# Electronic level structure of $\mathrm{Th}^+$ in the range of the $^{229m}\mathrm{Th}$ isomer energy

## TL;DR

This study identifies numerous new energy levels in Th+ ions within a specific energy range, providing insights that could facilitate nuclear excitation of the $^{229m}$Th isomer through electronic bridge processes.

## Contribution

The paper reports the discovery of 166 previously unknown energy levels in Th+ ions and compares experimental results with ab-initio calculations, advancing understanding of electronic structure near the $^{229m}$Th isomer energy.

## Key findings

- Discovered 166 new energy levels in Th+ ions between 7.8 and 9.8 eV.
- Classified high-lying levels by total angular momentum.
- Observed level density suggests enhanced electronic bridge excitation.

## Abstract

Using resonant two-step laser excitation of trapped $^{232}\mathrm{Th}^+$ ions, we observe 166 previously unknown energy levels of even parity within the energy range from 7.8 to 9.8 eV and angular momenta from $J=1/2$ to $7/2$. We also classify the high-lying levels observed in our earlier experiments by the total angular momentum and perform ab-initio calculations to compare their results with the observed level density. The observed levels can be relevant for the excitation or decay of the $^{229m}\mathrm{Th}$ isomeric nuclear state which lies in this energy range. The high density of electronic levels promises a strongly enhanced electronic bridge excitation of the isomer in $^{229}\mathrm{Th}^+$.