Two-photon laser excitation of trapped 232Th+ ions via the 402 nm resonance line

TL;DR

This paper demonstrates two-photon laser excitation of 232Th+ ions in a trap via a 402 nm resonance line, studying decay pathways, buffer gas effects, and achieving excitation to high-lying states relevant for nuclear resonance research.

Contribution

It introduces a novel two-photon excitation scheme for 232Th+ ions, including detailed modeling of decay and quenching processes, and demonstrates excitation to high-lying states for nuclear resonance studies.

Findings

Two-photon excitation to 49960 cm^{-1} state achieved

Buffer gases influence metastable state populations

A four-level rate equation model matches experimental data

Abstract

Experiments on one- and two-photon laser excitation of 232Th+ ions in a radiofrequency ion trap are reported. As the first excitation step, the strongest resonance line at 402 nm from the 6d^2 7s J=3/2 ground state to the 6d7s7p J=5/2 state at 24874 cm^{-1} is driven by radiation from an extended cavity diode laser. Spontaneous decay of the intermediate state populates a number of low-lying metastable states, thus limiting the excited state population and fluorescence signal obtainable with continuous laser excitation. We study the collisional quenching efficiency of helium, argon, and nitrogen buffer gases, and the effect of repumping laser excitation from the three lowest-lying metastable levels. The experimental results are compared with a four-level rate equation model, that allows us to deduce quenching rates for these buffer gases. Using laser radiation at 399 nm for the second step, we demonstrate two-photon excitation to the state at 49960 cm^{-1}, among the highest-lying classified levels of Th+. This is of interest as a test case for the search for higher-lying levels in the range above 55000 cm^{-1} which can resonantly enhance the excitation of the 229Th+ nuclear resonance through an inverse two-photon electronic bridge process.