He-atom interaction with the (001) surfaces of LiF and NaCl

Calculations are presented of the potential energy of a He atom near the (001) surfaces of LiF and NaCl. Ingredients in this interaction are a repulsion, computed from the substrate charge density with the effective-medium theory, a sum of damped dispersion attractions contributed by the individual ions, and a small induced dipole energy. The net interaction has a qualitatively reasonable attractive strength. The corrugation, however, is ∼1.5-2 times larger than is inferred from He-beam diffraction intensities. Hypothetical explanations are briefly discussed.