Clocking Femtosecond Collisional Dynamics via Resonant X-Ray Spectroscopy

Abstract

Electron-ion collisional dynamics is of fundamental importance in determining plasma transport properties, nonequilibrium plasma evolution, and electron damage in diffraction imaging applications using bright x-ray free-electron lasers (FELs). Here we describe the first experimental measurements of ultrafast electron impact collisional ionization dynamics using resonant core-hole spectroscopy in a solid-density magnesium plasma, created and diagnosed with the Linac Coherent Light Source x-ray FEL. By resonantly pumping the 1s→2p transition in highly charged ions within an optically thin plasma, we have measured how off-resonance charge states are populated via collisional processes on femtosecond time scales. We present a collisional cross section model that matches our results and demonstrates how the cross sections are enhanced by dense-plasma effects including continuum lowering. Nonlocal thermodynamic equilibrium collisional radiative simulations show excellent agreement with the experimental results and provide new insight on collisional ionization and three-body-recombination processes in the dense-plasma regime.

Screening potential and continuum lowering in a dense plasma under solar-interior conditions

“…An accurate description of the plasma screening potential is of crucial importance for investigations of the atomic processes in dense plasmas (Liu et al 2018). Recent experimental investigations on electron-impact ionization show that the cross-sections and rates are enhanced by dense-plasma effects and cannot be explained by currently available atomic collision theory (Vinko et al 2015;van den Berg et al 2018). Photoionization cross-sections are also increased, and thus the radiative opacity is increased as well for hot, dense plasmas (Bailey et al 2015).…”

“…The screening potential induced by the plasma environment plays pivotal roles in both microscopic atomic processes and macroscopic physical properties (Basu & Antia 2008;Bahcall et al 2004). Microscopically, it affects the atomic structure and all kinds of radiative and collisional atomic processes, such as photo-excitation and photoionization, as well as electron-impact excitation and the ionization of ions (Vinko et al 2015;van den Berg et al 2018). Macroscopically, the ionization balance, and hence, the physical properties of equations of state, opacities, and thermal and electronic conductivities can be significantly modified by the dense plasma environment (Bailey et al 2015; Tables of the numerical data used for the figures are only available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/ cat/J/A+A/634/A117 1994; Iglesias & Rogers 1991;Rose 1992;Hansen et al 2005;Vinko et al 2012).…”

Inverse problem instabilities in large-scale modeling of matter in extreme conditions

“…X-ray spectroscopy X-ray spectroscopy is a popular diagnostic technique across the physical sciences, widely used in astrophysical investigations [26][27][28], for elemental analysis in laserinduced breakdown spectroscopy [29], in applied condensed matter physics [30,31], in fundamental plasma physics [32,33], laser-plasma interactions [34][35][36], and in inertial confinement fusion (ICF) research [37][38][39][40].…”

Validating Continuum Lowering Models via Multi-Wavelength Measurements of Integrated X-ray Emission

“…The intense x-ray pulse can drive x-ray photoionization of inner-shell electrons, provided the photon energy is higher than the shell’s ionization edge, and bound-bound transitions leading to excited atomic configurations, if the resonance energies are within the bandwidth of the x-ray pulse. Recombination into the core holes created by this interaction produces strong x-ray emission that has been spectrally resolved to identify ionization edges and deduce levels of CL 14 , 15 , observe non-linear processes 21 , measure x-ray opacities 16 and extract electron collisional ionization rates 23 , 24 .…”