P. Kaukasoina

Observation of top-site adsorption for Xe on Cu(111)

Abstract A low-energy electron diffraction study of Cu(111)–(√3×√3)R30°–Xe at 50 K indicates that Xe atoms occupy the top sites. The equilibrium Xe–Cu interlayer spacing is 3.60±0.08 A and the spacings of the three top Cu layers are essentially bulk-like. The Xe–Cu spacing agrees well with estimates based on hard-sphere packing. Top-site adsorption for Xe on Cu(111) was unexpected on the basis of previous experimental and theoretical results for noble gas adsorption on metal surfaces. This result is discussed in the light of earlier studies of physisorbed atoms, anticorrugated potentials in He-atom scattering, and possible links to alkali metal adsorption.

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.

LEED determination of the structures of Ni(111) and the p(2*2) overlayer of potassium on Ni(111)

The authors have used dynamical low-energy electron diffraction (LEED) to determine the structure of both the clean Ni(111) surface and the p(2*2) structure of potassium adsorbed on Ni(111). The result of the clean surface study indicates that the Ni(111) is essentially a truncation of the bulk crystal. The p(2*2) structure of potassium on this surface consists of the potassium atoms adsorbed on top of the Ni atoms with a slight reconstruction of the top-layer Ni atoms combined with vertical relaxations of the first and second layers of Ni. The potassium-nickel bond length is 2.82+or-0.04 AA corresponding to an effective potassium radius of about 1.57 AA. This result fits well in the bond length versus coordination number trend observed for other alkali metal overlayers.

Structural determination of an intermixed (1×2) Au film on Pd(110) by dynamical low-energy electron-diffraction analysis

Abstract The geometric structure of a (1 × 2) Au film on Pd(110) has been determined by dynamical low-energy electron-diffraction analysis. The concentration profile of Au in the surface region is determined in the analysis, and the results clearly reveal the presence of intermixed layers beneath two layers of essentially pure Au. As for the optimum geometry of the film, the topmost layer is of the missing-row type, the second layer is row-paired by 0.10 A, the third layer is rumpled by 0.07 A, and the first three interlayer spacings are relaxed by Δd 12 = −6.6%, Δd 23 = +5.1%, and Δd 34 = +3.6% relative to the bulk-truncated substrate. (Deeper interlayer spacings are at or near the bulk value.) The optimum geometry of the intermixed Au film differs considerably from that of the (1 × 2) missing-row reconstruction of bulk Au(110), but can be rationalized in terms of conservation of volume density as a response to the 4.8% lattice mismatch between Au and Pd.

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.

LEED and the effective surface dipole moment of Ni(111)-p(2×2)K

Abstract In most of the low-energy electron diffraction (LEED) studies of the geometric structure of alkali metals on metal surfaces, the level of agreement between experimental and theoretical intensity-energy curves has been relatively low. It has been proposed [1] that the origin of the low level of agreement is due to the induced dipole potential at the position of the adatom, and that the agreement between the experimental and theoretical curves can be improved by the inclusion of a point dipole potential in the LEED calculations. Such improvement was demonstrated for the case of K/Ni(100) [1]. In this paper, we show that inclusion of a point dipole in the calculated potential does not improve the level of agreement in the case of Ni(111)p(2 × 2)K.