  We have made a high resolution study of the specularity of the atomic
reflection from an evanescent wave mirror using velocity selective Raman
transitions. We have observed a double structure in the velocity distribution
after reflection: a peak consistent with specular reflection and a diffuse
reflection pedestal, whose contribution decreases rapidly with increasing
detuning. The diffuse reflection is due to two distinct effects: spontaneous
emission in the evanescent wave and a roughness in the evanescent wave
potential whose amplitude is smaller than the de Broglie wavelength of the
reflected atoms.
