Two-photon laser excitation of trapped232Th+ions via the 402-nm resonance line

Abstract

Experiments on one- and two-photon laser excitation of ${}^{232}$Th${}^{+}$ ions in a radio-frequency ion trap are reported. As the first excitation step, the strongest resonance line at 402 nm from the $(6{d}^{2}7s)J=3/2$ ground state to the $(6d7s7p)J=5/2$ state at 24874 cm${}^{\ensuremath{-}1}$ 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, which 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 49 960 cm${}^{\ensuremath{-}1}$, 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 55 000 cm${}^{\ensuremath{-}1}$ which can resonantly enhance the excitation of the ${}^{229}$Th${}^{+}$ nuclear resonance through an inverse two-photon electronic bridge process.