Electronic ground states of Fe$_2^+$ and Co$_2^+$ as determined by x-ray absorption and x-ray magnetic circular dichroism spectroscopy

TL;DR

This study experimentally determines the electronic ground states of Fe$_2^+$ and Co$_2^+$ diatomic cations using x-ray absorption and magnetic circular dichroism spectroscopy, revealing discrepancies with theoretical models.

Contribution

First experimental assignment of the electronic ground state of Co$_2^+$ and identification of candidate states for Fe$_2^+$, challenging existing theoretical predictions.

Findings

Co$_2^+$ has a $^6\Pi$ ground state.

Fe$_2^+$ has multiple candidate states with large orbital angular momentum.

Ground states of transition metal diatomic cations may not follow simple one-electron relationships.

Abstract

The $^6\Pi$ electronic ground state of the Co$_2^+$ diatomic molecular cation has been assigned experimentally by x-ray absorption and x-ray magnetic circular dichroism spectroscopy in a cryogenic ion trap. Three candidates, $^6\Phi$, $^8\Phi$, and $^8\Gamma$, for the electronic ground state of Fe$_2^+$ have been identified. These states carry sizable orbital angular momenta that disagree with theoretical predictions from multireference configuration interaction and density functional theory. Our results show that the ground states of neutral and cationic diatomic molecules of $3d$ transition elements cannot generally be assumed to be connected by a one-electron process.