Effect of an applied magnetic field on  ⁸⁴Sr n¹D_2 ultralong-range dimers

Ultralong-range Rydberg dimer molecules, which comprise a Rydberg atom whose electron cloud contains a ground-state atom weakly bound by scattering of the Rydberg electron, can be created by photoassociation. In the present work, we use ⁸⁴Sr to examine the effects of an external magnetic field on ¹D₂ Rydberg dimers and probe the transition from the "weak" field regime, where electron-atom scattering causes the electronic orbital angular momentum to couple to the molecular axis, to the "strong" field regime, where coupling to the magnetic field dominates. In zero field, a well-defined series of dimer vibrational levels is seen. As the field is initially increased (B ≤ 0.3 G), the parent atomic state undergoes Zeeman splitting, whereas the dimer levels remain essentially unchanged. At higher applied fields, where the Zeeman interaction energy is comparable to the molecular binding energy, the photoassociation spectra undergo a series of dramatic changes, and separate vibrational series appear associated with each Zeeman level. The present results are analyzed using a theoretical model that accounts for both intramolecular angular momenta mixing and the presence of an external field. The model accounts for many of the features seen in the experimental data and provides new insights into the transition from the weak- to the strong-field regime of dimer formation.