D.L. Adams

The missing-row model for the reconstructed Pt(110)−(1 × 2) surface: A leed intensity analysis showing multilayer distortions

Abstract The (1 × 2) missing-row reconstruction of clean Pt(110) is studied with a new low-energy electron diffraction (LEED) intensity analysis. In order to obtain theoretical intensity-voltage curves in good agreement with experiment, it is necessary to consider distortions from the basic missing-row model in the theoretical treatment. In our calculations, alternating row-pairing and vertical buckling distortions down to the fourth layer are allowed for the first time. Large contractions are seen in the first three layer spacings, and significant distortions are found even in the fourth layer. The distortions are similar to those seen in Ir and Au LEED results as well as those predicted by the embedded atom method (EAM) for Pt. In the optimum geometry, the top-layer spacing contracts by 18%. The atoms in the second layer pair towards the missing rows, in agreement with Ir and Au LEED but in the direction opposite to that predicted by EAM. The R-factors are especially sensitive to the third-layer buckling. The fourth-layer pairing is also towards the missing rows, in agreement with the EAM prediction. It is shown that full dynamical LEED calculations of this kind can be realistically run on a personal computer with suitable coprocessor board.

LEED study of the Pt(110)-(1 × 2) surface

Abstract The results of a quantitative LEED study of the clean, reconstructed Pt(110)-(1 × 2) surface are described, in which experimental intensity-energy spectra for ten diffracted beams have been compared, by an γ-factor analysis, to intensity spectra calculated using a dynamic theory for various structural models. Despite considerable effort in ensuring the reliability of the experimental and calculated intensities, a satisfactory level of agreement has not been achieved for any of the model structures considered although a “missing-row” model leads to the closest agreement between experimental and calculated intensities.

Oscillatory relaxation of the Cu(110) surface

The surface structure of Cu(110) is determined by means of an r -factor comparison of experimental and calculated LEED intensity-energy spectra for nine diffracted beams in the energy range 20–360 eV at normal incidence. The surface structure is found to exhibit a damped, oscillatory relaxation of the surface interlayer spacings, with the first interlayer spacing contracted by 8.5 ± 0.6%, and the second interlayer spacing expanded by 2.3 ± 0.8% with respect to the bulk value.

The surface structure of V(100)

The structure of the clean V(100)−(1×1) surface is determined, based on an r-factor comparison of experimental LEED intensity-energy spectra with the results of multiple-scattering model calculations. Minimization of the r-factor with respect to the calculational variables leads to optimum values of the first and second interlayer spacings of d1=1.41 ± 0.01 A and d2=1.53 ±0.01 A, corresponding respectively to a contraction of 7% and an expansion of 1% with respect to the bulk value of dB=1.5141 A. Preliminary studies of the adsorption of O2 and CO confirm that the V(100)−(5×1) structure observed during the process of cleaning the crystal is not characteristic of the clean surface, as suggested recently by Davies and Lambert (Surface Science 107 (1981) 391), but rather is associated with the presence of a significant concentration of oxygen in the surface region.

Oscillatory relaxation of the Al( 110) surface

The surface structure of Al(110) is determined by means of r-factor comparisons of new measurements at 100K of LEED intensity-energy spectra with spectra calculated by multiple-scattering theory. The effect of correlations between structural and non-structural variables on the process of structure determination is analysed in detail. The surface structure is found to exhibit a damped, oscillatory relaxation of the interlayer spacings. The relaxations of the first three interlayer spacings, expressed as deviations from the bulk value, are determined to be Delta d1=-8.6%, Delta d2=+5.0% and Delta d3=-1.6%. These results are discussed in the context of previous LEED studies of Al(110) and in the light of recent model calculations of surface structure.

A novel procedure for fast surface structural analysis based on LEED intensity data

By evaluating LEED intensities from different diffraction beams taken only at discrete energy intervals (which may be as large as 15–20 eV) the same degree of reliability in surface structure determination can be reached as with the conventional techniques based on analysis of continuous I/V-spectra. The minimum of the corresponding R-factor can be found by a least-squares fit method, as will be exemplified with a system in which 8 structural parameters were subject to simultaneous refinement.

Multilayer relaxation of the Al(331) surface

Abstract The surface structure of Al(331) has been determined by means of a systematic optimization of the agreement between experimental LEED intensity-energy spectra, measured at normal incidence and 115 K, and spectra calculated using multiple-scattering theory, as a function of the structural and non-structural variables of the calculations. Relaxation from the bulk structure has been investigated for the first six atomic layers. Substantial relaxations of the first four interlayer spacings d1 to d4 and the first interlayer registry r1 have been found. The relaxations, expressed as deviations from the respective distances in the bulk, are determined to be: Δd1 = −11.7±2.3%, Δd2 = −4.1±3.1%, Δd3 = +10.3±2.7%, Δd4 = −4.8±4.1% and Δr1 = −2.8±3.7%. These results are discussed in the context of recent model calculations of surface structure.

Oscillatory relaxation of the Ni(110) surface: a LEED study

The surface structure of Ni(110) has been determined by means of an r-factor comparison of experimental and calculated LEED intensity-energy spectra from two different crystals, for nine diffracted beams, in the energy range 60-360 eV at normal incidence. The surface structure is found to exhibit a damped, oscillatory relaxation of the surface interlayer spacings, with relaxations of Delta d1=-8.7%, Delta d2=+3.0% and Delta d3=-0.5% with respect to the bulk value. These results are discussed in the context of previous studies of the relaxation of metal surfaces.

Multilayer relaxation of the Ni(311) surface: A new LEED analysis

Abstract The results of a new LEED analysis of the surface structure of Ni(311) are reported, in which relaxation of the first three interlayer vectors from their bulk value has been considered. The surface structure is found to exhibit a damped, oscillatory relaxation of the surface interlayer spacings, with deviations from the bulk value determined to be Δd1 = −15.9%, Δd2 = +4.1% and Δd3 = −1.6% for the first three interlayer spacings. However, the registries between the first four layers parallel to the surface are found to be unrelaxed to within the estimated uncertainties. The latter results are in contrast to recent theoretical predictions, and experimental LEED determinations, of large relaxations of interlayer registries for certain open surfaces of Al, Na and Fe.

The preparation and surface structure of clean V(110)

The first quantitative determination of the surface structure of a group VB metal is reported. A procedure is described for the preparation of a clean, well-ordered surface of V(110), free from the major bulk impurity, oxygen. The clean surface exhibits the two-dimensional periodicity of the corresponding bulk plane. Experimental LEED intensity measurements are compared to the results of multiple-scattering model calculations, with very good agreement being obtained for a value of the first interlayer spacing d, close to the bulk value. Minimization of the r factor for the comparison of experimental and calculated intensity spectra leads to a value of d = 2.12 ± 0.02 A, compared to the bulk value of 2.14 A.