  Rapid progress in the experimental control and interrogation of molecules,
combined with developments in precise calculations of their structure, are
enabling new opportunities in the investigation of nuclear and particle physics
phenomena. Molecules containing heavy, octupole-deformed nuclei such as radium
are of particular interest for such studies, offering an enhanced sensitivity
to the properties of fundamental particles and interactions. Here, we report
precision laser spectroscopy measurements and theoretical calculations of the
structure of the radioactive radium monofluoride molecule, $^{225}$Ra$^{19}$F.
Our results allow fine details of the short-range electron-nucleus interaction
to be revealed, indicating the high sensitivity of this molecule to the
distribution of magnetization, currently a poorly constrained nuclear property,
within the radium nucleus. These results provide a direct and stringent test of
the description of the electronic wavefunction inside the nuclear volume,
highlighting the suitability of these molecules to investigate subatomic
phenomena.
