ATOMIC DIFFRACTION BY A THIN PHASE GRATING

We present a semiclassical perturbation method for the description of atomic diffraction by a weakly modulated potential. It proceeds in a w ay similar to the treatment of light diffraction by a thin phase,orati ng, and consists in calculating the atomic wavefunction by means of ac tion integrals along the classical trajectories of the atoms in the ab sence of the modulated part of the potential. The capabilities and the validity condition of the method are illustrated on the well-known ca se of atomic diffraction by a Gaussian standing wave. We prove that in this situation the perturbation method is equivalent to the Raman-Nat h approximation, and we point out that the usually-considered Raman-Na th validity condition can lead to inaccuracies in the evaluation of th e phases of the diffraction amplitudes. The method is also applied to the case of an evanescent wave reflection grating, and an analytical e xpression for the diffraction pattern at any incidence angle is obtain ed for the first time. Finally, the application of the method to other situations is briefly discussed.