D. W. Jepsen

Determination of the structure of ordered adsorbed layers by analysis of LEED spectra

Abstract Recent work on the structure of ordered overlayers of chalcogens on nickel is reviewed, and discussed as a convincing example of the use of LEED intensity spectra for surface structure determination. Guidelines are suggested for successful application of LEED to surface structure, and the special suitability of the muffin-tin potential for the LEED calculation is brought out. The model of the complex crystal potential used for the calculation, and the systematic determination of the four parameters of the model from the data for clean Ni are described. The application to chalcogen overlayers is illustrated by the case of c(2 × 2)S and c(2 × 2)O on Ni(001) surfaces, and the correspondence between experiment and theory is exhibited for the intensity versus energy spectra of 1 2 1 2 beams. The correspondence is close and detailed; the complex shapes of broad features are reproduced and the average of the magnitude of the deviation between peaks in measured and calculated spectra is less than 2 eV, whereas the accidental correspondence of two spectra is shown to fluctuate around 4 eV. The observed values of the bond lengths of O, S, Se, Te on Ni are shown to be plausible on the basis of the covalent radii and the expected deviations from them. Results for other surfaces and other structures are tabulated. In all cases but one [O/Ni(110)] the chalcogen occupies sites that the next layer of Ni atoms would use.

Determination of the c(2 × 2) structure of sulfur on Fe{001} by low-energy electron diffraction

In the early stages of reaction with sulfur, a clean Fe{001} surface develops a c(2 × 2) superstructure. A low-energy electron diffraction analysis of this structure leads to a model in which the S atoms lie in the four-fold symmetrical sites on the Fe{001} surface, the S-Fe interplanar spacing being 1.09 ± 0.05 A and corresponding to an effective radius of 1.06 A for the chemisorbed S atoms. In contrast to Fe{001} 1 × 1-O, the first interlayer spacing of the substrate here is not significantly expanded.

Atomic underlayer formation during the reaction of Ti{0001} with nitrogen

Abstract Changes in the {0001} surface of Ti during exposure to nitrogen gas are monitored by low-energy electron diffraction (LEED) and Auger electron spectroscopy (AES). Evidence is provided for the formation of a Ti{0001}1 × 1-N phase followed by a Ti {001} 3 × 3−30°-N structure. A LEED intensity analysis of the Ti{0001}1 × 1-N phase reveals that the N atoms penetrate into the octahedral interstitial holes underneath the first layer of Ti atoms. This is the first ordered monoatomic underlayer found in the earliest stages of any solid-gas interaction. The surface structure bears close resemblance to that of {111} planes in bulk TiN. We find a Ti-N bond length of 2.095 A to be compared with the value of 2.120 A in bulk TiN. The analysis of the Ti{0001}1 × 1-N structure indicates that Ti {0001} 3 × 3−30°-N is not a low-coverage phase. The importance of recognizing the existence of 1 × 1 phases prior to the formation of superstructures is emphasized, and some procedures for extracting the information from experimental observations are discussed.

Atomic structure of Fe{111}

Abstract A clean Fe {111} surface was prepared and studied with LEED (low-energy electron diffraction) and AES (Auger electron spectroscopy). A LEED intensity analysis was carried out with a new computational scheme (THIN) specially designed for short interlayer spacings. The results are, for the fust interlayer spacing, d 12 = 0.70 ± 0.03 A and for the inner potential V 0 = 11.1 ± 1.1 eV, the confidence intervals referring to 95% confidence level. Thus, the Fe {111} surface is contracted 15.4% with respect to the bulk (0.827 A).

The structure of overlayers: III. Nitrogen on Mo {001}

Abstract The Mo {001} c (2 × 2) — N structure formed upon adsorption of a monolayer of nitrogen on a clean Mo {001} surface is studied by means of low-energy electron diffraction (LEED). Diffracted intensities for a large number of integral-order and half-order beams are calculated with the layer KKR method for different structural models and compared with extensive experimental data. Best overall agreement between calculated and observed LEED spectra is found for a model in which N atoms are adsorbed in the four-fold pyramidal hollows formed by four adjacent Mo atoms on a net consistent with c (2 × 2) symmetry. However, the fit between theory and experiment is good only for certain beams, much less good for others. Attempts at improving the fit by varying the values of structural and non-structural parameters used in the calculations have not been succesful. We raise the question of the significance of a structure model for which the fit with experiment is wholly satisfactory only for certain beams.

Overlayer structure determination by LEED at low energies: Na on Ni (001)

Analysis of new LEED data by Anderson (1975) shows that Na atoms in a c(2*2) overlap on Ni(001) sit in the four-fold hollows but, contrary to a previous analysis of Andersson and Pendry (see abstr. A28985) at a distance of 2.23+or-0.1AA from the surface. The sensitivity of the low-energy spectra to nonstructural features of the LEED model allows discrimination between different adsorbate potentials, inner potentials in the overlayer and surface barriers.

The determination of surface debye temperatures from low-energy electron diffraction data

Abstract The temperature dependence of low-energy electron diffraction (LEED) intensities has often been interpreted with kinematic theory in terms of an effective Debye temperature θ D eff of the surface atoms. The validity of this procedure, often questioned in the literature, is tested with a computer experiment in which LEED spectra are calculated from dynamical theory (layer-KKR method) for a model of Ag{111} with a given value of θ D eff and then the usual kinematic formulae are used to re-extract the value of θ D eff . The results of the experiment indicate that this procedure yields rough values of the surface Debye temperature for electron energies higher than about 40 eV, which fluctuate substantially and tend to be somewhat smaller than that originally introduced into the model. At energies lower than about 40 eV the kinematically deduced values of θ D eff are too large by 10 to 15 %.

Application of the solution of the dynamical scattering problem for electrons to surface structure analysis

Abstract The solution of the low-energy electron diffraction (LEED) problem at a crystal-vacuum interface by the layer-KKR method is shown to apply readily to transmission and reflection of electrons at crystal-crystal interfaces. A simple model of the scattering potential is described, which can be readily constructed for any configuration of atoms. The LEED spectra it yields are shown to give good agreement with experiment for several simple metals, when the effects of electron absorption and lattice motion are included. It is shown that LEED probes the first few surface layers of a crystal and is sensitive to changes in these layers, including parallel and perpendicular displacements of the first layer. Thus it is concluded that, in conjunction with multiple scattering calculations as described above, LEED is potentially a valuable tool for the determination of the structures of these layers.

LEED analysis of the Co{001}c(2 × 2)O structure

Abstract A LEED intensity analysis of the c(2 × 2) structure obtained upon adsorption of oxygen gas on Co {00l} is reported. Three structural models have been tested on the basis of a total of 13 LEED spectra for three angles of incidence and one azimuth. The correct model has oxygen atoms chemisorbed in the four-fold symmetrical hollows formed by four adjacent substrate atoms. Atomic arrangement and metal-oxygen distances are analogous to those found in Ni{001}c(2 × 2)O.

LEED structure analysis of a Co{001} surface

A {001} surface of face-centered-cubic cobalt was cleaned to the point of elimination of all impurities except carbon and oxygen, which were reduced to minimum terminal amounts. A LEED structure analysis of this surface, using 12 intensity spectra at 3 angles of incidence, reveals that the atomic arrangement corresponds to truncation of the bulk structure but with about 4% contraction of the first interlayer spacing along 〈001〉 with respect to the bulk.