Comment on: "Hyperfine structure measurements of Co I and Co II with Fourier transform spectroscopy" by Fu et al. [JQSRT 2021, 107590]

TL;DR

This comment critiques Fu et al.'s hyperfine structure measurements of Co II, highlighting prior work, identifying errors, and discussing discrepancies due to analysis of weak transitions and spectrum quality.

Contribution

It clarifies the inaccuracies in Fu et al.'s analysis and emphasizes the importance of comprehensive literature review and high-quality spectra in hyperfine constant measurements.

Findings

Most of Fu et al.'s claimed new measurements were previously published.

Discrepancies in 17 hyperfine constants likely due to analysis of weak, unclassified transitions.

Poorer spectrum quality contributed to measurement disagreements.

Abstract

This comment points out errors in the analysis of 61 magnetic hyperfine structure ($A$) constants of Co II energy levels by Fu et al. [JQSRT 2021, 107590]. The paper was published without full awareness of the extensive literature already available for Co II hyperfine $A$ constants at the time; 57 of 58 $A$ constants that were claimed to have been measured for the first time had already been measured by the prior work of Ding \& Pickering [ApJS 2020, 251:24], who had published $A$ constants for 292 levels of Co II. The $A$ constant of 3d$^6$4s$^2$ a$^5$D$_4$ has been determined by Fu et al. [JQSRT 2021, 107590] for the first time to be $12.0\pm1.8$ mK (1 mK $=$ 0.001 cm$^{-1}$), which was found to agree with line profiles observed by Ding \& Pickering [ApJS 2020, 251:24]. Discrepancies in 17 $A$ constants of Fu et al. [JQSRT 2021, 107590] were found, which are likely due to the analysis of weak, experimentally unclassified transitions with Ritz wavenumbers 25453.966 cm$^{-1}$ and 25149.948 cm$^{-1}$ by Fu et al. [JQSRT 2021, 107590] for the $A$ constants of the energy levels 3d$^7$($^2$G)4s a$^3$G$_5$ and 3d$^7$($^2$P)4s c$^3$P$_2$ respectively. Fewer transitions and poorer quality spectra analysed by Fu et al. [JQSRT 2021, 107590] are also concluded to have contributed to disagreements in the 17 $A$ constants.