Site-selective adsorption of xenon on a stepped Ru(0001) surface

Abstract

The adsorption of xenon has been studied with UV photoemission (UPS), flash desorption (TDS) and work function measurements on differently conditioned Ru(0001) surfaces at 100 K and at pressures up to 3 × 10 −5 Torr. Low energy electron diffraction (LEED) and Auger electron spectroscopy (AES) served to ascertain the surface perfectness. On a perfect Ru(0001) surface only one Xe adsorption state is observed, which is characterized by Xe5 p 3 2 , 1 2 electron binding energies of 5.40 and 6.65 eV, an adsorption energy of E ad≈ 5 kcal/mole and dipole moment of μ' T ≈ 0.25 D. On a stepped-kinked Ru(0001) surface, the terrace-width, the step-height and step-orientation of which are well characterized with LEED, however, two coexisting xenon adsorption states are distinguishable by an unprecedented separation in Xe 5 p 3 2 , 1 2 electron binding energies of 800 meV, by their different UPS intensities and line shapes, by their difference in adsorption energy of ΔE ad ≈ 3 kcal/mole and finally by their strongly deviating dipole moments of μ S = 1.0 D and μ T = 0.34 D. The two xenon states (which are also observed on a slightly sputtered surface) are identified as corresponding to xenon atoms being adsorbed at step and terrace sites, respectively. Their relative concentrations as deduced from the UPS intensities quantitatively correlate with the abundance of step and terrace sites of the ideal TLK surface structure model as derived from LEED. Furthermore, ledge-sites and kink-sites are distinguishable via E ad. The E ad heterogeneity on the stepped-kinked Ru(0001) surface is interpreted in terms of different coordination and/or different charge-transfer-bonding at the various surface sites. The enormous increase in Xe 5p electron binding energy of 0.8 eV for Xe atoms at step sites is interpreted as a pure surface dipole potential shift. —The observed effects suggest selective xenon adsorption as a tool for local surface structure determination.