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There is strong interest in laser spectroscopy of trapped Th ions because of the low-energy (8.3 eV) isomer that exists in 229Th [1]. With an energy that is accessible for laser excitation, this nuclear resonance is attractive as the reference of an optical clock that combines high accuracy with a strong sensitivity for effects of new physics that may be sought in frequency comparisons with atomic clocks [2].
As a step towards laser excitation of 229Th we have developed a tunable vacuum-ultraviolet (VUV) laser source based on four-wave frequency mixing in xenon. Using seed radiation from two contin-uous-wave lasers, the system allows for precise control of the VUV frequency. Tunable in the wave-length range 148-155 nm, the source produces pulses of 6-10 ns duration with up to 40 μJ energy and is coupled via a vacuum beamline to a linear RF ion trap. In a first implementation of VUV laser spectroscopy of trapped Th+ ions we excite three previously unknown resonance lines to electronic levels in the vicinity of the 229Th isomer energy. An analysis of the lineshape is used to estimate the linewidth of the VUV radiation to be about 6 GHz, dominated by phase noise that is enhanced in harmonic generation and in the four-wave mixing process.
Trapping of 229Th ions in charge states 1+, 2+ and 3+ has been demonstrated with the ions produced in laser ablation from solid 229Th targets [3,4], but the efficiency of the method decreases substantially with increasing charge. In preparation of experiments with laser-cooled 229Th3+ ions we have developed an apparatus for the trapping of Th3+ recoil ions from the alpha decay of 233U. The ion source in a helium buffer gas cell is linked to a linear RF trap in ultrahigh vacuum, where the ions are cooled sympathetically by laser cooled 88Sr+ ions. 88Sr+ has been selected as the coolant ion because of its convenient laser cooling transitions and because its charge to mass ratio is similar to that of 229Th3+, so that Coulomb crystals are produced where the two species are closely coupled.
[1] K. Beeks et al., Nature Rev. Phys. 3, 238 (2021).
[2] E. Peik et al., Quant. Sci. Techn. 6, 034002 (2021).
[3] K. Zimmermann, M.V. Okhapkin, O.A. Herrera-Sancho, E. Peik, Appl Phys B 107, 883 (2012).
[4] C. J. Campbell, A. G. Radnaev, and A. Kuzmich, Phys. Rev. Lett. 106, 223001 (2011).
