Laser spectroscopy of Th+ above 7 eV excitation energy for electronic bridge excitation of the Th-229 nucleus

The possibility to realize a highly precise nuclear clock based on laser excitation of the isomeric state in Th-229 has motivated experiments with thorium ions in traps. The application of highly precise methods of laser spectroscopy to a nuclear excitation promises to open a new field of research at the border between atomic and nuclear physics. To facilitate the search for the nuclear transition within the present wide uncertainty range around 8 eV, two-photon excitation in the dense electronic level structure of Th⁺, which enables the nuclear excitation via a resonantly enhanced electronic bridge process, was investigated.