Experiment
Production
The ionization potential of AcF was predicted to lie above the threshold for efficient ionization by contact with a hot surface. In addition, without knowledge of its electronic structure, resonance laser ionization could not be used to produce ion beams of AcF+. The forced-electron-beam-induced arc-discharge (FEBIAD)-type ion source was chosen and operating parameters were identified for the production of AcF+. The UC x target in the FEBIAD-type ion-source unit was irradiated for 114.4 hours (4.77 days) before the start of the experiment, receiving 1.7 × 1018 protons, or a total of 73.774 μAh. During irradiation, the target unit was kept under vacuum (about 1 × 10−6 mbar) and the target container was resistively heated to slightly above room temperature to prevent condensation during irradiation. At the start of the experiment, the tantalum cathode of the ion source was resistively heated to 1,950 °C to facilitate electron emission. An anode voltage of 100 V was applied to the anode grid to accelerate the electrons and induce ionization within the anode volume, which was also maintained at 100 V with a magnetic-confinement field induced by applying a current of 2.8 A to the ion-source electromagnet. A bias voltage of 40 kV was applied to the target and ion-source unit, such that the ion beam was extracted to the ground potential of the beamline with an energy of 40.1 keV.
The target temperature was increased from about 1,300 °C at the start of the experiment up to 2,000 °C by heating in steps on the order of 10 A to maintain a continuous supply of AcF+. A mix of 10% CF 4 and 90% Ar gas was added to the target via a leak of 1.5 × 10−4 mbar l s−1 calibrated for He, injecting 0.065 nmol s−1 of CF 4 for the formation of fluoride molecules.
An extensive beam purity investigation was performed using α-decay spectroscopy of implanted ions and multi-reflection time-of-flight mass spectrometry using the ISOLTRAP apparatus23,49. The main expected isobaric contaminant, 227Ra19F+, was eliminated owing to the asynchronous proton irradiation and nuclide extraction, taking advantage of the drastically longer half-life of 227Ac (21.8 years) compared with 227Ra (42 minutes). The time-of-flight spectra in Fig. 1g and Supplementary Fig. 3 show that the ion beam delivered for study was purely composed of 227Ac19F+, with no identifiable contaminants above background.
Collinear resonance ionization spectroscopy
At CRIS, the molecular beam was temporally and spatially overlapped in a collinear geometry with pulsed lasers that step-wise excited the molecular electron to ionization. At the end of the laser–molecule interaction region, the ionized molecules were deflected from the path of the residual neutral beam onto a single-ion detector. The excitation spectra were produced by monitoring the ion count rate on the detector as a function of the laser excitation wavenumbers.
Prior ab initio calculations of the excitation energies in AcF (ref. 27) predicted the (8)1Π state to lie at 26,166(450) cm−1 above X1Σ+. The 1σ error of ±450 cm−1 required a scanning range of 1,800 cm−1 to have 95% probability of discovering the predicted transition. Such a wide range is challenging for continuous scanning of light produced from a single-pass β-barium borate SHG crystal, as the SHG crystal angle requires active stabilization to ensure optimal frequency doubling for all fundamental wavenumbers, while small deviations from the optimal crystal angle also lead to the doubled light exiting the crystal at an angle. The latter issue is exacerbated by the distance between the laser table and the beamline, exceeding 15 m, which means that small exit angles from the SHG crystal lead to the laser light not entering the CRIS beamline.
To compensate for both issues, an active crystal-angle stabilization system was constructed using a ThorLabs PIAK10 piezoelectric inertia actuator, controlled with a proportional-integral-derivative loop reading a fraction of the SHG power output with the help of a beam sampler. To ensure that the second-harmonic light always followed the optimal trajectory for interaction with the molecules in the CRIS interaction region, a commercial active laser-beam stabilization system from MRC Systems was also installed, as shown in Fig. 1. This extended the continuous scanning range from 5 cm−1 without stabilization to about 1,450 cm−1.
The observed excitation wavenumber for (8)1Π ← X1Σ+ was determined by simultaneously monitoring the laser wavenumber and the ion acceleration voltage, defining the kinetic energy of the beam. The wavenumber of the fundamental Ti:Sa laser was monitored with a four-channel HighFinesse WSU-2 wavemeter and the acceleration voltage of the 227AcF+ ions delivered by CERN-ISOLDE was monitored with a 7.5-digit digital multimeter (Keithley DMM7510) with a precision of 100 mV. To trace long-term drifts of the wavemeter, a grating-stabilized diode laser (TOPTICA dlpro) locked to a hyperfine transition in a Rb vapour cell (TEM CoSy) was also continuously monitored by the wavemeter. The small difference in wavelength between the Rb line (about 780 nm) and the fundamental wavelength of the Ti:Sa in this work (about 774 nm) provided confidence that the drift correction is valid in the fundamental wavelength region where the (8)1Π ← X1Σ+ transition was discovered.
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