The geometry of xenon and krypton on Ru(001): a LEED IV investigation

Detailed LEED IV analyses of the (root 3 x root 3)R30 degrees-Xe and the (3 x 3)-Kr ordered structures on the hexagonally close-packed Ru(001) surface have been performed by conventional dynamic and tensor LEED calculations. The most important finding is that both rare gases tend to adsorb on or close to top sites. In the submonolayer structure of xenon, all atoms adsorb on well-defined top sites with unexpectedly short bonding distance. For the close-packed Kr structure, the majority of the atoms are found to sit close to top sites, with these atoms having the smallest distances from the substrate atoms; only the atoms locking into the substrate corrugation are found to occupy three-fold hollow sites. The formation of an island structure of two different domains in which the superstructure locks into fcc and hcp sites with equal probability is found. The mean layer distances between adsorbate and first Ru layer were found to be 3.54 +/- 0.06 Angstrom for xenon and 3.70 +/- 0.03 Angstrom for krypton. The distance between the first two Ru layers, d(12), is compressed beyond that of the clean surface, to 2.07 +/- 0.03 Angstrom (Xe) and 2.05 +/- 0.03 Angstrom (Kr). We describe special problems occurring in the structural analysis of this type of system (radiation sensitivity, number of angular momenta included in the description of electron scattering, importance of early adjustment of adsorbate Debye temperature due to very soft lateral vibrations), and discuss the results in context with the question of the bonding mechanism of the heavy rare gases on transition metal surfaces. (C) 1998 Elsevier Science B.V. All rights reserved.