Pinning down electron correlations in RaF via spectroscopy of excited states and high-accuracy relativistic quantum chemistry

We report the spectroscopy of the 14 lowest excited electronic states in the radioactive molecule radium monofluoride (RaF). The observed excitation energies are compared with fully relativistic state-of-the-art Fock-space coupled cluster (FS-RCC) calculations, which achieve an agreement of ≥ 99.64% (within ∼12 meV) with experiment for all states. Guided by theory, a firm assignment
of the angular momentum and term symbol is made for 10 states and a tentative assignment for 4 states. The role of high-order electron correlation and quantum electrodynamics effects in the excitation energy of excited states is studied, found to be important for all states. Establishing the simultaneous accuracy and precision of calculations is an important step for research at the intersection of particle, nuclear, and chemical physics, including searches of physics beyond the
Standard Model, for which RaF is a promising probe.