Resonant Electron Impact Excitation of 3d levels in Fe$^{14+}$ and Fe$^{15+}$

TL;DR

This study measures and compares laboratory spectra of Fe$^{14+}$ and Fe$^{15+}$ ions, confirming resonant excitation effects and highlighting discrepancies in non-resonant cross sections that impact spectral line ratio models.

Contribution

It provides experimental spectra of Fe ions and compares them with relativistic theoretical calculations, revealing good resonance agreement but discrepancies in non-resonant cross sections.

Findings

Resonant excitation confirmed at specific electron energies.

Good agreement between experimental and theoretical resonance strengths.

Discrepancy in non-resonant cross sections for Fe$^{14+}$.

Abstract

We present laboratory spectra of the $3p$--$3d$ transitions in Fe$^{14+}$ and Fe$^{15+}$ excited with a mono-energetic electron beam. In the energy dependent spectra obtained by sweeping the electron energy, resonant excitation is confirmed as an intensity enhancement at specific electron energies. The experimental results are compared with theoretical cross sections calculated based on fully relativistic wave functions and the distorted-wave approximation. Comparisons between the experimental and theoretical results show good agreement for the resonance strength. A significant discrepancy is, however, found for the non-resonant cross section in Fe$^{14+}$. %, which can be considered as a fundamental cause of the line intensity ratio problem that has often been found in both observatory and laboratory measurements. This discrepancy is considered to be the fundamental cause of the previously reported inconsistency of the model with the observed intensity ratio between the $^3\!P_2$ -- $^3\!D_3$ and $^1\!P_1$ -- $^1\!D_2$ transitions.