Critical analysis of the possible experimental evidence for an on-top adsorption site for xenon on Pt(111)

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Abstract
The recent theoretical evidence for an on-top adsorption site for xenon on Pt(111) has led us to investigate the possible influence of the adsorption site on the diffraction pattern of the uniaxially compressed phase in this system. As pointed out by Gottlieb adsorption in the threefold hollow or on-top site, respectively, might lead to different domain-wall structures with different characteristic diffraction patterns. This is due to the fact that on the hexagonal surface lattice there are two hollow sites per unit cell (which were assumed to be degenerate) but only one on-top site. Here we show how the diffraction signature is modified if the threefold hollow sites are no longer energetically degenerate [i.e., if the true three-dimensional character of the Pt(111) surface is taken into account]. We give a quantitative evaluation of the nature of the domain-wall phase in terms of domain-wall energies and the difference of the adsorption energy at the two different hollow sites. In conclusion, we find that the inference of an on-top adsorption site from the measured diffraction pattern has to be taken with caution.