Diffraction of atoms by light: The near-resonant Kapitza-Dirac effect

We have observed the Kapitza-Dirac effect in the scattering of sodium atoms by a nearly resonant standing-wave laser field. The data clearly show diffraction peaks of the atomic momentum transfer at even multiples of the photon momentum. Theoretical predictions for an off-resonant, adiabatic interaction with a two-state system are in reasonable agreement with the data.