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In the past few years, spectroscopy of radioactive molecules has been performed at ISOLDE (CERN) using the Collinear Resonance Ionization Spectroscopy (CRIS) experiment [1]. Given their structure and chemical properties, radioactive molecules are promising candidates for studies in different fields [2], including for more efficient extraction of refractory elements from ISOLDE targets [3].
Many reference electric quadrupole moments (EQM) of stable isotopes [4] have been extracted from measurements of the coupling constant of isotopes in diatomic molecules, e.g., KF (for 39,41K), HCl (for 35,37Cl), and HI (for 127I) [5,6,7], in combination with accurate calculations of the electric field gradient in the molecule. For some of these elements, the electric-field gradient is too small in atomic systems, or the element is not accessible for laser spectroscopy in its atomic form; thus, molecular spectroscopy is the only option for studying the quadrupole moments of their radioactive isotopes.
Following the successful high-resolution studies of 225,226RaF [8,9], the CRIS experiment at ISOLDE performed the first hyperfine-resolved resonance ionization spectroscopy of 223RaF, yielding the first measurement of the EQM in a short-lived radioactive molecule (t1/2 = 11.4 days) for 223Ra (I = 3/2). Additionally, the change in charge radii of 223,225,226Ra have been extracted from spectroscopy of RaF molecules with a precision comparable to atomic studies [10,11]. Finally, the magnetic dipole moment and finite magnetization contribution of 223,225RaF were extracted in a nuclear model-independent way.
This contribution will focus on the measured nuclear moments of 223RaF and compare them with atomic and ionic measurements reported in the literature. Our measurement, in combination with state-of-the-art relativistic coupled-cluster calculations of the electric field gradient in the molecule [12,13,14], provides an accurate and precise value for the 223Ra quadrupole moment. Thus, serving as a reference for the extraction of quadrupole moments of other isotopes. This work also demonstrates how molecular laser spectroscopy could offer a new pathway for extracting unknown nuclear moments of radioactive isotopes, not accessible in atomic form, using suitable radioactive molecules [15].
[1] Garcia Ruiz, R.F., et al. "Spectroscopy of short-lived radioactive molecules." Nature 581.7809 (2020): 396-400. https://doi.org/10.1038/s41586-020-2299-4
[2] Opportunities for Fundamental Physics Research with Radioactive Molecules, Rep Prog Phys. 2024 Jul 12;87(8). https://doi.org/10.1088/1361-6633/ad1e39
[3] Au, Mia. Production of actinide atomic and molecular ion beams at CERN-ISOLDE. No. CERN-THESIS-2023-228. 2023.
[4] Pyykkö, P. (2018). Year-2017 nuclear quadrupole moments. Molecular Physics, 116(10), 1328-1338.
[5] Kello, Vladimir. "Determination of the quadrupole moment of the halogen nuclei (Cl, Br, I) from molecular data." Molecular Physics 89.1 (1996): 127-137. https://doi.org/10.1080/002689796174047
[6] Kellö, Vladimir, and Andrzej J. Sadlej. "The quadrupole moment of the 39K and 41K nuclei from microwave data for KF and KCl." Chemical physics letters 292.4-6 (1998): 403-410. https://doi.org/10.1016/S0009-2614(98)00680-0
[7] Bieroń, Jacek, et al. "Nuclear quadrupole moments of bromine and iodine from combined atomic and molecular data." Physical Review A 64.5 (2001): 052507. https://doi.org/10.1103/PhysRevA.64.052507
[8] Wilkins, S. G., Udrescu, S. M., Athanasakis-Kaklamanakis, M., Garcia Ruiz, R. F., Au, M., Belošević, I., ... & Zülch, C. (2025). Observation of the distribution of nuclear magnetization in a molecule. Science, 390(6771), 386-389. https://doi.org/10.1126/science.adm7717
[9] S.-M. Udrescu, et al., Precision spectroscopy and laser-cooling scheme of a radium-containing molecule”, Nature Physics (2024) online January 9, https://doi.org/10.1038/s41567-023-02296-w
[10] Wansbeek, L. W., Schlesser, S., Sahoo, B. K., Dieperink, A. E. L., Onderwater, C. J. G., & Timmermans, R. G. E. (2012). Charge radii of radium isotopes. Physical Review C—Nuclear Physics, 86(1), 015503. https://doi.org/10.1103/PhysRevC.86.015503
[11] Lynch, K. M., Wilkins, S. G., Billowes, J., Binnersley, C. L., Bissell, M. L., Chrysalidis, K., ... & Yang, X. F. (2018). Laser-spectroscopy studies of the nuclear structure of neutron-rich radium. Physical Review C, 97(2), 024309. https://doi.org/10.1103/PhysRevC.97.024309
[12] Kudashov, A. D., Petrov, A. N., Skripnikov, L. V., Mosyagin, N. S., Isaev, T. A., Berger, R., & Titov, A. V. (2014). Ab initio study of radium monofluoride, RaF, as a candidate to search for P-and T, P-violation effects. Phys. Rev. A 90, 052513. https://doi.org/10.1103/PhysRevA.90.052513
[13] Petrov, A. N., & Skripnikov, L. V. (2020). Energy levels of radium monofluoride RaF in external electric and magnetic fields to search for P-and T, P-violation effects. Physical Review A, 102(6), 062801. https://doi.org/10.1103/PhysRevA.102.062801
[14] Skripnikov, L. V. (2020). Nuclear magnetization distribution effect in molecules: Ra+ and RaF hyperfine structure. The Journal of Chemical Physics, 153(11). https://doi.org/10.1063/5.0024103.
[15] Dognon, Jean-Pierre, and Pekka Pyykkö. "Determining nuclear quadrupole moments of Bi and Sb from molecular data." Physical Chemistry Chemical Physics 25.4 (2023): 2758-2761. https://doi.org/10.1039/D2CP04747K