Observation of an unexpected negative isotope shift in 229Th+ and its theoretical explanation

TL;DR

This study reports an unexpected negative isotope shift in 229Th+ and introduces a new theoretical method to explain it, enhancing understanding of heavy atom isotope effects and nuclear-electronic interactions.

Contribution

The paper presents the first measurement of hyperfine structure and isotope shifts in 229Th+ and develops a novel all-order theoretical approach for heavy atoms.

Findings

Negative isotope shift observed in 229Th+ at 399.6 nm

Theoretical calculations explain the negative isotope shift

Good agreement between calculated and experimental isotope shifts

Abstract

We have measured the hyperfine structure and isotope shifts of the 402.0 nm and 399.6 nm resonance lines in 229Th+. These transitions could provide pathways towards the 229Th isomeric nuclear state excitation. An unexpected negative isotope shift relative to 232Th+ is observed for the 399.6 nm line, indicating a strong Coulomb coupling of the excited state to the nucleus. We have developed a new all-order approach to the isotope shift calculations that is generally applicable to heavy atoms and ions with several valence electrons. The theoretical calculations provide an explanation for the negative isotope shift of the 399.6 nm transition and yield a corrected classification of the excited state. The calculated isotope shifts are in good agreement with experimental values.