Resolving the rotationally excited states of ultralong-range Rydberg molecules

We report experimental observations of rotational structure in photo-associative 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 does photo-associative spectroscopy to low-lying electronic states [1]. At such separations (R_n=1400a₀ at n=31), the kinetic energy of the initial colliding pairs supports higher partial wave scattering channels, which, coupled with the recoil momentum resulting from photoexcitation, allows creation of rotationally excited ULRRMs. The visibility of the rotational structure is further enhanced by the suppression of the s-wave channel due to the near-resonant scattering properties of ⁸⁶Sr (a_s=811a₀). Results are discussed with the aid of theory that accounts for the recoil momentum associated with photoexcitation and the large s-wave scattering length in the entrance channel.