  We report on a measurement of splitting in the excitation spectrum of a
condensate driven by an optical travelling wave. Experimental results are
compared to a numerical solution of the Gross Pitaevskii equation, and analyzed
by a simple two level model and by the more complete band theory, treating the
driving beams as an optical lattice. In this picture, the splitting is a
manifestation of the energy gap between neighboring bands that opens on the
boundary of the Brillouin zone.
