We report an experiment showing that atomic diffraction at grazing incidenc
e from an evanescent wave mirror results from polarization gradients in the
evanescent wave which induce transitions among atomic internal states. The
resulting grating can produce large angle coherent beam splittings. We als
o demonstrate atomic interference in the form of a Stuckelberg oscillation
in the diffraction efficiency which is very sensitive to the atom wall van
der Waals potential. [S0031-9007(98)07831-4].