  We consider in the unidirectional approximation the propagation of an ultra
short electromagnetic pulse in a resonant medium consisting of molecules
characterized by a transition operator with both diagonal and non-diagonal
matrix elements. We find the zero-curvature representation of the reduced
Maxwell-Bloch equations in the sharp line limit. This can be used to develop
the inverse scattering transform method to solve these equations. Finally we
obtain two types of exact traveling pulse solutions, one with the usual
exponential decay and another with an algebraic decay.
