Abstract
Molecules containing heavy radioactive nuclei are predicted to be extremely sensitive to violations of the fundamental symmetries of nature. The nuclear octupole deformation of certain radium isotopes massively boosts the sensitivity of radium monofluoride molecules to symmetry-violating nuclear properties. Moreover, these molecules are predicted to be laser coolable. Here we report measurements of the rovibronic structure of radium monofluoride molecules, which allow the determination of their laser cooling scheme. We demonstrate an improvement in resolution of more than two orders of magnitude compared to the state of the art. Our developments allowed measurements of minuscule amounts of hot molecules, with only a few hundred per second produced in a particular rotational state. The combined precision and sensitivity achieved in this work offer opportunities for studies of radioactive molecules of interest in fundamental physics, chemistry and astrophysics.
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Data availability
The processed spectra used for the analysis and supporting the findings of these studies are provided in ref. 57. The complete raw data is available from the corresponding authors upon request. Source data are provided with this paper.
Code availability
The Python script used for fitting individual peaks as well as a PGOPHER file containing a fitted spectra for the 0′ ← 0″ and 1′ ← 1″ rovibronic transitions are provided in ref. 57. The code used for processing the raw data is available from the corresponding authors upon request.
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Acknowledgements
This work was supported by the Office of Nuclear Physics, US Department of Energy, under grants DE-SC0021176 and DE-SC0021179 (S.M.U., S.G.W., R.F.G.R., A.J.B.); the MISTI Global Seed Funds (S.M.U.); Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) – Projektnummer 328961117 – SFB 1319 ELCH (A.A.B., R.B., K.G., T.F.G.); STFC grants ST/P004423/1 and ST/V001116/1 (M.L.B., K.T.F., H.A.P., J.R.R., J.W.); Belgian Excellence of Science (EOS) project No. 40007501 (G.N.); KU Leuven C1 project No. C14/22/104 (M.A.-K., T.E.C., R.P.d.G., G.N.); FWO project No. G081422N (M.A.-K., G.N.); International Research Infrastructures (IRI) project No. I001323N (M.A.-K., T.E.C., R.P.d.G., A.D., S.G., L.L., G.N., B.v.d.B.); the European Unions Grant Agreement 654002 (ENSAR2); LISA: European Union’s H2020 Framework Programme under grant agreement no. 861198 (M.A., D.H., M.N., J.W.); The Swedish Research Council (2016-03650 and 2020-03505) (D.H., M.N.). The National Key RD Program of China (No: 2022YFA1604800) (X.F.Y.) and the National Natural Science Foundation of China (No:12027809). (X.F.Y.). We thank R. Field, T. Isaev, L. Skripnikov and A. Zaitsevskii for insightful discussions.
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S.M.U. and S.G.W. contributed equally to this work. S.M.U. led the data analysis and S.G.W. led the experiments. S.M.U., S.G.W., A.A.B., M.A.-K., R.F.G.R., M.A., I.B., R.B., M.L.B., C.L.B., A.J.B., K.C., T.E.C., A.D., S.F., K.G., S.G., T.F.G., R.H., A.K., S.K., L.L., M.N., H.A.P., J.R.R., S.R., B.v.d.B., A.R.V., Q.W., J.W. and C.Z. performed the experiment. S.M.U. and A.A.B. performed the data analysis. S.M.U. prepared the figures. S.M.U., S.G.W. and R.F.G.R. prepared the manuscript. All authors discussed the results and contributed to the manuscript at different stages.
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Extended data
Extended Data Fig. 1 Example of rovibronic spectra of the second step used in the experimental scheme.
The red dots represent the measured data while the blue line is the best fit to the data. The x-axis corresponds to the wavenumber of the second laser used in the excitation-ionization scheme, Doppler-corrected to the molecular rest frame and shifted by \({{{{T}}}}_{{\Pi }_{1/2,0}}=13284.427\,{{{{\rm{cm}}}}}^{-1}\) (see Methods for the details of the fit). The y-axis shows the rate in arbitrary units (a.u.). The errorbars show one standard deviation statistical uncertainty.
Extended Data Fig. 2 Example of measured spectra for the \({1}^{{\prime} }\leftarrow {1}^{{\prime\prime} }\) transitions.
In the centre, in blue, we present the simulated RaF spectrum for J ≤ 100, over a range of ~ 70 cm−1 (J is the rotational quantum number of the rotational levels in the X2Σ+ electronic level). Figures in magnified views show measured spectra for different regions (note the broken x-axis present in some of the figures). The connected red dots show the experimental data, whereas the continuous blue line represents the best fit to the data. The errorbars indicate one standard deviation statistical uncertainty. For each spectrum we also show the covered range of J-values (see the main text and Methods for the details of the fit). The values on the x-axis correspond to the wavenumber of the first laser used in the excitation-ionization scheme, Doppler-corrected to the molecular rest frame and shifted by \({{{{T}}}}_{{\Pi }_{1/2,0}}=13284.427\,{{{{\rm{cm}}}}}^{-1}\). On the y-axis we show the rate in arbitrary units (a.u.).
Extended Data Fig. 3 Location of the bandheads in the \({0}^{{\prime} }\leftarrow {0}^{{\prime\prime} }\) rovibronic transitions.
The bandhead locations are indicated with green arrows. The red dots represent the measured data while the blue line is the best fit to the data (see Methods for the details of the fit). The x-axis corresponds to the wavenumber of the first laser used in the excitation-ionization scheme, Doppler-corrected to the molecular rest frame and shifted by \({{{{T}}}}_{{\Pi }_{1/2,0}}=13284.427\,{{{{\rm{cm}}}}}^{-1}\). The y-axis shows the rate in arbitrary units (a.u.). The errorbars show one standard deviation statistical uncertainty.
Extended Data Fig. 4 Example of \({0}^{{\prime} }\leftarrow {0}^{{\prime\prime} }\) rovibronic spectra for different second-step laser wavenumbers.
The red, blue and green dots correspond to separate scans of the first step laser, while the second-step laser wavenumber, Doppler-shifted to the molecular rest frame, was kept fixed at 15485.23(2) cm−1, 15485.39(2) cm−1, and 15485.56(2) cm−1, respectively. Increasing the wavenumber of the second-step laser facilitated the observation of new transitions starting from levels with higher rotational quantum numbers, J, in the A2Π1/2 ← X2Σ+ spectrum (the new peaks appearing on the left). The maximum J-value of the shown spectra increases from \({{{{J}}}}_{\max }=25.5\) to \({{{{J}}}}_{\max }=27.5\). The x-axis corresponds to the wavenumber of the first laser used in the excitation-ionization scheme, Doppler-corrected to the molecular rest frame and shifted by \({{{{T}}}}_{{\Pi }_{1/2,0}}=13284.427\,{{{{\rm{cm}}}}}^{-1}\). The y-axis shows the rate in arbitrary units (a.u.). The errorbars show one standard deviation statistical uncertainty.
Source data
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Statistical source data.
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Source Data Extended Data Fig. 1
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Source Data Extended Data Fig. 4
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Udrescu, S.M., Wilkins, S.G., Breier, A.A. et al. Precision spectroscopy and laser-cooling scheme of a radium-containing molecule. Nat. Phys. 20, 202–207 (2024). https://doi.org/10.1038/s41567-023-02296-w
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DOI: https://doi.org/10.1038/s41567-023-02296-w
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