Gerald S. Leatherman

The adsorption sites of rare gases on metallic surfaces: a review

During the past six years, the adsorption geometries of several rare gases in structures having several different symmetries on a variety of substrates were determined using low-energy electron diffraction (LEED). In most of these studies, a preference is found for the rare gas atoms to adsorb in the low-coordination sites. Only in the case of adsorption on graphite has a clear preference for a high-coordination site for a rare gas atom been found. This unexpected behaviour is not yet completely understood, although recent density functional theory (DFT) calculations for these and similar surfaces suggest that this is a general phenomenon. This paper reviews the early studies that were presages of the discovery of top site adsorption for rare gases, the discovery itself, and the present state of understanding of this curiosity. It also details some of the features of the LEED experiments and analysis that are specific to the case of rare gas adsorption.

Low-energy electron diffraction study of the thermal expansion of Ag(111)

The temperature dependence of the first three interlayer distances of the Ag(111) surface was studied by low-energy electron diffraction (LEED) over the temperature range 128K to 723 K. The first three interlayer spacings and the effective Debye temperatures were extracted from the LEED analysis. At the lowest temperature, the first two interlayer spacings are slightly (0.5 percent) contracted. All three interlayer spacings increase with temperature, finally reaching expansions relative to the bulk of about 0.8 percent at the highest temperature studied. The effective surface Debye temperature is lowest for the outermost layer, increasing toward the bulk value for successive layers.

The coverage dependence of the adsorption structures of Cs on Ag(111)

Abstract The structures of five different submonolayer commensurate phases of Cs on Ag(111) have been determined by low energy electron diffraction (LEED). This paper presents the results of two of these studies: for the primitive (2 3 ×2 3 )R30° and ( 7 × 7 )R19° structures which form at coverages of 1/12 and 1/7 respectively. The adsorption site was found to be the fcc hollow in both cases. These structures are accompanied by a substrate rumple which has the effect of allowing the Cs atoms to push deeper into the substrate. The structures determined here, along with the earlier structure determinations of three other submonolayer phases, indicate that the CsAg bond length does not change over the coverage range from 1/12 to the monolayer saturation coverage of 1/3.

ADSORPTION SITE CHANGE FOR Cs, Rb OR K ADSORPTION ON Ag(111)

The adsorption geometries for the primitive (3×3), (2×2) and ( structures of K, Rb and Cs on Ag(111) have been determined using low-energy electron diffraction. In the lower-coverage (3×3) and (2×2) structures, the adatoms occupy fcc hollow sites, while in the ; structure they occupy the hcp hollow sites. The fcc hollow structures are accompanied by significant substrate rumpling. There is no significant coverage-dependent or site-dependent change in chemisorption bond length. However, there is a large coverage-dependent anisotropy of vibrational amplitude of the adatoms, with the parallel component as much as five times larger than the perpendicular component at low coverages.

Rare gas coadsorption with Cs or K on Ag(111)

Abstract Rare gas (RG) coadsorption with submonolayer amounts of Cs or K on Ag(111) was studied using low-energy electron diffraction (LEED). A crossover in the alkali-RG interaction from repulsive to attractive was observed as a function of alkali species and of alkali coverage. The KRG interaction was observed to be repulsive at all coverages, while the CsRG interaction was observed to be attractive at low Cs coverages and apparently repulsive at high Cs coverages. For the K + RG adsorption system, desorption data were analyzed to determine the spreading pressure in the alkali layer, thus showing that RG can be used as a 2D manometer in some coadsorption systems. From the spreading pressure it is possible to obtain some information about the properties of the adsorbed alkali such as the energy differences between commensurate and incommensurate phases. We also demonstrate that work function measurements from such coadsorption systems do not necessarily have a simple interpretation.