Diffraction of He Atoms at a Si(111) 7 × 7 Surface

We report the diffraction of He atoms at the Si (111) 77 reconstructed surface. The atom diffraction patterns consist of numerous sharp peaks filling out the seventh-order net observed by low-energy electron diffraction (LEED). In contrast to low-energy electron diffraction, the diffraction intensities in this seventh-order net are determined by only the outermost exposed layer of silicon atoms. These results should be sufficient to allow a semiclassical scattering calculation to test critically the various structural models which have been proposed for this reconstruction.