P M Marcus

Low-energy electron diffraction analysis of clean Fe (001)

A structure analysis of the clean (001) surface of body-centred cubic iron by low-energy electron diffraction (LEED) is described. The surface was cleaned by a series of argon-ion bombardments followed by annealing treatments. The intensity analysis of 16 non-degenerate beams at three angles of incidence in the energy range from about 30 to 160 eV revealed that the structure of Fe (001) essentially corresponds to simple truncation of the bulk, with a probable contraction of 1.4% in the first interlayer spacing compared to bulk. The uncertainty in this figure, however, is +or-3% of the interlayer spacing.

The atomic structure of Fe(110)

A detailed low-energy electron diffraction analysis of a clean Fe(110) surface is described. The experimental data base consists of intensity-versus-energy curves (spectra) at 5 directions of incidence with a total of 34 spectra at energies up to 210 eV. A statistical analysis carried out with the reliability factor (r-factor) of Zanazzi and Jona (1977) gave a first interlayer spacing d12=2.04+or-0.04 AA (95% confidence interval, the bulk value is 2.03 AA), and an (energy-independent) inner potential, V0=11.5+or-0.4 eV, with an overall r-factor of 0.10. The effects of using an approximate procedure to vary V0, a rigid translation along the energy axis, are compared with exact calculations.

Structural analysis of oxygen adsorption on Fe(001)

LEED data from the stable 1*1 structure formed by exposing alpha -Fe(001) to oxygen at room temperature have been compared with full dynamical calculations. The oxygen atoms are found to lie at 0.53+or-0.06 AA above the surface in the four-fold hollows, and the spacing between the first and second iron layers is increased by about 7.5% to 1.54+or-0.06 AA.

On the structure of reconstructed Si(001)2×1 and Ge(001)2×1 surfaces

Full dynamical LEED intensity calculations have been carried out on Si(001) and Ge(001) surfaces distorted from bulk in the first three layers as in the surface-dimer model proposed previously. Comparison with experimental data indicates that for Si(001), the model produces the best agreement achieved so far, but the overall agreement is not good enough to qualify as satisfactory. For Ge(001) there is practically no agreement with experiment. Variations of the dimer model are suggested which introduce distortions with 2*2 and c(4*2) periodicities and explain qualitatively a number of experimental observations, although no quantitative model has been found that reproduces satisfactorily the experimental LEED intensity data available.

A low-energy electron diffraction intensity analysis of Cu(001)c(2×2)-Cl

A LEED intensity analysis of the Cu(001)c(2*2)-Cl structure finds the Cl atoms in the fourfold symmetrical hollow sites with a Cl-Cu interlayer spacing of 1.60+or-0.03 AA and a slightly expanded Cu-Cu first interlayer spacing of 1.85+or-0.03 AA (bulk value 1.807 AA). The calculated Cu-Cl bond length is 2.41+or-0.02 AA, in fair agreement with the value 2.37+or-0.02 AA determined with SEXAFS by Citrin and co-workers (1978, 1982). The effect of the Cl scattering potential on the LEED intensities, and hence on the structural parameters, is quantitatively analysed with r-factor calculations. The results obtained in this work speak in favour of the same overlayer model found by an earlier LEED analysis of the closely related Ag(001)c(2*2)-Cl structure but contradicted by photoemission studies, and thus help resolve the discrepancy between LEED and photoemission about the atomic geometry of this system in favour of the LEED structure.

The structure of the clean Ti(0001) surface

The atomic arrangement in the basal plane of Ti is determined by analysis of low-energy electron diffraction (LEED) intensities. Twenty-five beams at four angles of incidence and at two azimuths have been measured and compared to calculations. Complications caused by the simultaneous presence on the surface of domains with different terminations of the bulk structure are discussed. The model that produces satisfactory correspondence with experiment has the Ti atom in the top layer located in the positions that correspond to simple termination of the bulks ABABA stacking characteristic of HCP structures. The spacing between the top and the second atomic layers is slightly (about 2%) contracted with respect to the bulk value. The values of the non-structural parameters entering the calculations are discussed.

Probable atomic structure of reconstructed Si(001)2×1 surfaces determined by low-energy electron diffraction

A LEED intensity analysis of the Si(001)2*1 structure leads to exclusion of two of the most widely discussed models for surface reconstruction. These are models involving surface dimers or surface vacancies. A different model is presented that provides acceptable agreement between calculated and observed intensity spectra. The model is based on the idea of conjugated surface chains advanced by Seiwatz (1964), and involves distortions of both the first and the second atomic layers. The model is consistent with the rare observation of four-fold periodicities on Si(001) and suggests that small surface concentrations of carbon may be important.

Al(111) revisited

A new, more refined analysis of the old LEED intensity data from Al(111) (Lang and Kohn, 1970) using new intensity calculations and reliability factors confirms the earlier LEED result about the first interlayer spacing being slightly expanded (2.2%) with respect to the bulk. This result is in striking contrast to the recent EXAFS study of Al(111) by Bianconi and Bachrach (1979), according to which the first interlayer spacing is contracted (8.1%) with respect to the bulk. Considerably poorer agreement between theory and experiment is found for 8.1% contraction.

The 2 × 1 reconstruction of the Ge(001) surface

Low-energy electron diffraction (LEED) analyses of several structure models for the Ge(001) 2 × 1 surface confirm that the reconstruction involves at least 3 layers, and demonstrate that models with buckled asymmetric dimers are favoured over models with symmetric dimers. Three models with different amounts of buckling are given, which show equivalent moderate agreement with experiment.

LEED versus SEXAFS in the problem of oxygen on aluminium

Suggestions are made to help resolve the discrepancies between LEED and SEXAFS results for the Al-O distance in the Al(111) 1*1-O structure. The need for proper characterisation of a surface structure and for an adequate data base in intensity analyses is emphasised. It is shown that with the LEED data available it is not possible to discriminate between the two proposed interatomic distances.