  We study experimentally superradiance in a Bose-Einstein condensate using a
two-frequency pump beam. By controlling the frequency difference between the
beam components, we measure the spectrum of the backward (energy-mismatched)
superradiant atomic modes. In addition, we show that the populations of these
modes display coherent time-dynamics. These results are compared to a
semi-classical model based on coupled Schroedinger-Maxwell equations.
