Precision spectroscopy of ultracold Sr 2 86 halo molecules

We demonstrate the creation and field-free precision spectroscopy of ultracold Sr 2 86 molecules in the halo regime, where vibrational binding energies are dominated by long-range interactions. Using one-photon photoassociation with optimized wavefunction overlap, we produce stable samples of halo and neighboring weakly bound molecular states. We measure vibrational splittings of the three least-bound states with sub-100-Hz uncertainty, enabling a high-precision determination of absolute binding energies. These results establish Sr 2 86 halo molecules as a new platform for precision studies of long-range molecular forces, including quantum electrodynamics effects such as Casimir-Polder interactions, and provide a direct basis for isotope-shift spectroscopy sensitive to nanometer-scale mass-dependent forces.