Observation of hyperfine interaction in photoassociation spectra of ultracold RbYb

We report on the creation of ultracold heteronuclear and electronically excited Rb*Yb molecules in a hybrid conservative trap by photoassociation of ultracold Rb-87 and Yb-176. The molecules are formed below the Rb-5p1(P-2(1/2)) + Yb-6s2(S-1(0)) dissociation limit and the resonances are detected using trap-loss spectroscopy. By addressing vibrational levels with binding energies down to E-B = -h x 2.2 THz, we study the change in hyperfine coupling of the diatomic molecule as a function of internuclear separation. We observe a decreasing hyperfine splitting for more tightly bound excited molecular states where the hyperfine splitting is reduced by more than 30% compared to the atomic value for Rb-87.