (IPD) [12,13,41,43], is not considered for simplicity. Note that for the given x-ray parameters the IPD values are estimated to lie in the range from 100 to 260 eV for charge states between +6 and +14 by employing a hybrid quantum-classical model [41]. These values are much smaller in comparison with the given photon energy of 7200 eV, so we expect that IPD has little influence on photoionization processes.…”