It is experimentally demonstrated that diffraction beams from the scattering of He atoms from surfaces can be observed at grazing final angles of up to ${\theta }_f\approx 90°$ with respect to the surface normal. For He atom scattering from Rh(311) under conditions in which a diffraction beam exits at a grazing angle, a broad scattered intensity appears that is interpreted as diffuse scattering from collisions with the small density of step defects resulting from the unavoidable miscut of the surface. Theoretical models for real and evanescent diffraction beams support the conclusion that this diffuse intensity is due to scattering by step defects from a diffraction beam under grazing exit conditions. For scattering in the plane perpendicular to the direction of the steps, this diffuse scattering signal is much stronger in one direction than in the opposite direction where the crystal azimuthal angle is rotated by 180°. The asymmetry in intensity is interpreted as due to the much larger number of steps up in one direction on the surface due to the crystal miscut, thus such measurements can uniquely determine the miscut direction. This experiment opens new possibilities for the characterization of surface defects with diffraction techniques.

• Received 17 July 2001

DOI:https://doi.org/10.1103/PhysRevB.65.165435