Laser spectroscopy and CP-violation sensitivity of actinium monofluoride.

The apparent invariance of the strong nuclear force under combined charge conjugation and parity (CP) remains an open question in modern physics1,2. Precision experiments with heavy atoms and molecules can provide stringent constraints on CP violation via searches for effects due to permanent electric dipole moments and other CP-odd properties in leptons, hadrons and nuclei3. Radioactive molecules have been proposed as highly sensitive probes for such searches4, but experiments with most such molecules have so far been beyond technical reach. Here we report the production and spectroscopic study of a gas-phase actinium molecule, 227AcF. We observe the predicted strongest electronic transition from the ground state, which is necessary for efficient readout in searches of symmetry-violating interactions. Furthermore, we perform electronic- and nuclear-structure calculations for 227AcF to determine its sensitivity to various CP-violating parameters, and find that a realistic, near-term experiment with a precision of 1 mHz would improve current constraints on the CP-violating parameter hyperspace by 3 orders of magnitude. Our results thus highlight the potential of 227AcF for exceptionally sensitive searches of CP violation.