  We study the effect of a one dimensional periodic potential on the dynamic
structure factor of an interacting Bose Einstein condensate at zero
temperature. We show that, due to phononic correlations, the excitation
strength towards the first band develops a typical oscillating behaviour as a
function of the momentum transfer, and vanishes at even multiples of the Bragg
momentum. The effects of interactions on the static structure factor are found
to be significantly amplified by the presence of the optical potential. Our
predictions can be tested in stimulated photon scattering experiments.
