DIFFUSE LEED THEORY FROM SURFACES PARTIALLY COVERED WITH MOLECULES .2. APPLICATION TO THE CASE OF THE ADSORPTION OF CO ON PT(111)

The aim of this paper is to investigate the relative influence of geometrical and order parameters on diffuse LEED intensities. In most diffuse LEED studies relative to molecules adsorbed at single crystal surfaces, there are probably several chemisorption sites or several adsorbed species. For this reason, this situation is examined in detail here. The elastic diffuse LEED intensity from a single crystal surface Pt(111) partially covered with CO molecules is calculated for different values of the parameters which characterize the adsorbate/substrate model. These parameters are separated into: geometrical parameters related to the local arrangement of atoms near chemisorption sites, and order parameters related to the statistical distribution of the occupied chemisorption sites. In the present case, the geometrical parameters are the C-Pt and the C-O bond lengths and the location in the 2D unit cell of the surface lattice (bridge and top sites). The order parameters are the site occupancy pair correlation functions or the quantities which determine them: surface coverage, surface temperature, pair interaction potential between adsorbates, etc. Even in the case of a shortrange order, the sensitivities of diffuse intensities to both kinds of parameters are found very similar. On the other hand, in the situation where several kinds of chemisorption sites exist, the diffuse LEED intensity is not the product of a form factor and a structure factor. So the use of the Y function method, which avoids the structure factor determination, is not possible. Thus, a determination of the statistical distribution of adsorbates at the substrate surface cannot be circumvented.