Diffraction of multi-level atoms by an evanescent wave reflective diff
raction grating is modeled by numerically solving the time-dependent S
chrodinger equation. We are able to explain the diffraction observed i
n experiments with metastable neon. This is not possible using a two-l
evel atom model. The multi-level model predicts sensitive dependence o
f diffraction on the laser polarization. For pure p-polarization of th
e lasers a dark state forms. Associated with this is high order diffra
ction.