LEED calculations for the NiO(100) surface

Dynamical calculations have been performed to determine the surface structure of NiO(100) from a comparison with experimental LEED data. As a preliminary to the structural determination an investigation was made of the effect of different assumptions in the construction of the muffin-tin potential. It was found that these were of secondary importance compared to the structural parameters thus giving us confidence in the final result which was that there is negligible rearrangement (<5% of the interlayer spacing) of the atoms at the NiO(100) surface.     

LEED studies on stepped W surfaces

Steps on single crystal surfaces have been shown to cause changes of various physical properties and to influence the behaviour towards chemical reactions. A proper knowledge of the step structure is required for the understanding of these phenomena. The following investigation concentrates on a detailed evaluation of the LEED patterns of various stepped tungsten surfaces. A formula is given for determining the terrace width of ordered step arrays from any diffraction order. Step height, step orientation, terrace width and the sample orientation have been deduced from the LEED patterns and the experimental errors involved are being discussed. The step height can be determined within 1% and the terrace width within 1 to 3% depending on the step density. It is concluded that the determination of the sample orientation as obtained from the LEED pattern is at least as precise as by using the Laue X-ray back reflection technique.     

LEED studies of vicinal surfaces of germanium

Clean germanium surfaces inclined at small angles to (111), (100) and (110) planes were investigated by LEED. Surfaces with orientations close to (111) and (100) are stepped and regular steps are retained in the whole temperature range investigated.Steps with (111) terraces and edges towards $\text{[2}\text{1}\text{1}\text{]}$ have a height of about one interplanar distance d₁₁₁ at all temperatures, and steps with edges towards $\text{[}\text{2}\text{11]}$ have a height of about two interplanar distances below 500°C and of about one interplanar distance above 500°C. Steps with (100) terraces and edges in the [011] direction have a height about two interplanar distances d₁₀₀. The surfaces with orientations close to (110) are facetted at room temperature. The (17 15 1) facets are present on the surfaces oriented in the $\text{[}\text{1}\text{10]}$ zone and the (10 9 2) facets on the surfaces oriented in the [001] zone. At high temperatures (about 480 and 770°C respectively) a reversible structural reconstruction of these surfaces into stepped ones takes place.     

LEED calculations of exchange reflections from antiferromagnetic NiO(100)

Multiple scattering LEED calculations have been performed for the intensities of the half-order features of the NiO(100) surface, assuming different exchange potentials on the different magnetic sublattices. Approximate methods have been tested on a model NaCl like structure, in which the different sublattices have opposite spin, and for which exact calculations can be performed. These techniques, thus validated, have been applied to the NiO(100) surface, and comparisons are made with the limited experimental data currently available.     

LEED studies of adsorption on vicinal copper surfaces

As a means for studying the role of atomic steps in adsorption phenomena LEED has been used to investigate the properties of vicinal copper surfaces. Single crystalline surfaces were cut at angles up to 20° from the (100) pole along [001] and [011&#x0304;] zones. The diffraction patterns obtained for the clean surfaces and after adsorption of oxygen, nitrogen ions, carbon and sulphur are described. The emphasis of the paper is on the method of interpretation of the geometry of the patterns, which may be done by straightforward kinematic analyses. In the case of nitrogen it is found that if the steps are widely separated the structure of the layer adsorbed on the terrace is the same as that on the low index surface. When the step spacing is small, and comparable with the crystalline parameter of the adsorbed layer, modifications occur which give rise to different superlattices which extend over several terraces. Adsorption of sulphur on 〈11〉 steps can produce a change in the periodicity of the adsorbed layer parallel to the step direction. The study of diffraction patterns for vicinal surfaces with different step spacings may provide an interesting technique for verifying the interpretation of patterns for low index surfaces.     

Electron attenuation anisotropy at crystal surfaces from LEED

Dynamical theory of electron scattering is used to describe the electron transport in the surface regions of crystals. The angle resolved attenuation length of electrons is derived from the transmitted LEED electron current decay. Electron attenuation length energy dependence and anisotropy in polar angle are found for crystalline Cu(1 1 1) for two high symmetry azimuths. Pronounced anisotropy in polar angle distributions of attenuation lengths is found to be in qualitative agreement with the results obtained from the photoelectron diffraction. Comparison with the attenuation lengths obtained from semiclassical simulations for amorphous copper is given. This comparison demonstrates that simple transfers of the smoothly behaving surface sensitivity from amorphous materials oversimplifies the electron attenuation process and can lead to incorrect results in quantitative analyses of crystalline surfaces.     

LEED spectra study of temperature effects in crystalline xenon surfaces

The temperature dependence of normalized LEED intensity data was obtained on well-ordered and chemically pure xenon single crystal (111) surfaces. It is concluded that the strongly kinematic character of the spectra makes it possible to obtain a useful and unambiguous evaluation of the surface Debeye-Waller factor. A tentative analysis of the surface Debeye temperatures for two well-defined Bragg peaks is compared to that based on a first principle calculation where specific surface effects are included.     

Single-beam trapping in front of reflective surfaces

We show that the optical trapping of dielectric particles by a single focused beam in front of a weakly reflective surface is considerably affected by interference of the incident and reflected beams, which creates a standing-wave component of the total field. We use the two-photon-excited fluorescence from a trapped dyed probe to detect changes in the distance between the trapped beam focus as the focus approaches the reflective surface. This procedure enables us to determine the relative strengths of the single-beam and the standing-wave trapping forces. We demonstrate that, even for reflection from a glass-water interface, standing-wave trapping dominates, as far as 5 mum from the surface.     

Spin-polarized LEED from low-index surfaces of platinum and gold

A relativistic theory of LEED has been applied, in the energy range from 0 to 200 eV, to (111) and (001) faces of Pt and Au. The calculated intensity versus energy profiles are in good agreement with experimental data, which, in particular, provides support for interpreting the observed surfaces as unreconstructed. As a consequence of strong spin-orbit coupling, large spin-polarization peaks are found, partly in conjunction with intensity maxima. As a rough guide to diffraction conditions, at which both polarization and intensity are sizeable, a simple kinematic model might be of some use.     

稳定In/Si表面的LEED研究

利用LEED图形拟合的方法对大量不同取向In/Si表面的稳定性和小面化进行了研究，新发现了In覆盖度在1/2单层原子以下的三个稳定表面：Si(214)-In，Si(317)-In和Si(436)-In，以及In覆盖度在1单层原子左右的两个稳定表面Si(101)-In和Si(313)-In.此外还确定了In覆盖度在1单层原子左右的6个稳定In/Si表面的家族领地以及In覆盖度在1/2单层原子以下的4个稳定In/Si表面的家族领地.特别值得注意的是 Si(103)-In的家族领地相当大，甚至比最稳定的Si(1 关键词： 硅表面 铟 稳定表面 家族领地     

LEED studies of clean high Miller index surfaces of germanium

Clean high Miller index Ge surfaces the poles of which lie on the sides of a unit stereographic triangle were studied by LEED. Clean surfaces of germanium may coincide with the atomic planes of the same indices or consist of the regular steps with (111) or (100) terraces, or contain the “hill and valley” configuration depending on the orientation. Surface atomic planes are reconstructed. On clean Ge surfaces at certain temperatures the reversible order-disorder and order-order transitions take place.     

LEED studies of UO2(∼111) vicinal surfaces

The ordering/disordering of terraces, ledges and kinks on three UO₂(~ 111) vicinal surfaces were examined by LEED. Optical transforms were used as an indication of the types of diffraction displays expected. On surfaces with high kink and ledge densities and unheated above 600 °C, the ledges on the average are equally spaced and parallel, but the kink-site positions are random. At higher temperatures of 700 < T < 900 °C, the ledges rotate and decompose irreversibly into {553} and {311} microfacets. By transform simulations it was inferred that ledges on the UO₂{553} surfaces are highly ordered and contain less than 7% kink-sites.     

LEED studies of clean high Miller index surfaces of silicon

Clean high Miller index surfaces of silicon were studied by LEED at different temperatures. Three types of surfaces were observed depending on orientation: flat surfaces, stepped ones with the (100) and (111) terraces and with the step heights of one or two interplanar distances, and “hill and valley” or facetted surfaces. All clean surfaces of silicon are reconstructed and contain surface structures with the periodicities different from the periodicities in the bulk. At certain temperatures the reversible order-order and order-disorder transitions take place on clean surfaces of silicon.     

A LEED study of UO2(∼100) vicinal surfaces

The ordering and faceting properties of UO₂(～100) vicinal surfaces have been studied via LEED and Auger measurements. The measurements have demonstrated a reduced tendency for step ordering on UO₂(～100) vicinal surfaces when compared to step ordering on UO₂(～111) vicinal surfaces. The UO₂(～100) vicinal surfaces were observed to decompose irreversibly into low-index facets, including prominent (100) facets, at temperatures below those needed for creation of lowest index faceting on UO₂(～111) vicinal surfaces. These properties suggest that (100) terraces, in contrast to (111) terraces, act as surface diffusion barriers that limit longrange surface communication while growing at the expense of intermediate faceting stages.     

A combined LEED,AES and XPS study of the ZnO {0001} polar surfaces: I. LEED,AES and XPS of contaminated surfaces

The {0001} polar surfaces of ZnO single crystals have first been examined after a chemical treatment involving HCl and H₃PO₄ and a 24 hr bakeout at 250 °C. The impurities detected on the (000$\text{1}$)-O surface with AES were carbon, chlorine, phosphorus and to a lesser extent sulphur. On the (0001)-Zn surface, carbon, chlorine and sulphur were the dominant impurities, while the phosphorus signal was less important. These results were confirmed by XPS measurements on frehsly etched surfaces. The AES spectra were recorded as distribution curves N(E). Averaging, curve-fitting and related numerical techniques were used to obtain high resolution spectra, enabling the identification of the phosphorus L₁-transitions. The etched surfaces were cleaned progressively using argon ion bombardment and ohmic heating. It has been consistently observed that the clean surfaces exhibit primitive (1 × 1) structures. Superstructures such as $\text{(}\text{3}\text{×}\text{3}\text{)}$ on the (000$\text{1}$)-O surface, and $\text{(4}\text{3}\text{× 4}\text{3}\text{)}$ and (3 × 3) on the (0001)-Zn surface, were repeatedly observed at discrete spots of contaminated surfaces. A clear correlation with impurities as observed by AES however could not be found. Facetting was observed after prolonged heating.     

Charge neutrality condition for solid surfaces

The charge neutrality condition for the plane contact of the two media is derived, using the technique of the surface Green functions. For illustration, a simple semiinfinite one-dimensional Sommerfeld model with a-function is discussed.     

Theoretical spin polarization and intensity profiles in LEED from low-index surfaces of tungsten

Subsequent to the recent measurement of spin polarization effects in LEED, a relativistic LEED theory has been modified such as to facilitate computations for energies up to about 200 eV. Application to several low-index surfaces of tungsten yields intensity-energy profiles in good agreement with experiment. The calculated spin polarization profiles exhibit large maxima and show encouraging agreement with the as yet limited experimental data. Contraction of the top layer spacing with respect to the bulk spacing is found to produce drastic changes in parts of the polarization profiles. LEED spin polarization analysis could therefore provide a sensitive means of surface structure determination. Further, some polarization maxima coincide in energy with sizeable intensity values. This offers promising prospects for the construction of a low-energy spin polarization detector on a double diffraction basis and of an intense source of polarized electrons.