Effect of Atom-Atom Scattering Length and Photon Recoil on the Rotational Distribution of Ultralong-range Rydberg Molecules

Ultralong-range Rydberg molecules (ULRRMs) comprise one or more weakly-bound ground-state atoms embedded in a Rydberg atom’s electron cloud. The production of rotationally-excited ULRRMs is largely unexplored. We present a study of rotational states produced in the two-photon excitation of dimer ULRRMs in 5sns ¹S₀ Rydberg states in ultracold ⁸⁴Sr and ⁸⁶Sr. Spectra for photoexcitation to these states agree with a simple model of the Franck-Condon factor based on the overlap of the molecular rotational state with thermally populated partial waves in the initial scattering state. Comparison of spectra for ⁸⁴Sr and ⁸⁶Sr samples shows suppression of the ground molecular rotational state in ⁸⁶Sr, demonstrating the effect of the large s-wave scattering length in ⁸⁶Sr. Momentum transfer to the system via photon recoil during excitation is also explored. The close matching of recoil momentum of 461nm and 413nm photons used for the two-photon excitation makes it possible to elucidate the role of photon momentum transfer which can lead to formation of N=1 rotational states, whose excitation is forbidden by symmetry considerations if recoil is neglected. The results agree well with theory that includes the effects of photon momentum recoil.