Theoretical study of the effect of beam misalignment in fast-atom diffraction at surfaces

Abstract

Typical applications of the fast-atom diffraction technique exploit incidence along low-index directions to extract accurate structural information on surfaces from the diffraction pattern. The data interpretation in this case is well developed and simplified by the axial channeling conditions, where the three-dimensional projectile-surface interaction appears effectively averaged over the fast-motion direction. We study theoretically the diffraction of fast-atom beams that are significantly misaligned with respect to a low-index direction at the surface. With full quantum wave-packet calculations, we address He atom scattering from the LiF(001) surface within a wide range of azimuthal and polar incidence angles. In particular, we are interested in the transitions between low-index directions upon the azimuthal rotation of the beam, and in the question of the choice of an adequate axial channeling approximation. We make use of the energy criterion to analyze the diffraction patterns and to discuss how the momentum exchange projected on the beam direction affects the diffraction.