# Electron-impact fine-structure excitation of Fe II at low temperature

## TL;DR

This study provides accurate electron-impact collision strengths for Fe II's low-lying fine-structure transitions at low temperatures, crucial for interpreting astronomical emission lines across various objects.

## Contribution

It introduces new, reliable collision strength data for Fe II's ground term transitions using advanced R-matrix methods, with comprehensive uncertainty estimates.

## Key findings

- Good agreement with previous R-matrix calculations.
- Provided recommended effective collision strengths with uncertainties.
- Focused on low-temperature environments like the interstellar medium.

## Abstract

Fe II emission lines are observed from nearly all classes of astronomical objects over a wide spectral range from the infrared to the ultraviolet. To meaningfully interpret these lines, reliable atomic data are necessary. In work presented here we focused on low-lying fine-structure transitions, within the ground term, due to electron impact. We provide effective collision strengths together with estimated uncertainties as functions of temperature of astrophysical importance ($10 - 100,000$ K). Due to the importance of fine-structure transitions within the ground term, the focus of this work is on obtaining accurate rate coefficients at the lower end of this temperature range, for applications in low temperature environments such as the interstellar medium. We performed three different flavours of scattering calculations: i) a intermediate coupling frame transformation (ICFT) $R$-matrix method, ii) a Breit-Pauli (BP) $R$-matrix method, and iii) a Dirac $R$-matrix method. The ICFT and BP $R$-matrix calculations involved three different AUTOSTRUCTURE target models each. The Dirac $R$-matrix calculation was based on a reliable 20 configuration, 6069 level atomic structure model. Good agreement was found with our BP and Dirac $R$-matrix collision results compared to previous $R$-matrix calculations. We present a set of recommended effective collision strengths for the low-lying forbidden transitions together with associated uncertainty estimates.