Direct experimental determination of precise electron affinities (EAs) of lanthanides
is a longstanding challenge to experimentalists. Considerable debate exists in
previous experiment and theory, hindering the complete understanding about the
properties of the atomic anions. Herein, we report the first precise photoelectron
imaging spectroscopy of europium (Eu), with the aim of eliminating prior
contradictions. The measured EA
(0.116 ± 0.013 eV) of Eu is in
excellent agreement with recently reported theoretical predictions, providing direct
spectroscopic evidence that the additional electron is weakly attached.
Additionally, a new experimental strategy is proposed that can significantly
increase the yield of the lanthanide anions, opening up the best opportunity to
complete the periodic table of the atomic anions. The present findings not only
serve to resolve previous discrepancy but also will help in improving the depth and
accuracy of our understanding about the fundamental properties of the atomic
anions.