Towards a nuclear clock with the ²²⁹Th isomeric transition

The 229Th nucleus with its 7.8 eV isomeric state is a unique candidate for the first nuclear optical clock at an exceptional accuracy of 10−19. Due to the low isomeric transition energy, the coupling to the atomic shell in the processes of internal conversion (IC) or electron bridge (EB) is very strong. In this work we investigate theoretically the IC and EB mechanisms and design novel laser-assisted schemes aimed at (i) a more accurate determination of the isomer energy Em and (ii) efficient excitation of the isomer. On the one hand, we show how IC occurring from excited electronic states of Th ions can be used to determine the isomeric transition energy. We also propose a new approach to measure the isomer energy with laser spectroscopy accuracy via laser-induced EB in 229Th3+ ions. On the other hand, we put forward possible isomer excitation schemes based on the EB process in highly charged Th ions or on laser-assisted nuclear excitation by electron capture of the 29.19 keV nuclear state lying directly above the isomer. Our results show that EB in highly charged ions which allow optical driving with high-power lasers should lead to efficient excitation. Such a setup using ion traps might become relevant for the future development of the nuclear clock