  It has recently been demonstrated that quantum degenerate gases of very
weakly bound molecules can be produced by atomic gases with Feshbach
resonances. More strongly bound molecules can be produced with Raman
photoassociation of a quantum gas, although this process has not yet been shown
to produce a quantum degenerate molecular gas. In principle, Feshbach resonance
and Raman photoassociation can be quantum-mechanically reversible, and lead to
collective coherent phenomena such as Rabi cycling between an atomic and a
molecular gas. However, such atom-molecular coherence has only partly been
realized experimentally. Effects that may limit coherence include thermalizing
elastic collisions, inelastic collisions, spontaneous Raman scattering, and
pairing field formation. Here, we demonstrate a method that circumvents these
limitations, based on Raman photoassociation of atoms in an optical lattice and
driven into a Mott insulator state. We find that the Raman photoassociation
transition is resolved into discrete lines corresponding to the quantized
lattice site occupancies, and demonstrate that this provides a new method to
accurately determine the distribution of site occupancies and the atom-molecule
scattering length. Furthermore, we observe a Raman-induced oscillation of the
central core of the gas, containing about 30% of the atoms, between an atomic
and a molecular gas.
