Grazing incidence fast-atom diffraction (GIFAD) uses keV atoms to probe the topmost layer of crystalline surfaces. The atoms are scattered by the potential energy landscape of the surface onto elastic diffraction spots located at the Bragg angles and on the Laue circle. However, atoms transfer a significant momentum to the surface, giving rise to possible phonon excitation. This causes the inelastic intensity to spread above and below the circle along the direction of the surface normal. The relative intensity of the elastic contribution is well fitted by the Debye-Waller model adapted to GIFAD, but the composite azimuthal line profile governing the ability to resolve diffraction spots has not been investigated in detail. The paper reports a series of diffraction measurements of helium on a LiF(001) surface revealing marked differences in the polar ($\theta$) and lateral ($\varphi$) inelastic profiles but also similarities in the evolution of their line widths ${\sigma }_{\theta }$ and ${\sigma }_{\varphi }$. We observe two regimes: When elastic diffraction is significant, the Laue circle appears as a reference for inelastic diffraction; the azimuthal inelastic line shape is an exponential decay and its width increases almost linearly as the scattering angle deviates from the specular condition. When elastic diffraction weakens, the inelastic line shape evolves towards a Gaussian and its width is no longer minimum on the Laue circle. As a possible difference with x ray, neutrons, and electrons, the in-plane motion of surface atoms may not be the dominant cause of the broadening of the lateral profile in GIFAD.

• Received 15 May 2023
• Accepted 6 July 2023

DOI:https://doi.org/10.1103/PhysRevB.108.035413