Ionization potential of radium monofluoride

The ionization potential (IP) of radium monofluoride (RaF) was measured to be 4.969(2)[10] eV, revealing a relativistic enhancement in the series of alkaline earth monofluorides. The results are in agreement with a relativistic coupled-cluster prediction of 4.969[7] eV, incorporating up to quantum electrodynamics corrections. Using the same computational methodology, an improved calculation for the dissociation energy ($D_{0}$) of 5.54[5] eV is presented. This confirms that radium monofluoride joins the small group of diatomic molecules for which $D_{0}>\mathrm{IP}$, paving the way for precision control and interrogation of its Rydberg states.

💡 Research Summary

The paper reports the first high‑precision measurement of the ionization potential (IP) of radium monofluoride (RaF) and provides an updated theoretical determination of its dissociation energy (D₀). Using the Collinear Resonance Ionization Spectroscopy (CRIS) setup at CERN’s ISOLDE facility, beams of ²²⁶Ra¹⁹F⁺ ions were produced, neutralized, and overlapped with pulsed laser beams in an ultra‑high‑vacuum interaction region. Two distinct resonance‑ionization schemes were employed: a two‑step scheme that excites the A²Π₁/₂(v = 0) ← X²Σ⁺(v = 0) transition followed by direct ionization, and a three‑step scheme that adds an intermediate E²Σ⁺ state before ionization. By scanning the ionization‑laser wavelength and recording the resulting RaF⁺ ion count, the authors extracted the ionization threshold from the saturation point of a fitted sigmoid curve. The two‑step data yielded an IP of 4.966 eV, while the three‑step data gave 4.970 eV; a weighted average gives a final experimental value of 4.969 eV with a combined statistical and systematic uncertainty of 0.010 eV.

On the theoretical side, the authors performed relativistic coupled‑cluster calculations (CCSD(T)) using the Dirac‑Hartree‑Fock Hamiltonian as implemented in the DIRAC19 code. They employed Dyall core‑valence correlation basis sets (cvn z, n = 2–4) augmented with diffuse functions (s‑aug‑cvn z) and extrapolated to the complete basis‑set limit (CBSL). Electron correlation was treated for 49 electrons with a virtual‑space cutoff of 50 a.u.; additional calculations correlating all 97 electrons with a larger cutoff were used to estimate the full‑core correction. Corrections for higher‑order excitations (full triples), Breit interaction, and quantum electrodynamics (QED) effects (Uehling vacuum polarization and self‑energy via the Flambaum‑Ginges potential) were added. The cumulative theoretical IP is 4.969 eV, matching the experimental result within the quoted uncertainties. Using the same methodology, the dissociation energy D₀ was computed as 5.54 eV; the experimental determination (5.61 ± 0.24 eV) is consistent with this value.

A key finding is that D₀ exceeds the IP (D₀ > IP), a rare property shared only by a few diatomic molecules such as BaF. This inequality implies that RaF can be excited to high‑lying Rydberg states without immediate fragmentation, enabling long‑lived, highly polarizable states suitable for quantum simulation, precision spectroscopy, and searches for physics beyond the Standard Model. The authors discuss the relevance of RaF for probing P‑ and T‑violating interactions, especially given the octupole deformation of certain radium isotopes, which enhances sensitivity to such effects.

The paper also places RaF in the broader context of alkaline‑earth monofluorides. Experimental IP values for CaF, SrF, BaF, and RaF show a non‑monotonic trend: the IP decreases from CaF to BaF and then rises again for RaF. This “relativistic enhancement” is attributed to the strong increase in electron mass and spin‑orbit coupling in the heavy radium atom, which raises the binding energy of the valence electron. The excellent agreement (within one standard deviation) between the measured IPs and the relativistic coupled‑cluster predictions across the series validates the theoretical approach and underscores the necessity of including relativistic and QED corrections for heavy‑element molecules.

In summary, the work delivers a benchmark experimental IP for RaF, confirms the rare D₀ > IP condition, and provides state‑of‑the‑art theoretical values that incorporate high‑order electron correlation, Breit, and QED effects. These results open the door to precision control of RaF’s Rydberg manifold, facilitating advanced experiments in quantum information processing, ultracold chemistry, and fundamental symmetry tests.