  We compute the scattering amplitude for Schr\"odinger operators at a critical
energy level, corresponding to the maximum point of the potential. We follow
the wrok of Robert and Tamura, '89, using Isozaki and Kitada's representation
formula for the scattering amplitude, together with results from Bony, Fujiie,
Ramond and Zerzeri '06 in order to analyze the contribution of trapped
trajectories.
