Double to tenfold M-shell photoionization of singly charged lanthanum ions

TL;DR

This work delivers experimental benchmarks for double to tenfold photoionization of La$^+$ across $820$–$1400$ eV, where inner-shell $3d$ and $3p$ excitations dominate. It combines high-resolution merged-beams measurements with large-scale JAC calculations to model both direct absorption and subsequent deexcitation cascades, testing their applicability to heavy, multi-electron ions relevant for kilonova plasmas. While the theory reproduces the main absorption features and resonance positions (with a modest energy shift), the cascade-derived product-ion charge-state distributions are somewhat narrower than observed, highlighting limitations from single-configuration treatments and metastable admixtures. Overall, the results demonstrate the feasibility of ab initio approaches for complex heavy ions and emphasize the need for advanced cascade modeling to support accurate nonequilibrium plasma data for astrophysical contexts.

Abstract

Using the photon-ion merged-beams technique at the PETRA\,III synchrotron light source, we have measured cross sections for double and up to tenfold photoionization of La$^{+}$ ions by a single photon in the energy range 820--1400~eV, where resonances and thresholds occur that are associated with the excitation or ionization of one $M$-shell electron. These cross sections represent experimental benchmark data for the further development of quantum theoretical methods, which will have to provide the bulk of the atomic data required for the modeling of nonequilibrium plasmas such as kilonovae. In the present work, we have upgraded the Jena Atomic Calculator (JAC) and pushed the state-of-the-art of quantum calculations for heavy many-electron systems to new limits. In particular, we have performed large-scale calculations of the La$^+$ photoabsorption cross section and of the deexcitation cascades, which set in after the initial creation of a $3d$ hole. Our theoretical results largely agree with our experimental findings. However, our theoretical product-ion charge state distributions are somewhat narrower than the experimental ones, which is most probably due to the simplifications necessary to keep the cascade calculations tractable.

Figures (4)

• Figure 1: Experimental cross sections (symbols with statistical error bars) $\sigma_q$ for net ($q$-1)-fold photoionization of La$^+$ ions in the photon-energy range 824--1400 eV. The photon-energy ($h\nu$) axis is compressed toward higher energies using $x = \log(h\nu/\mathrm{eV} - 750)$ for the abscissa in order to provide a better view of the resonance structures at lower energies. The various panels have different cross section scales. Cross sections of the same color are plotted on the same scale. The top panel shows the sum cross section $\sigma_\Sigma$ (symbols), which was scaled to the theoretical absorption cross section for neutral La atoms of Chantler2005 (dashed magenta line) to put the experimental cross sections on an absolute scale. The gray shaded band marks the $\pm$20% systematic uncertainty of the thus calibrated experimental cross section. The magenta diamonds represent the present theoretical cross sections for direct photoionization. The vertical arrows mark the approximate locations of the $3d$ and $3p$ ionization thresholds. The experimental data is available on Zenodo under an open-source Creative Commons Attribution license: [doi:10.5281/zenodo.17977213]https://doi.org/10.5281/zenodo.17977213.
• Figure 2: Experimental sum cross section (black circles) and theoretical absorption cross section near the $3d$ ionization threshold. The dashed magenta line is the cross section by Chantler2005 multiplied by a factor of 0.7. The magenta diamonds represent the present theoretical cross section for direct ionization. The dark blue shaded curve is the calculated resonant cross section due to $3d\to 4f$ excitation for a [Xe]$\,6s^2$ initial configuration. The light blue curve represents the resonant cross section accounting for configuration interaction between [Xe]$\,5d^2$ and [Xe]$\,4f^2$ initial configurations (shifted by -1 eV). Each cross section for resonant absorption consists of several resonances which were individually represented by Voigt profiles with 0.6-eV Lorentzian width and 1.0-eV Gaussian full width at half maximum. In order to (coarsely) account for direct ionization, constant offsets of 0.3 Mb and 1.3 Mb were added to the resonant cross sections below and above 846 eV, respectively.
• Figure 3: Experimental charge state fractions $f_q$ (Equation \ref{['eq:fq']}, upper panel) and mean charge state $\overline{q}$ (Equation \ref{['eq:qmean']}, lower panel) resulting from photoionization of La$^+$.
• Figure 4: Experimental (black stars connected by full lines) and theoretical (symbols connected by dashed lines) product-ion charge state fractions $f_q$ (Equation \ref{['eq:fq']}) for the photon energies $h\nu$ given. The experimental error bars are smaller than the size of the symbols. The computational models A (blue diamonds) and B (magenta circles) in panel c) are explained in the text. They predict mean charge states of 5.48 and 5.73, respectively.