The hyperfine structure of radium monofluoride has been precisely measured by scientists in the US and internationally, allowing the effects of the finite size of nuclear magnetisation to be observed for the first time. This increases the likelihood that scientists could utilize the molecule to search for extensions to the standard model of particle physics, including signs of dark matter or origins of matter–antimatter asymmetry.
Certain theories of physics that extend beyond the standard model suggest that such signs may be identified through minute shifts in the frequencies of energy levels of specific states within atoms and molecules. Molecules are particularly effective for this purpose, as they are significantly more responsive to external fields. ‘What we desire in our experiments is to stretch the electric field,’ explains Silviu-Marian Udrescu from Johns Hopkins University in Maryland. ‘In an atom, you must apply a massive electric field to achieve that since the electron cloud is highly symmetrical. In a molecule, the cloud is already considerably polarised.’
Molecules are expected to be particularly responsive if they include an atom with a nucleus exhibiting a rare octupole deformation, resulting in a pear-shaped form. ‘This phenomenon exists in just a few areas of the nuclear charts, all of which are radioactive,’ states Shane Wilkins at Michigan State University. This makes the synthesis, cooling, and examination of the molecules before they disintegrate quite difficult.
In the latest research, Udrescu, Wilkins, and their colleagues from Ronald Fernando Garcia Ruiz’s team at the Massachusetts Institute of Technology, along with global collaborators, utilized the collinear resonance ionisation spectroscopy apparatus at Cern. They bombarded a uranium carbide target with high-energy protons and subsequently introduced tetrafluoromethane gas to create octupole-deformed nucleus radium-225 monofluoride ions. These were rapidly extracted, sorted by mass, and excited using three different lasers to evaluate electronic transitions in the molecules.
Analysis of the hyperfine structure – the splitting of the energy levels due to interactions between electronic and nuclear spins – uncovered something particularly intriguing. ‘In most of our experiments, we treat the nucleus as a point-like [magnetic] dipole,’ asserts Wilkins. ‘What’s remarkable about this molecule is that … we see a departure from that view because the electron spends significant time within the nucleus.’ Thus, the electron interacts with the entire nucleus structure.
Wilkins states this effect, which has not been previously observed in a molecule, supports theoretical predictions and demonstrates that electron energy shifts could potentially aid in distinguishing between different nuclear structure models in the future. He is currently developing techniques to cool the molecules at Michigan State University, enabling higher-precision spectroscopy that may have implications for chemistry and astrophysics.
‘This is an exceptionally impressive experimental achievement, but I would characterize it as evolutionary, not revolutionary,’ comments chemical physicist David Nesbitt at the University of Colorado Boulder. ‘Many of these measurements have been conducted before, and they confirm high-level ab initio calculations with improved accuracy than has been achieved to date… I don’t perceive a direct link to how this will challenge the standard model.’ The researchers reply that even though the molecule has been examined in earlier studies, none have achieved the precision necessary to elucidate the hyperfine structure. ‘Without our measurements, which are the first of this nature in any molecules with such short half-lives, there would be no future precision investigations into the fundamental symmetries of the universe in 225RaF,’ states Wilkins.
On 31 October 2025 the association of Shane Wilkins was revised.