Substrate induced specificities in atom-adsorbate van der Waals scattering

Abstract

We investigate the substrate induced specificities of the van der Waals potentials acting between gas phase atoms and chemisorbed CO molecules, and the effect of these specificities on the enhancement of the magnitude of the total scattering cross section in He → CO/metal collisions. The role of these effects in the relative enhancement of the cross section can be quantitatively estimated for prototype systems of weak and strong CO chemisorption, viz. CO/Cu and CO/Ni, respectively, for which other electronic and structural properties are known. Since the strengths of the atom-adsorbate van der Waals potentials depend on the spectra of virtual electronic excitations in each electronic subsystem involved, the specificity of the cross section arises from the particularities of the screening properties of the substrate surface, on the one hand, and on the strength of the adsorbate-substrate chemisorptive bond, on the other. Using the response functions of Cu and Ni surfaces and the valence electronic properties of the 2π∗ derived resonances of CO chemisorbed on Cu and Ni substrates, we are able to calculate and discuss qualitatively the scattering potentials and the corresponding cross sections for He → CO/Cu and He → CO/Ni collision events. The results of our calculations demonstrate that the strong chemisorption system CO/Ni represents also a stronger scattering centre for thermal He beam atoms than the weak chemisorption system CO/Cu, and that for equal kinetic parameters of the collision the total cross sections may in these sysems differ by up to 10%.