Dissociation energy of the strontium van der Waals dimer

Alkaline-earth metals in the electronic ground state form weakly bound molecules due to the van der Waals attraction. The closed-shell nature of these molecules gives rise to narrow dipole-forbidden transitions between ground vibrational states that are insensitive to external fields. We leverage this to determine the dissociation energy of the strontium dimer (⁸⁸Sr₂), improving the measurement uncertainty by 5 orders of magnitude over early experiments with rare-gas matrices and heat-pipe ovens. We achieve this through frequency-comb-assisted Raman spectroscopy of the vibrational splitting between the absolute ground state and the least bound vibrational state, and two-color photodissociation to reference the binding energy of the latter with respect to the atomic threshold.