Observation of velocity-tuned resonances in the reflection of atoms from an evanescent light grating

We report on the observation of velocity-tuned (doppleron) resonances in the grazing-incidence reflection of thermal sodium atoms from a moving evanescent light grating created by interference between two counterpropagating laser beams. The relative velocity between the atoms and the grating can be varied continuously by adjusting the frequency of one laser. We observe an exchange of photons between copropagating and counterpropagating fields in an evanescent light grating. The results are in general agreement with a dressed-state model for the diffraction of atoms from such a grating.