Relativistic effects on LEED intensities from Au(111)

Comparison of relativistically and nonrelativistically calculated intensity versus energy profiles in low energy electron diffraction (LEED) from the (111) surface of Au (Z = 79) reveals that relativistic corrections are quite significant. They can however, be obtained in very good approximation by quasirelativistic calculations, in which spin-averaged relativistic phase shifts are used as input for the nonrelativistic multiple scattering formalism. Further, relativistic effects on intensities are found to be comparable to differences arising from different approximations to the exchange part of the ion core potential.     

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.     

LEED and UPS study of Ni ≅ (001) vicinal surfaces

Two stepped surfaces of nickel in the vicinity of the (001) plane have been studied by LEED and UPS. On clean surfaces, diffraction patterns show reversible step rearrangements when temperature and contamination are varied. A simple model is proposed describing this reconstruction. UPS spectra measured on these vicinals show no remarkable differences with corresponding EDC's for the clean simple Ni(001) surface, suggesting a large similarity in their respective local density of states. Some results concerning the stability of the surface morphologies upon sulphur adsorption are described.     

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.     

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.     

Irrationally related lattices and LEED intensities

A study of Nb layers deposited on a Cu {001} substrate provides an example of irrationally related lattices. The low-energy electron diffraction (LEED) intensity spectra from the clean substrate are almost unaffected by the presence of the Nb overlayer.     

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.     

LEED intensities from clean and hydrogen covered Ni(100) and Pd(111) surfaces

Hydrogen adsorbs on Ni(100) and Pd(111) surfaces without the formation of additional diffraction spots in the LEED patterns. Measurements of LEED intensities revealed that adsorbed hydrogen layers cause considerable changes even in such cases where displacements of surface atoms (“reconstructive adsorption”) may be excluded. After hydrogen adsorption on Ni(100) the intensities of Bragg beams are uniformly lowered whereas the background intensity increases which is attributed to the formation of a disordered adsorbed layer. With Pd(111) adsorbed hydrogen causes a slight decrease of the background intensity and characteristic modifications of the intensity/voltage curve of the (0,0) beam, suggesting the formation of an ordered 1 × 1 structure. In the latter case energy shifts of the primary Bragg maxima were observed and are interpreted as being caused by an expansion of the layer spacing in the surface region by about 2% owing the partial dissolution of the hydrogen.     

Is there a future for LEED intensities?

The application of LEED intensities within surface crystallography is illuminated with respect to its today's standard, frontier and hopes. It is shown that structure determination is possible with high accuracy in case of simple surfaces. For more complex surfaces with about a handful of atoms in the surface unit-cell structural parameters are still reliably determined, however, with tremendously increasing computational amount of work. Some approximative approaches which could show a way out of the misery are described. However, importance is attached to other and newer developments of LEED as well. So the possibility to extract structural informations from diffuse intensities in case of disordered adsorption is demonstrated as well as the use of intensities to probe the critical phenomena and kinetics of surface phase transitions. Examples are given which make the future of LEED intensities appear quite promising.     

STM and LEED studies of CaF2 submonolayer coverage on Si(001)

In the present work we report on the initial stage of CaF₂ growth on Si(001) at 750 °C. Samples with submonolayer coverage are grown and in situ studied by low energy electron diffraction and scanning tunneling microscopy. The unusual diffraction patterns with asymmetrically split fractional reflections and streaks are ascribed to the antiphase relations at the surface characteristic for the studied CaF₂ coverage. A statistical model built to describe the partially ordered array of 3× cells shows good agreement with the observed diffraction patterns. Similarities between CaF₂/Si(001) and metallic Ca/Si(001) interfaces are discussed.     

Diffuse LEED intensities of disordered crystal surfaces: III. LEED investigation of the disordered (110) surface of gold

The LEED pattern of clean (101) surfaces of Au show a characteristic (1 × 2) superstructure. The diffuseness of reflections in the reciprocal [010] direction is caused by one-dimensional disorder of chains, strictly ordered into spatial [101̄] direction. There is a transition from this disordered superstructure to the normal (1 × 1) structure at 420 + 15°C. The angular profiles of the $\text{(0}\text{1}\text{2}\text{)}$ and (01) beam are measured at various temperatures and with constant energy and angles of incidence of the primary beam. The beam profiles are deconvoluted approximately with the instrument response function.     

Displacement disorder and reconstruction of the (001) face of tungsten

The reconstruction of the (001) border of tungsten is examined taking into consideration random static displacements of surface atoms in the high-temperature (1 × 1) phase. A microscopic model is proposed, in which the creation of the c(2 × 2) phase is described as a transition of the Jahn-Teller type and an ordering of static displacements. It is shown that displacement disorder induces instability of (001) tungsten with respect to reconstruction. The effect of a uniform electric field on a disordered reconstructing surface is examined. A possible reason is given for pronounced differences in the results of investigations of the structural conversion of the (001) face in tungsten when different experimental methods are used.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 8, pp. 24–35, October, 1991.     

Adsorption of silver on (001), (011) and (111) oriented tungsten single crystals

Work function changes caused by silver adsorption were measured on (001), (011) and (111) tungsten single crystal planes using electron beam method. The decrease of the work function was observed on all planes studied in the small coverage region. The result evidences that the increase previously observed for the average work function in this coverage region can not be connected with negative dipole formation with silver atoms.     

A LEED investigation of the chemisorption of nitrous oxide on a tungsten (100) surface

Nitrous oxide has been adsorbed on the tungsten (100) surface at 295 and 500°K. Flash desorption experiments show two binding states of nitrogen which desorb at 1005 and 1275°K. When both nitrogen states and the oxygen state from the dissociated nitrous oxide molecule are on the surface, there are no new LEED features. When the 1005°K nitrogen state is desorbed, a (4 × 1) LEED pattern occurs. When all the nitrogen is desorbed, a (2 × 1) oxide pattern remains. The oxygen is desorbed as a volatile tungsten oxide. A model of the sorbate surface which is consistent with the facts currently available on the nitrous oxide-tungsten system is described.     

On the contraction of the W(001)-(1 × 1) surface using LEED intensity analysis

Three recent independent attempts at deducing the W(001)-(1 × 1) surface structure by LEED beam intensity analysis have yielded contractions of the topmost layer spacing of 6 ± 6%, 11 ± 2%, 4.4 ± 3% normal to the surface plane. We investigate possible reasons for the discrepancies by comparing published experimental and theoretical profiles of these workers as well as our own. Our main conclusions are that the direct comparison of experimental data of different investigators shows deviations which are comparable to the changes in the calculated profiles for various surface contractions. Also the deviations between calculated intensity profiles using different (but still realistic) assumed scatteting potentials are comparable to the changes in the calculated profiles for various surface contractions. The main uncertainty in the scattering potential is the choice between the Slater free electron exchange-correlation term (coefficient α = 1) or the Kohn-Sham version ($\text{α =}\text{2}\text{3}$) or a value in between. For tungsten the corrections due to relativistic atomic scattering must also be considered. These uncertainties in the calculated and experimental profiles lead to the conclusion that the surface layer contraction of W(001)-(1 × 1) is not known at the present time. To assess the potential of LEED in deducing surface structures of this type further comprehensive analyses are required where the uncertainties in the theoretical scattering potential are also examined.     

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

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.     

Determination of the Co(001)c(2 × 2)-S superstructure by comparison of LEED intensity data

A cross-comparison of LEED intensity data for Co(001)c(2 × 2)-S and Ni(001)c(2 × 2)-S gives evidence that the sulfur atoms are chemisorbed on Co(001) in a fourfold site, at 1.3 Å from the first substrate layer.