Transition frequency shifts with fine structure constant variation for Fe II: Breit and core-valence correlation correction

TL;DR

This study calculates how Fe II transition frequencies depend on the fine structure constant b, including corrections from correlations and Breit interaction, confirming Fe II's suitability for probing b variation over cosmic time.

Contribution

The paper provides new calculations of Fe II transition frequency sensitivities to b, incorporating significant corrections from core-valence correlations and Breit interaction, which were previously underestimated.

Findings

Corrections from correlations and Breit interaction are larger than expected.

Fe II lines have large, sign-changing q-factors.

Fe II remains a viable probe for b variation in cosmology.

Abstract

Transition frequencies of Fe II ion are known to be very sensitive to variation of the fine structure constant \alpha. The resonance absorption lines of Fe II from objects at cosmological distances are used in a search for the possible variation of \alpha in cause of cosmic time. In this paper we calculated the dependence of the transition frequencies on \alpha^2 (q-factors) for Fe II ion. We found corrections to these coefficients from valence-valence and core-valence correlations and from the Breit interaction. Both the core-valence correlation and Breit corrections to the q-factors appeared to be larger than had been anticipated previously. Nevertheless our calculation confirms that the Fe II absorption lines seen in quasar spectra have large q-factors of both signs and thus the ion Fe II alone can be used in the search for the \alpha-variation at different cosmological epochs.