Scattering of helium from O/Ni(001): Cluster-model studies

Abstract

A cluster model has been used to study the interaction of He with O/Ni(001) at c(2×2) coverage. The Ni11O2 cluster which was used contains two nearest-neighbor O atoms. The self-consistent-field interaction energies of He with this cluster, Ni11O2He, were obtained for several He geometries along the [100] direction between the He directly over an O atom, the on-top site, to He between the two O atoms, the central site. These energies were obtained in the frozen orbital (FO) and relaxed orbital (RO) approximations. The FO and RO corrugation heights between these sites for He distances with repulsive interaction energies of 40, 60, and 100 meV are all near 0.4 AI, very close to the value obtained from experiment. The corrugation curve along [100] at 60 meV was obtained in the FO approximation and has a full width at half maximum of 1.86 AI. The nature and consequence of the charge redistributions associated with the RO interaction were investigated. It is shown that, due to the size of the He atom, it is not possible to equate the variation of the charge-density contours directly to the corrugation as is done when using the Esbjerg and No/rskov approximation which gives the interaction energy as directly proportional to the surface charge density. We show that there are different proportionality constants for different positions of the He atom on the O/Ni(001) surface. A significant conclusion is that there will not be a unique constant when the surface charge density is highly corrugated with respect to the size of the He atom. However, the Esbjerg-No/rskov relation for superimposed atomic charge densities may be adequate for semiquantitative estimates. © 1987 The American Physical Society.