Coverage dependence of adsorption-site geometry in the Cs/Ru(0001) system: A low-energy electron-diffraction analysis

Abstract

The ordered overlayer structures formed by Cs adsorbed on a Ru(0001) surface were analyzed by use of low-energy electron diffraction (LEED). The phase diagram reflects the dominance of dipole-dipole repulsions between the adparticles and comprises quasiliquid configurations characterized by diffraction rings up to a coverage 6=0.17, followed by a (2 X 2) structure with maximum intensity of the diffraction spots at 6=0.23. Beyond 6=0.25, a series of structures with rotated unit cells is identified which are followed by a (&3X&3)R30' structure around 6=0.33 ( =completion of the first monolayer). In the 0 (2X2) phase the Cs atoms are located in on-top sites with a Ru-Cs bond length of 3.25+0.08 A, corresponding to a hard-sphere radius of 1.9 A for the Cs atom. In the (&3X &3)R 30' structure, on the other hand, the adatoms occupy threefold hollow hcp sites with Ru-Cs bond lengths of 3.52+0.02 A, corre-0 sponding to a Cs hard-sphere radius of about 2.2 A. The increase in bond length and effective radius of the adparticle is paralleled by the transition of the character of bonding from more "ionic" at 6=0.25 (large dipole moment) to more "metallic" at 6=0.33 (dipole moment reduced by about 30%). The associated change of the type of adsorption site (from on-top to hollow) is qualitatively rationalized by a model according to which inherently less favorable sites may become preferred due to improved effective screening of the dipole-dipole repulsion by the location of substrate atoms in the region between neighboring adatoms.

Alkali Promotion ofN2Dissociation over Ru(0001)

“…For the Cs͞Ru͑0001͒ system, where there is an experimental structure determination [19], the agreement with experiment is excellent. By varying the size of the unit cell, we can vary the coverage between 1 8 for the ͑2 3 4͒ cell and 1 4 for the ͑2 3 2͒ cell.…”

Coverage-dependent adsorption sites in the K/Ru(0001) system: a low-energy electron-diffraction analysis

“…This behaviour is also found in the (fi x fi)R30" phase of the system Cs/Ru(OOOl) [2] and generally is observed for alkali atoms adsorbed in highly coordinated fourfold hollow sites [17]. The K-Ru bond length in the p(2 x 2) phase of 3.25 is 0.04 A shorter than the corresponding bondlength in the (6 x fi)R30"…”

“…However, this enhancement is mainly caused by a switching of the Cs coordination number of the adsorption site from 1 (on top) to 3 (hcp) [2].…”

“…As a consequence, incommensurate phases may result, but even with commensurate structures in which all adparticles are located on identical sites the sites may alter with varying coverage. An illustrative example is represented by the Cs/Ru(OOOl) system, for which very recently a LEED structural analysis revealed that the on-top site was preferred for the p(2 X 2) phase at 0 = 0.25, while for the (6 X fi)R30" phase at 0 = 0.33 the threefold hcp site was occupied [2]. Typically the adsorption geometry for adsorbed alkali metals is governed by a repulsive dipole-dipole interaction between Cs atoms.…”

“…Details about their computation can be found in ref. [2]. For the Ru layers, the phase shifts were temperature corrected using a fixed effective Debye temperature of 420 K. The Debye temperature of K was kept fixed at 160 K. The agreement between experimental and theoretical intensity data was quantified both by the Ron-factor introduced by Kleinle et al [13] and by Pendry's R-factor R, [14].…”