Heteronuclear Rydberg Molecules

We report the creation of heteronuclear ultralong-range Rydberg-molecule dimers by excitation of minority Sr-88 atoms to 5sns(3)S(1) Rydberg states (n = 31-39) in a dense background of Sr-84. We observe an isotope shift of the v = 0 vibrational state over this range of n and compare our measurements with a theoretical prediction and a simple scaling argument. At low principal quantum number, the isotope shift is sufficiently large to produce heteronuclear dimers with almost perfect fidelity. When the spectral selectivity is limited, we obtain a lower bound on the ratio of heteronuclear to homonuclear excitation probability of 30 to 1 by measuring the scaling of the molecular excitation rate with varying relative densities of Sr-88 and Sr-84 in the ultracold mixture.