Electronic level structure investigations in Th⁺ and nuclear properties of ^229mTh

The ²²⁹Th isotope possesses a unique, low-energy nuclear isomeric state at about 7.8(5) eV. This fact has stimulated the development of novel ideas in the borderland between atomic and nuclear physics, for example the use as an optical nuclear clock. Since the required precise information on the isomer energy is not yet available, it is intensely searched for using different experimental approaches. For the excitation of the nuclear isomer via electronic bridge or NEET processes, we investigate two-photon laser excitation of high-lying electronic levels in Th⁺. We recently expanded our search range to higher energies and measured more than 100 previously unknown energy levels with J=1/2, 3/2 and 5/2 in the range from 7.8 eV to 9.8 eV. We also present the recent progress of the hyperfine structure measurement of ^229mTh²⁺ ions and the determination of ^229mTh nuclear properties.