Sensitivity of helium diffraction to surface geometry

In order to examine the sensitivity of He diffraction to surface structure, we develop explicit expressions for the principal features of the He scattering potential in terms of simple atomic and lattice parameters. A measure of structural sensitivity evolves naturally out of these relationships. In particular, the surface lattice constant emerges as the most crucial factor in determining when He diffraction will provide useful structural information. For surface lattice constants less than ∼5–6 Å, the potential is often well approximated by a simple universal form, and has little sensitivity to the precise structure. The conclusions are illustrated with recent experimental results for selected metal, semiconductor, and adsorbate-covered surfaces.