Observation of electric-dipole transitions in the laser-cooling candidate Th- and its application for cooling antiprotons

RL Tang and R Si and ZJ Fei and XX Fu and YZ Lu and T Brage and HT Liu and CY Chen and CG Ning, PHYSICAL REVIEW A, 103, 042817 (2021).

DOI: 10.1103/PhysRevA.103.042817

Despite the fact that the laser-cooling method is a well-established technique to obtain ultracold neutral atoms and atomic cations, it has rarely if ever been applied to atomic anions due to the lack of suitable electric-dipole transitions. Efforts of more than a decade have until recently only resulted in La- as a promising anion candidate for laser cooling, but our previous work Tang et al., Phys. Rev. Lett. 123, 203002 (2019) showed that Th- is also a potential candidate. Here we report on a combination of experimental and theoretical studies to determine the frequencies and rates, as well as branching ratios, for the relevant transitions in Th-. The resonant frequency of the laser- cooling transition is determined to be. = 123.455(30) THz lambda = 2428.4(6) nm. The transition rate is calculated as A = 1.17 x 10(4) s(-1). Since the branching fraction to dark states is negligible, 1.47 x 10(-10), this represents an ideal closed cycle in Th- for laser cooling. Furthermore, the zero nuclear spin of 232Th makes the cooling process possible in a Penning trap, which can be used to confine both antiprotons and Th- ions. The presented ion dynamics simulations show that the laser-cooled Th- anions can effectively cool antiprotons to a temperature around 10 mK.

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