Electrons reveal nuclear magnetism inside a molecule for the first time
Researchers from CERN and MIT have recorded, for the first time inside a molecule, how magnetism is distributed within a radioactive nucleus by using electrons as sensitive probes. The result — reported in Science on Oct. 23 — is the first molecular observation of the Bohr–Weisskopf effect, a subtle nuclear magnetization effect previously seen only in isolated atoms.
Why this matters
The Bohr–Weisskopf effect describes how the internal distribution of magnetization in a nucleus alters the magnetic interaction with surrounding electrons, producing tiny shifts in atomic or molecular energy levels (hyperfine structure). Observing this effect inside a molecule opens a new pathway to ultra-precise measurements that can amplify tiny symmetry violations in nature and potentially reveal physics beyond the Standard Model.
How the experiment worked
The team studied radium monofluoride (RaF), a short-lived molecule made from radium and fluorine. Radium nuclei of the isotope radium-225 exhibit an octupole deformation — a pear-like asymmetry — that enhances sensitivity to certain symmetry-breaking effects. Because such nuclei are radioactive and rare, they are difficult to produce and study: the radium isotopes used here decay on timescales of days, and the molecules themselves survived only fractions of a second.
At CERN’s ISOLDE facility, high-energy protons struck a uranium target to generate radium-225, which was then combined with fluorine gas to create RaF. The experiment registered roughly 50 usable molecules per second in the right quantum state for spectroscopy.
Measuring the effect
Researchers illuminated the molecular beam with multiple laser frequencies and recorded extremely small shifts in absorbed and emitted light to produce a high-resolution spectrum. In molecules, electrons shuttle between two nuclei, which can blur nuclear magnetic signals; in RaF, the fluorine atom is electronically simple, allowing signals from the heavier radium nucleus to be isolated.
“The electron actually probes inside the nucleus, so you can no longer treat it as a long-range interaction. Instead, it starts to sense the internal properties of the radium nucleus itself,” said Shane Wilkins, lead author and physicist at MIT.
Some of the observed spectral shifts could be explained only if the electrons were sensitive to the internal magnetization of the radium nucleus — a clear signature of the Bohr–Weisskopf effect in a molecular environment. The team also modeled the effect theoretically and found good agreement with the measurements.
Next steps and significance
With RaF’s internal structure mapped, the molecule becomes a promising platform for higher-precision tests. The next experimental steps include laser slowing and trapping of these molecules to extend interaction times and improve measurement sensitivity. Such advances could help probe tiny symmetry violations — for example, those connected to why the universe contains far more matter than antimatter.
Bottom line: This work demonstrates that electrons in a molecule can directly probe the internal magnetic structure of a nucleus, opening new experimental routes for precision tests of fundamental physics.
