Surface structure determination by quasidynamical LEED intensity evaluation

The quasidynamical method for LEED intensity calculations is applied to Ir(100)1 × 1 over a broad energy range, 100–500 eV. The comparison with experiment and full dynamical calculations shows that peak positions and peak widths are well reproduced, but relative peak heights and therefore also shapes of peak groups can be considerably modified. This behaviour is discussed and explained in terms of the forward and backward diffraction properties of single layers. Finally, a structure determination is tried using the Pendry r-factor for the quantitative comparison of the experimental and quasidynamical data. The resulting value for the surface relaxation agrees with that determined from the full dynamical evaluation within the usual error of structure determination by LEED.     

Surface structure of MgO(001) surface studied by LEED

MgO(001) surfaces prepared by three different heat treatments were studied by LEED. The first was the surface just after the vacuum cleavage; the second was that after the annealing of the first one at 300°C in UHV. The third was prepared by cleaving in air and heating in oxygen until the carbon Auger peak disappear. LEED I-V curves of the diffraction spots, (10), (11) and (20), were recorded and compared carefully. No remarkable difference corresponding to the preparing methods was found. Therefore, the atomic structures of the surfaces seem to be similar to each other. These experimental I-V curves were compared with the theoretical curves obtained by dynamical calculations for several grades of surface relaxation. The theoretical curves with no relaxation or with the smallest one in the calculation (2.5% expand) fitted best with the experimental curves. The experimental curves were also compared with the theoretical ones calculated for the rumpled surface. But no effect induced by the rumpling was found in the experimental curves.     

LEED analysis of the surface structure of Mo(001)

A detailed comparison between theoretical and experimental LEED data for the Mo(001) surface, using the quantitative r-factor method of Zanazzi and Jona, yields a value for the surface layer contraction of 9.5 ± 2.0%, consistent with the 11.5% found previously by Ignatiev et al. Non-structural parameters are also in agreement, although the surface Debye temperature, 230 K, is slightly higher than found before. r-Factors were used to investigate the effects of all the parameters, and show that an energy-dependent inner potential gives better agreement than a fixed value and that the second layer is contracted by about 1% of the bulk layer spacing. The effect of using different ion core potentials is examined and discussed. A superposition potential which models the surface ion environment is found to have a significant effect on the layer displacement concluded.     

Irreversible structure transitions in Gd monolayers on Mo(112)

Low-energy electron diffraction (LEED), Auger electron spectroscopy (AES) and contact potential difference (CPD) methods have been used to investigate the structure of Gd monolayers deposited on Mo(112) at T = 78 K and the changes upon annealing in a wide temperature range, up to the beginning of desorption. In the submonolayer coverage range (θ < 0.67), the film structures p(1.3×1) and p(2×1) already formed at T = 78 K, testifying that Gd adatoms possess some mobility at rather low temperatures. The p(1.3×1) structure was found to appear at 0.07 < θ < 0.25, but it irreversibly turned into the p(2×1) structure when the annealing temperature, T_an, exceeded 500 K. Above θ = 0.25, the p(2×1) structure emerged immediately at 78 K. Formation of step arrays was observed in the range of T_an = 500–1200 K and is attributed to surface alloying. The suggestion of surface alloying is corroborated by data on annealing induced variations of the work function and Auger peak of Gd. In the coverage range 0.5 < θ < 0.67, the phase p(2×1) was found to coexist with the phase c(1.5×2), which corresponds to a physical monolayer. No evidence of surface alloy in the complete monolayer was revealed. Distinction between ordering scenarios for the systems Gd/Mo(1 1 2) and Dy/Mo(112) is discussed.     

Oxygen submonolayers on Mo(112): structure and work function

Correlation between the work function change and the structure of oxygen submonolayers onthe Mo(112) surface are studied using low-energy electron diffraction (LEED), Augerelectron spectroscopy (AES) and contact potential difference (CPD) methods. Oxygen wasadsorbed at temperatures T = 78?300 K and thereafter the adlayers wereannealed in a wide temperature range up to oxygen desorption. Temperature inducedirreversible and reversible phase transitions are investigated. With coverage growth,formation of the monolayer proceeds through three first-order phase transitions, one ofwhich is featured by a specific change in the course of the work function dependence oncoverage. It is suggested that during this transition the oxygen adatoms may change theirsites on the substrate from those of a short-bridge type to quasi-threefold ones, thusincreasing their coordination number from two to three.     

Surface barrier determination using spin-polarized LEED: W(001)

Spin-polarized LEED intensities have been calculated for the W(001) surface for a variety of surface barriers and compared with both spin-polarized measurements (15° ≤ angle of incidence Θ≤45°) and high resolution unpolarized data (Θ=48°). Observed features, in particular the striking differences between spin-up and spin-down intensities, are reproduced satisfactorily by a barrier with an image plane 3.3 bohr from the outermost atomic layer and with a value of 70% of the bulk inner potential at that layer.     

Structure determination of the clean Co(112&#x0304;0) surface by LEED

The atomic structure of the (112&#x0304;0) surface of cobalt has been determined by LEED using six intensity spectra at normal incidence. The surface exhibits the truncated bulk structure with a contraction of the first interlayer spacing by about 8.5% with respect to the bulk value. Quantitative evaluation of the LEED spectra was done using Zanazzi and Jona's and Pendry's r-factors. The minimum averaged r-factors are $\text{r}{}_{\mathrm{<mtext>ZJ</mtext>}}\text{= 0.09}$ and $\text{r}{}_{\mathrm{<mtext>P</mtext>}}\text{= 0.22}$. No change of the interatomic distances within the plane could be detected and no rearrangement of the surface structure takes place up to temperatures shortly below the transition temperature.     

Chlorine superstructures on Mo(110) investigated by LEED and AES

C1₂ was adsorbed onto a clean Mo(110) surface at room temperature. Four well-defined structures occurred as coverage increased. Below half a monolayer (θ <0.5) a p'(2 × 1) pattern (with respect to the rectangular surface mesh) with p2mg glide-plane symmetry formed. Near θ = 0.5, a p'(1 × 1) structure formed, next p'(1 × 2) and at saturation (θ?0.8) p'(1 × 3). Physically plausible models based on compression structures (relatively even distribution influenced by adatom-adatom repulsion) with bonding sites restricted to the vicinity of quasi-four-fold hollows and three-fold wells are devised to explain these patterns. These models are suitable for the reinterpretation of other investigations of the related systems S on Mo(110) and Cl on Ta(110).     

Relaxation of the Mo(112) and W(112) surfaces

Relaxation of the Mo(112) and W(112) surfaces has been simulated within DFT in local density approximation. It has been found that the surface relaxation, which can be described as a 14% contraction of the topmost surface layer with a small (0.1%) shift of surface atomic rows, results in a strong decrease of the surface energy with respect to the bulk truncated crystal surfaces (from 0.2 to 0.17 eV/Å2 for the Mo(112) and from 0.36 to 0.33 eV/Å2 for the W(112)). The surface relaxation is accompanied by the redistribution of the surface density of states, associated with the transformations of surface states.     

Atomic structure of a thin silica film on a Mo(112) substrate: a two-dimensional network of SiO4 tetrahedra

The structure of a thin single crystalline SiO(2) film grown on Mo(112) has been studied by scanning tunneling microscopy, infrared reflection absorption spectroscopy, and x-ray photoelectron spectroscopy. In excellent agreement with the experimental results, density functional theory calculations show that the film consists of a two-dimensional network of corner sharing [SiO(4)] tetrahedra, with one oxygen of each tetrahedron binding to the protruding Mo atoms of the Mo(112) surface.     

Surface layer relaxation on Mo(111) measured by low energy alkali ion scattering

The surface of clean Mo(111) has been studied using Li⁺ ion scattering at 1000 eV. The dependence of single scattering intensity was measured as a function of incident polar angle in the [12&#x0304;1] and the [21&#x0304;1&#x0304;] azimuths for various total scattering angles. Very pronounced intensity cut-offs are observed and are readily assignable to shadowing and blocking effects in scattering from first, second or third layer atoms. After taking certain precautions to avoid interference from deeper layers, the measured positions of the features yield the first-second layer spacing which is found to be strongly contracted by (18 ± 2)% compared to the bulk spacing. The method also indicates that the second-third layer spacing is possibly expanded (4 ± 4)%, but this result is uncertain due to possible contributions from deeper layers which make this value an upper limit of the layer spacing. The physical implications of these results and the uncertainties in the technique are discussed.     

Surface parameters of clean nickel (001) determined by LEED averaging

The low-energy electron diffraction from clean Ni(001) has been studied in detail over a large range of temperature and diffraction geometries with particular emphasis on the reproducibility of the data and internal consistency of the analysis. Averaging the data at constant momentum transfer extracts the kinematic intensity for both the specular and non-specular beam. The results are analyzed to obtain both structural and electron diffraction parameters. The electron total attenuation coefficient and its energy dependence are determined from both the widths of peaks in the averaged data and from the integrated elastic intensity and are consistent with one another and with previous determinations for Ni(111) and Ni(001). While both the inner potential and the relaxation of the interplanar spacing cause shifts in peak positions, their effects can be separated in the analysis because the latter also causes changes in the relative peak intensities and in the peak shapes. The resulting inner potential, as well as its energy dependence, agree with earlier determinations for Ni(111) and with theoretical calculations to within an uncertainty of about 2 V. From analysis of peak shapes in the averaged data, it is concluded that the surface interplanar spacing is within ±4% of the bulk interplanar spacing at 380 K. This result agrees with estimates obtained from dynamical theories. It is shown that the excess atomic thermal vibrations near the surface, and not experimental uncertainties, limit the accuracy to which this spacing can be determined.     

Surface plasmons on Mo (100)

A new low energy surface plasmon on Mo (100) is revealed by inelastic electron spectroscopy. The significance for XPS spectra is discussed.     

The interplay between the surface band structure and possible surface reconstructions of Mo(112)

The experimental band structure of Mo(112) and the effects by temperature and adsorbate are presented. A surface resonance, identified as crossing the Fermi level at about 1/3 from to of surface Brillouin zone, was observed to be very sensitive to both contamination and temperature. We find evidence of adsorbate and temperature induced reconstruction of the Mo(112) surface. Examination of low-energy electron diffraction (LEED) and scanning tunneling microscopy (STM) data provides evidence for an adsorbate induced reconstruction of the Mo(112) surface with periodicities consistent with the Fermi level crossing of the surface resonance. The reconstruction is found to occur at coverages as low as 0.03 Langmuirs of oxygen or carbon. The reconstruction and/or adsorbate affects the density of states and bands near the Fermi level of a ₁ symmetry. Received 3 March 1999 and Received in final form 1 October 1999     

Surface diffusion of potassium on theW (112) plane

Dependent on temperature and coverage, numerous spectral density functionsW(f) of the field-emission flicker noise of potassium adsorbed on the tungsten (112) plane were determined. The analysis in terms of the Timm and van der Ziel model gives surface diffusion energies between 0.55 and 0.83 eV for (average) coverages from 0.3 to 1.0 and diffusion coefficients between 2×10^–10 and 3×10^–9 cm²/s at 400 K. The results are compared with those obtained previously for the tungsten (111) region. Some conclusions as to the mechanism of diffusion and the manifestation of phase transitions between commensurate and incommensurate adlayer structures are discussed.On leave from Wrocaw University     

The use of the Patterson function in surface structure determination by LEED

Recently, Clarke, Mason and Tescari have proposed a modified Patterson function for deriving surface structures from LEED intensity data. The technique is analysed in this paper by applying it to “averaged” experimental data and to data from kinematical calculations. It is found that truncation errors (associated particularly with the inner potential) prevent the technique from being generally useful, and their absence in the work of Clarke et al. was due to their use of an unrealistic inner potential. In addition, the complex form of scattering factors applicable to LEED provide a fundamental barrier to the use of Patterson functions for the analysis of LEED data in the most interesting case of adsorbate structures on surfaces.     

The influence of sulfur adsorption on the step structure of vicinal Mo(100): a LEED and STM study

The effect of sulfur adsorption on the step structure of vicinal Mo(100) surfaces has been studied with scanning tunneling microscopy (STM) and low energy electron diffraction (LEED). STM was used for low densities of steps on a nominally flat (100) surface. LEED was used to study the more highly and regularly stepped (910), (911), and (28,4,1) oriented surfaces. Steps and the sulfur adsorbate were found to have a strong interaction. The presence and orientation of steps on the surface governed the formation of ordered domains of sulfur, and sulfur modified the structure of the steps on the surface. Both techniques show that monatomic steps predominate on the clean surfaces. When sulfur was adsorbed on the surfaces, however, steps coalesced to form groups of steps, double atomic height steps, and multiple height steps, depending on sulfur coverage and initial step density. The results are discussed in the framework of the theory of equilibrium crystal shape and provide information on how the Mo step-step interactions are affected by sulfur adsorption.     

A combined LEED/RHEED Auger study of oxygen adsorption on the W(112) surface

LEED, RHEED and Auger spectroscopy have been used to study the adsorption of oxygen on to a clean and carbon contaminated (112) face of tungsten. At room temperature all the features reported previously were observed together with a p(1 × 4) surface structure which appeared at an exposure of about 1. 4L just before the formation of the p(1 × 2). Previously a p(1 × 4) structure has been reported only after heating to 2000K. RHEED showed this p(1 × 4) structure clearly; using LEED, the structure was difficult to distinguish. This appears to confirm suspicions that in some situations involving gas adsorption, RHEED has a greater sensitivity than LEED. Possibly most of these situations involve, as does the present p(1 × 4) structure, monolayer islands where the differing coherence widths of the RHEED and LEED beams account for the differing sensitivities. Carbon on the (112) surface also appears to exist as thin islands, either of the previously reported c(6 × 4) structure, or in smaller amounts, on a surface showing (1 × 1) symmetry. Removal of all carbon by heat treatment alone was found to be impossible in a reasonable time and heating in oxygen was necessary. Oxygen adsorption on a carbon contaminated surface did not give rise to any new structures but rather a reduction in the visibility/formation of the clean surface/oxygen structures.