Effects of intermediate state detuning, atom-atom interactions, and electron spin in the two-photon excitation 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 rotational excitations of ULRRMs are largely unexplored. We present the rotational-excitation-resolved two-photon excitation spectra of dimer ULRRMs in cold ⁸⁶Sr and ⁸⁴Sr gases. Studies using ⁸⁶Sr 5sns ³S₁ (m=+1) dimer ULRRMs created in a week magnetic field (0.5G) show unexpected strong variations of rotational-state distributions that depend sensitively on the intermediate state detuning. Photoexcitation spectra for dimer ULRRMs of ⁸⁴Sr 5sns ³S₁ (m=+1), ⁸⁶Sr 5sns ¹S₀, and ⁸⁴Sr 5sns ¹S₀ states, however, yield rotational-state distributions that agree with the predictions of a model that considers both atom-atom scattering in the parent cold gas and the photon momentum transfer that accompanies Rydberg excitation. The reasons for the unexpected behavior in ⁸⁶Sr 5sns ³S₁ are the subject of ongoing studies. The observations suggest that ground-state atom-atom interactions, spin angular momentum, and excitation pathway all play important roles in determining the final distribution of rotational states.