  We demonstrate coherent one-color photoassociation of a Bose-Einstein
condensate, which results in Rabi oscillations between atomic and molecular
condensates. We attain atom-molecule Rabi frequencies that are comparable to
decoherence rates by driving photoassociation of atoms in an $^{88}$Sr
condensate to a weakly-bound level of the metastable $^{1}S_{0}+^{3}P_{1}$
molecular potential, which has a long lifetime and large Franck-Condon overlap
integral with the ground scattering state. Transient shifts and broadenings of
the excitation spectrum are clearly seen at short times, and they create an
asymmetric excitation profile that only displays Rabi oscillations for blue
detuning from resonance.
