The isotope Th is the only nucleus known to possess an excited stateTh in the energy range of a few electronvolts-a transition energy typical for electrons in the valence shell of atoms, but about four orders of magnitude lower than typical nuclear excitation energies. Of the many applications that have been proposed for this nuclear system, which is accessible by optical methods, the most promising is a highly precise nuclear clock that outperforms existing atomic timekeepers. Here we present the laser spectroscopic investigation of the hyperfine structure of the doubly charged Th ion and the determination of the fundamental nuclear properties of the isomer, namely, its magnetic dipole and electric quadrupole moments, as well as its nuclear charge radius. Following the recent direct detection of this long-sought isomer, we provide detailed insight into its nuclear structure and present a method for its non-destructive optical detection.

Experiments on one-and two-photon laser excitation of 232 Th + 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 (6d 7s7p)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 lowestlying 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 229 Th + nuclear resonance through an inverse two-photon electronic bridge process.

The thorium nucleus with mass number A = 229 has attracted much interest because its extremely low lying first excited isomeric state at about 8 eV opens the possibility for the development of a nuclear clock. However, neither the exact energy of this nuclear isomer nor properties, such as nuclear magnetic dipole and electric quadrupole moment are known to a high precision so far. The latter can be determined by investigating the hyperfine structure of thorium atoms or ions. Due to its electronic structure and the long lifetime of the nuclear isomeric state, Th 2+ is especially suitable for such kind of studies. In this letter we present a combined experimental and theoretical investigation of the hyperfine structure of the 229 Th 2+ ion in the nuclear ground and isomeric state. A very good agreement between theory and experiment is found for the nuclear ground state. Moreover, we use our calculations to confirm the recently presented experimental value for the nuclear magnetic dipole moment of the thorium nuclear isomer, which was in contradiction to previous theoretical

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