The isotope 229Th is the only nucleus known to possess an excited state 229mTh in the energy range of a few electronvolts—a transition energy typical for electrons in the valence shell of atoms, but about four orders of magnitude lower than typical nuclear excitation energies. Of the many applications that have been proposed for this nuclear system, which is accessible by optical methods, the most promising is a highly precise nuclear clock that outperforms existing atomic timekeepers. Here we present the laser spectroscopic investigation of the hyperfine structure of the doubly charged 229mTh ion and the determination of the fundamental nuclear properties of the isomer, namely, its magnetic dipole and electric quadrupole moments, as well as its nuclear charge radius. Following the recent direct detection of this long-sought isomer, we provide detailed insight into its nuclear structure and present a method for its non-destructive optical detection.
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We thank C. Mokry, J. Runke, K. Eberhardt and N. G. Trautmann for the production of the 233U source and M. Ehlers, S. Hennig and K. Kossert for the PTB 229Th source. We acknowledge discussions with C. Tamm and B. Lipphardt and thank T. Leder, M. Menzel and A. Hoppmann for technical support. We acknowledge financial support from the European Union’s Horizon 2020 Research and Innovation Programme under Grant Agreement number 664732 (nuClock), from DFG through CRC 1227 (DQ-mat, project B04) and TH956-3-2 and from the LMU Department of Medical Physics via the Maier–Leibnitz Laboratory.Reviewer Information
Nature thanks E. Hudson, M. Safronova and the other anonymous reviewer(s) for their contribution to the peer review of this work.
Extended data figures and tables
Transitions and electronic configurations of the initial (g), intermediate (i) and excited (e) states relevant to the experiment are shown, labelled by their energy in cm−1 and the electronic angular momentum J. Hyperfine sub-levels are indicated by their total angular momentum F and Fm. Transitions belonging to the same intermediate hyperfine level are depicted with the same colour. The hyperfine intervals are calculated from the hyperfine constants A and B presented in Table 1.
The spectroscopy laser of the first step excitation (484 nm) is locked to the wavemeter, which is calibrated by a Rb-stabilized ECDL at 780 nm. The second-step (1,164 nm) laser tuning is monitored with the confocal cavity. The ECDL at 459 nm is used to detect the number of ions in the traps. The loading of Th2+ in the PTB trap is provided by ablation (nanosecond Nd:YAG laser at 1,064 nm) and further three-photon ionization. The first step uses a 402-nm ECDL, pulsed via an acousto-optical modulator (AOM), and the second and third steps involve third-harmonic generation (THG) of a nanosecond Ti:Sa laser. Molecular compounds of Th+ are photodissociated by pulses from a Q-switched diode-pumped solid-state laser (Q-DPSS).
The resonances recorded for different positions of the 484-nm ECDL show the observed isomeric peaks for the case of co-propagating beams (labelled ‘i’). The resonances that originate from collisions of ions in the intermediate state are labelled ‘c’. The description of the peaks and their total angular momenta are given in Extended Data Table 1. Black lines show the recorded data and blue lines represent a multi-Lorentz fit with fixed width, which is used to extract the line centres and frequency intervals. Source data
The experimental points represent amplitudes and positions of the two-step resonances obtained by setting the 484-nm laser at certain frequencies and tuning the 1,164-nm laser. The frequency of the 484-nm laser is changed in steps of about 120 MHz. The resonance groups shown with the same colour correspond to transitions from the same intermediate state with total angular momentum F, which is populated from different ground-state hyperfine components. The graphs show the HFS transitions of 229Th2+ in the ground state (a) and the isomer (b). Source data
Extended Data Fig. 5 Pressure dependence of collision-induced changes in the intermediate-state HFS.
The two-step excitation resonances of Th2+ were obtained with the first laser stabilized at −800 MHz detuning with respect to the 229Th HFS centre and the second laser scanned. The measurement is performed for two different He buffer-gas pressures and shows a decrease in the relative amplitude of the collisional resonances for the reduction of the buffer-gas pressure. We note that the isomeric resonance is not affected by the change in He pressure. Source data
This file contains Supplementary Figures 1 to 60 and Supplementary Tables 1 and 2. The figures show 29 of the 35 two-step excitation spectra recorded using the trap at LMU with copropagating beam configuration (six spectra do not contain detectable resonance features) and all spectra with counterpropagating configuration. The tables list the resonances and the corresponding transitions. See contents page for details