Near L-Edge Single and Multiple Photoionization of Doubly Charged Iron Ions

TL;DR

This study combines experimental measurements and theoretical calculations to analyze the photoionization of Fe$^{2+}$ ions near the L-edge, providing insights into ionization processes and aiding in identifying iron in the interstellar medium.

Contribution

The paper presents new experimental cross-section data and improved theoretical models for Fe$^{2+}$ photoionization near the L-edge, including detailed Auger cascade calculations.

Findings

Good agreement between experiment and theory after energy shifts

Identification of L-shell absorption features for interstellar iron detection

Enhanced understanding of charge-state distributions after ionization

Abstract

Using the photon-ion merged-beams technique at a synchrotron light source, we have measured relative cross sections for single and up to five-fold photoionization of Fe$^{2+}$ ions in the energy range 690--920 eV. This range contains thresholds and resonances associated with ionization and excitation of $2p$ and $2s$ electrons. Calculations were performed to simulate the total absorption spectra. The theoretical results show very good agreement with the experimental data, if overall energy shifts of up to 2.5 eV are applied to the calculated resonance positions and assumptions are made about the initial experimental population of the various levels of the Fe$^{2+}$([Ar]$3d^6$) ground configuration. Furthermore, we performed extensive calculations of the Auger cascades that result when an electron is removed from the $2p$ subshell of Fe$^{2+}$. These computations lead to a better agreement with the measured product-charge-state distributions as compared to earlier work. We conclude that the $L$-shell absorption features of low-charged iron ions are useful for identifying gas-phase iron in the interstellar medium and for discriminating against the various forms of condensed-phase iron bound to composite interstellar dust grains.