Observation of the 717 nm electric quadrupole transition in triply charged thorium

TL;DR

This study reports the first observation of the 717 nm electric quadrupole transition in triply charged thorium ions, measuring its frequency and lifetime, and explores its potential for nuclear isomer excitation via electron-bridge transitions.

Contribution

It provides the first measurement of the 717 nm electric quadrupole transition in Th$^{3+}$ and demonstrates its use for nuclear isomer excitation schemes.

Findings

Transition frequency: 417,845,964(30) MHz

Metastable level lifetime: 0.60(7) seconds

Potential for laser excitation of nuclear isomer

Abstract

We excite the 717 nm electric quadrupole 6$D_{3/2} $ $\leftrightarrow$ 7$S_{1/2}$ transition in a laser-cooled $^{232}$Th$^{3+}$ ion crystal. The transition frequency and the lifetime of the metastable 7$S_{1/2}$ level are measured to be 417 845 964(30) MHz and 0.60(7) s, respectively. We subsequently employ the 7$S_{1/2}$ level to drive the ions with nanosecond-long 269 nm laser pulses into the 7$P_{1/2}$ level. The latter is connected to the 7$S_{1/2}$ electronic level within the $^{229}$Th nuclear isomer manifold by the strongest available electron-bridge transition, forming a basis for its laser excitation.