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

We have measured the hyperfine structure and isotope shifts of the 402.0 nm and 399.6 nm resonance lines in 229 Th + . These transitions could provide pathways towards the excitation of the 229 Th low-energy isomeric nuclear state. An unexpected negative isotope shift relative to 232 Th + is observed for the 399.6 nm line, indicating a strong Coulomb coupling of the excited state to the nucleus. We have developed a new all-order approach to the isotope shift calculations that is generally applicable to heavy atoms and ions with several valence electrons. The theoretical calculations provide an explanation for the negative isotope shift of the 399.6 nm transition and yield a corrected classification of the excited state. The calculated isotope shifts are in good agreement with experimental values.

The isotope 229 Th is unique in that it possesses an isomeric state of only a few eV above the ground state, suitable for nuclear laser excitation. An optical clock based on this transition is expected to be a very sensitive probe for variations of fundamental constants, but the nuclear properties of both states have to be determined precisely to derive the actual sensitivity. We carry out isotope shift calculations in Th + and Th 2+ including the specific mass shift, using a combination of configuration interaction and all-order linearized coupled-cluster methods and estimate the uncertainty of this approach. We perform experimental measurements of the hyperfine structure of Th 2+ and isotopic shift between 229 Th 2+ and 232 Th 2+ to extract the difference in root-mean-square radii as δ r 2 232,229 = 0.299(15) fm 2 . Using the recently measured values of the isomer shift of lines of 229m Th, we derive the value for the mean-square radius change between 229 Th and its low lying isomer to be 229m Th δ r 2 229m,229 = 0.0105(13) fm 2 .

Using resonant two-step laser excitation of trapped 232 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 multiconfiguration Dirac-Fock (MCDF) calculations to compare their results with the observed level density. The observed levels can be relevant for the excitation or decay of the 229m 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 Th + .

No abstract