Thorium-229 (²²⁹Th) possesses an optical nuclear transition between the ground state (^229gTh) and low-lying isomer (^229mTh). A nuclear clock based on this nuclear-transition frequency is expected to surpass existing atomic clocks owing to its insusceptibility to surrounding fields^1,2,3,4,5. In contrast to other charge states, triply charged ²²⁹Th (²²⁹Th³⁺) is the most suitable for highly accurate nuclear clocks because it has closed electronic transitions that enable laser cooling, laser-induced fluorescence detection and state preparation of ions^1,6,7,8. Although laser spectroscopic studies of ²²⁹Th³⁺ in the nuclear ground state have been performed⁸, properties of ^229mTh³⁺, including its nuclear decay lifetime that is essential to specify the intrinsic linewidth of the nuclear-clock transition, remain unknown. Here we report the trapping of ^229mTh³⁺ continuously supplied by a ²³³U source and the determination of nuclear decay half-life of the isolated ^229mTh³⁺ to be \({{\rm{1,400}}}_{-300}^{+600}\,{\rm{s}}\) through nuclear-state-selective laser spectroscopy. Furthermore, by determining the hyperfine constants of ^229mTh³⁺, we reduced the uncertainty of the sensitivity of the ²²⁹Th nuclear clock to variations in the fine-structure constant by a factor of four. These results offer key parameters for the ²²⁹Th³⁺ nuclear clock and its applications in the search for new physics.

Thorium-229 ( ²²⁹ Th) 具有基态 ( ^229g Th) 和低位异构体 ( ^229m Th) 之间的光学核跃迁。基于这种核跃迁频率的核钟预计将超越现有的原子钟，因为它不受周围场的影响^1,2,3,4,5。与其他电荷态相比，三重电荷²²⁹ Th ( ²²⁹ Th ³⁺ ) 最适合高精度核钟，因为它具有闭合电子跃迁，可实现激光冷却、激光诱导荧光检测和离子^{1,6的状态准备,7,8}。尽管已经对核基态的²²⁹ Th ³⁺进行了激光光谱研究^{8 ，但229m} Th ³⁺的特性（包括其核衰变寿命（对于指定核钟跃迁的固有线宽至关重要））仍然未知。在这里，我们报告了由²³³ U 源连续提供的^229m Th ³⁺的捕获，并将孤立的^229m Th ³⁺的核衰变半衰期确定为\({{\rm{1,400}}}_{-300 }^{+600}\,{\rm{s}}\)通过核态选择性激光光谱。此外，通过确定^229m Th ³⁺的超精细常数，我们将²²⁹ Th 核钟对精细结构常数变化的敏感性的不确定性降低了四倍。这些结果为²²⁹ Th ³⁺核钟及其在寻找新物理学中的应用提供了关键参数。