Hartree-Fock studies of helium-surface interaction potentials

Abstract

Elastic helium-atom surface scattering is currently being developed into a surface structural tool. To relate experimental diffraction data with atomic positions one requires a helium-surface interaction potential. An approximate relation due to Esbjerg and No/rskov derived from effective-medium theory has been used for calculating these potentials. However, the relation contains a proportionality constant whose value has been somewhat uncertain. We provide a more exact treatment of the He-metal interaction potential using the ab initio cluster model with Hartree-Fock wave functions. We calculate the repulsive potential and show that it decays exponentially (the decay constant is usually called the softness parameter). We considered appropriate clusters to model the bridge and the on-top site for He-Cu(110) interaction. The surface atoms directly involved in bonding with the helium atom were treated in an all-electron picture whereas the environmental atoms were treated using the effective-core-potential approximation. In both cases we calculated the interaction energy curves as a function of the helium-metal distance. Using these curves, we derived the softness parameter and found that the softness parameter is site dependent. This has an important physical consequence of projecting a rougher (more corrugated) potential surface to the He atom at higher incident energies. Our calculations also provide a proof for the validity of the Esbjerg-No/rskov relation. © 1985 The American Physical Society.