Theoretical study of interactions among surface defects by He scattering technique: the reason for the failure of the cross section geometric overlap hypothesis

Abstract

Abstract The study of interactions among surface defects by He scattering technique is mainly based on the hypothesis of geometric overlap (GO) of defect cross sections. Recently, quantum calculations have shown that this hypothesis is not valid. This result has been obtained by considering He scattering from targets consisting of a Pt(111) surface containing two defects, i.e. two CO admolecules or two vacancies arranged at different mutual distances, d def . Quantum cross sections Σ 2 , calculated by sudden approximation for both kinds of defects and in a wide d def range, were in large disagreement — up to 34%, namely more than the 3xperimental and sudden errors on cross section — with the corresponding quantities Σ 2 Δ obtained on a pure geometric ground by the GO hypothesis. Therefore, the main purpose of the present paper is to detect the reason for the GO hypothesis failure. To this end, because a more fruitful comparison can be made between homogeneous quantities, and Σ 2 and Σ 2 Δ are not, suitable target models have been built, containing two CO admolecules or two vacancies on Pt(111) arranged as in the case of the above-mentioned targets. The new targets are such that calculations on He scattering from them give the cross sections σ 2 and σ 2 Δ — both obtained by taking into account He-target interaction, and therefore homogeneous — whose trends versus d def are completely analogous to those of Σ 2 and Σ 2 Δ , respectively. In this way the comparison between σ 2 and σ 2 Δ , is useful to demonstrate that the GO hypothesis fails because it implies — translated in terms of atom-target interaction — that, in the region between the two defects, i.e. where their cross sections overlap, He scattering process could be described by taking into account the interaction of impinging atom with just one of the two defects and neglecting the contemporaneous effect of the other one.