  We report the creation of heteronuclear ultralong-range Rydberg-molecule
dimers by excitation of minority $^{88}\text{Sr}$ atoms to $5sns\,^3S_1$
Rydberg states ($n=31-39$) in a dense background of $^{84}\text{Sr}$. We
observe an isotope shift of the $\nu=0$ vibrational state over this range of
$n$ and compare our measurements with a theoretical prediction and a simple
scaling argument. With the appropriate choice of 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 $^{88}\text{Sr}$ and $^{84}\text{Sr}$ in the ultracold
mixture.
