1 Observation of Electric-Dipole Transitions in the Laser-Cooling Candidate Th − Rulin Tang 1,# , Ran Si 2,3,# , Zejie Fei 4 , Xiaoxi Fu 1 , Yuzhu Lu 1 , Tomas Brage 2,3 , Hongtao Liu 4,* , Chongyang Chen 3,* , and Chuangang Ning 1,5,* 1 Department of Physics, State Key Laboratory of Low-Dimensional Quantum Physics, Tsinghua University, Beijing 100084, China 2 Division of Mathematical Physics, Department of Physics, Lund University, P.O. Box 118, 221 00 Lund, Sweden 3 Shanghai EBIT Lab, Key Laboratory of Nuclear Physics and Ion-beam Application (MOE), Institute of Modern Physics, Department of Nuclear Science and Technology, Fudan University, Shanghai 200433, China 4 Key Laboratory of Interfacial Physics and Technology, Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201800, China 5 Collaborative Innovation Center of Quantum Matter, Beijing 100084, China Abstract: Despite the fact that the laser cooling method is a well-established technique to obtain ultra-cold neutral atoms and atomic cations, it has so far never been applied to atomic anions due to the lack of suitable electric-dipole transitions. Efforts of more than a decade currently have La – as the only promising candidate for laser cooling. Our previous work [Tang et al., Phys. Rev. Lett. (2019) accept] showed that Th – is also a potential candidate. Here we report on a combination of experimental and theoretical studies to determine the relevant transition frequencies, transition rates, and branching ratios in Th – . The resonant frequency of the laser cooling transition is determined to be /c= 4118.0 (10) cm –1 . The transition rate is calculated as A=1.1710 4 s 1 . The branching fraction to dark states is very small, 1.47×10 –10 , thus this represents an ideal closed cycle for laser cooling. Since Th has zero nuclear spin, it is an excellent candidate to be used to sympathetically cool antiprotons in a Penning trap. The achievement of Bose-Einstein condensation, precision spectroscopy, and tests of fundamental symmetries has opened a new chapter in atomic and molecular physics. The main driving force behind this achievement is the ability to cool atoms and positive ions to K or even lower temperatures via laser cooling techniques. Although laser cooling is a well-established technique for producing ultra- cold neutral atoms and positive ions, it has not yet been achieved for negative ions. In principle, once we produce ultracold ensembles of a specific anion system, we can use them to sympathetically cool any anions, ranging from elementary particles to molecular anions, which will promote the research of cold plasma[1], ultracold chemistry[2], and fundamental-physics tests[3-8]. In contrast to neutral atoms and positive ions, which have an infinite number of bound states, negative ions have only a single bound state in most cases. The reason is that in atomic anions, the excess