Anomalies in the rotational spectra of ⁸⁶Sr ULRRM dimers

We report a study of the rotational spectra of ultra-long-range Rydberg dimer molecules (ULRRMs) produced by two-photon excitation in an ultracold strontium gas. These investigations involved both singlet and triplet molecular states in the ⁸⁴Sr and ⁸⁶Sr isotopes. In the case of ⁸⁶Sr triplet dimers, we observed an anomalous phenomenon that distinguishes it from other dimer species studied. The distribution of rotational states is observed to vary with the detuning of the intermediate state, and the population of N=3 rotational states is unexpectedly large. Despite considering factors such as Rabi oscillations, polarization, magnetic field effects, and the presence of some accidental resonance we have, as yet, no definitive hypothesis as to the physical mechanism(s) responsible for this novel behavior. Possible explanation might involve the unusually large ⁸⁶Sr s-wave atom-atom scattering length, or the coupling of spin and rotational angular momentum.