  We report on the experimental investigation of the response of a
three-dimensional Bose-Einstein condensate (BEC) in the presence of a
one-dimensional (1D) optical lattice. By means of Bragg spectroscopy we probe
the band structure of the excitation spectrum in the presence of the periodic
potential. We selectively induce elementary excitations of the BEC choosing the
transferred momentum and we observe different resonances in the energy
transfer, corresponding to the transitions to different bands. The frequency,
the width and the strength of these resonances are investigated as a function
of the amplitude of the 1D optical lattice.
