Resolving rotationally excited states of ultralong-range Rydberg molecules

We report experimental observations of the rotational structure in photoassociative spectroscopy of ultralong-range Rydberg molecules (ULRRMs) in an ultracold gas of 86Sr. ULRRM spectroscopy probes scattering wave functions at much larger internuclear separations than photoassociative spectroscopy of low-lying electronic states. At such separations photon recoil momentum can lead to the transfer of significant angular momentum to the Rydberg molecule, evidence of which is provided through the distribution of excited rotational states. The visibility of the rotational structure is enhanced because, for collisions between ground-state 86Sr atoms, the large, near-resonant s-wave scattering length approaches the size of the ULRRM. Similar enhancement is not seen for 84Sr, which has a much smaller scattering length. Results are interpreted with the aid of a theory that accounts for the recoil momentum associated with photoexcitation and the large s-wave scattering length in the entrance channel. While the observed rotational splittings and the qualitative trends in the relative intensities of the spectral features are well described by theory, the effects of recoil momentum are more prominent in the measured data than in the theory.

Automated summary

This study reports experimental observations of the rotational structure in ultralong-range Rydberg molecules (ULRRMs) in an ultracold gas of 86Sr. It is found that at larger internuclear separations, the recoil momentum of photons can transfer significant angular momentum to the Rydberg molecule, which is confirmed through the distribution of excited rotational states. Additionally, the visibility of the rotational structure is enhanced due to the near-resonant s-wave scattering length approaching the size of the ULRRM.