  Diffraction of multi-level atoms by an evanescent wave reflective diffraction
grating is modeled by numerically solving the time-dependent Schr\"{o}dinger
equation. We are able to explain the diffraction observed in experiments with
metastable Neon. This is not possible using a two-level atom model. The
multi-level model predicts sensitive dependence of diffraction on the laser
polarization and on the intensity ratio of incoming and reflected laser beams.
