Surface Structure Determination Research Articles

Surface structure determination using ion scattering spectroscopy

Ion scattering spectroscopy (ISS) using low (<10 keV) energy noble gas ion beams [also known as low energy ion scattering (LEIS)] has been used for nearly a decade and a half in the study of surface composition and structure. Recent advances in both the technology associated with ISS and in the understanding of scattering from solids demonstrate that ISS is about to mature as a spectrometric technique and join LEED as an established tool for surface structure determination. The primary difficulty with ISS analysis has been bombardment damage to surfaces, which can be reduced to undetectable levels by using time-of-flight energy analysis or by monitoring the total (energy-integrated or partially filtered) yield of backscattered particles. Present technology is illustrated with an example of a shadow and blocking cone analysis of time-of-flight ISS data from a GaSb (001) surface. Near-future developments in ISS are anticipated by a computer simulation of a total-yield blocking cone analysis of scattering from a Cu (001) surface.

Core level photoelectron spectroscopy for surface crystallography

Inner shell photoionisation as a basis for surface structure determination is reviewed. During the recent decade, this field has received tremendous impetus from the advent of synchrotron radiation. Core level photoelectron spectroscopy allows the detection at submonolayer levels of foreign adsorbate atoms and the resolution of the first layers from the bulk for a clean single crystal surface as well. The surface-bulk core level binding energy shift is shown to be surface geometry dependent and may be employed to monitor clean surface reconstruction. Shifting following chemisorption, surface core levels deliver information on adsorption binding site geometry. The emission intensity is modulated by photoelectron diffraction effects along k-vector variation obtained by varying photon energy or in measurements resolved in azimuthal and polar angles. Interference patterns are sensitive to bond lengths. Finally correlation with surface extended X-ray fine structure achieved by detecting photon stimulated Auger excitation or ion emission is discussed.

5421. Surface structure determination: D P Woodruff,Vacuum,31 (10–12), 1981, 399–405

5421. Surface structure determination: D P Woodruff,Vacuum,31 (10–12), 1981, 399–405

New Approaches to Surface Structure Determinations

Progress in understanding the many scientifically interesting and technologically important processes that occur at surfaces has been slowed by the absence of basic structural information. A variety of new techniques are being developed to attack this central problem in surface science. With the new surface probes it is now possible to quantitatively determine the arrangement of atoms in the first monolayers of a solid. This provides the basis for exciting advances in surface science.

Surface structure determination

Recently many techniques for the determination of surface structures have been proposed, and in some cases developed. This short review attempts to compare some of the relative strengths of a limited number of these techniques (particularly LEED, SEXAFS, photoelectron diffraction, HREELS and ESDIAD.) In particular the information which each can provide on different types of surface structures is discussed in the context of adsorption structures lacking long range order, those forming commensurate and incommensurate ordered structures, adsorption at surface defects, and the elucidation of clean surface reconstruction.

Fourier-transform analysis of normal photoelectron diffraction data for surface-structure determination

A direct method for surface-structure determination from normal emission photoelectron diffraction (NPD) data is presented. Fourier transforms of the calculated NPD intensities yield peaks at adsorbate-substrate normal interlayer distances. Applications are demonstrated, using theoretical NPD curves for the Se/Ni system calculated by dynamical theory. These results show that interplanar spacings between the overlayer and as many as four substrate layers can be determined with an accuracy of better than 3%.

Sexafs: New horizons in surface structure determinations

Abstract As early as the 1930s, it had been observed that the absorption cross section in the X-ray regime had a complex structure as a function of energy extending as far as 1000 eV above an absorption threshold.1 These measurements were made by using a low‐power conventional X-ray tube, a dispersive Bragg spectrometer, and film as a detector. After some correct and incorrect speculation concerning the origin of this extended X-ray absorption fine structure (EXAFS), the field remained essentially dormant through the 1950s. In the late 1960s the measurements were revived,2 this time using modern counting and automation techniques. The previous interpretation3–5 of the structure as arising from the backscattering of the ejected photoelectron from the atom near the absorbing site was strongly supported. The idea that this phenomenon might be used as a local structural probe was born. However, even with the experimental advances the measurements were difficult and time-consuming because of the low power of t...

MeV ion scattering for surface structure determination

Abstract In the last few years a number of groups around the world have been using energetic ion beam scattering from solids as a surface structure tool. These studies have been with energetic particles of greater than 0.1 MeV where scattering cross–sections and the flux distribution of the beam in the solid are well–understood. Low–energy ion scattering, ∼lKeV, which is very surface sensitive, continues as an important structural probe. However, the physics of the atomic collisions and the method of extraction of information are sufficiently different in these two regimes for each to obtain an individual character.

Calculated beam intensities for low energy positron diffraction

The intensity of the specular beam for low energy positron diffraction (LEPD) from the Cu(111) surface is calculated in the energy range 20–150 eV. Close agreement is found with the first experimental data for LEPD published recently. This result indicates that LEPD may be exploited as a technique for determining surface structures in the near future. It is suggested that LEPD may have advantages in surface structure analysis in comparison with low energy electron diffraction (LEED).

Calculated beam intensities for low energy positron diffraction

The intensity of the specular beam for low position diffraction (LEPD) from the Cu(111) surface is calculated in the energy range 20–150 eV. Close agreement is found with the first experimental data for LEPD published recently. This result indicates that LEPD may be exploited as a technique for determining surface structure in the near future. It is suggested that LEPD may have advantages in surface structure analysis in comparison with low energy electron diffraction (LEED).

Site-selective adsorption of xenon on a stepped Ru(0001) surface

The adsorption of xenon has been studied with UV photoemission (UPS), flash desorption (TDS) and work function measurements on differently conditioned Ru(0001) surfaces at 100 K and at pressures up to 3 × 10 −5 Torr. Low energy electron diffraction (LEED) and Auger electron spectroscopy (AES) served to ascertain the surface perfectness. On a perfect Ru(0001) surface only one Xe adsorption state is observed, which is characterized by Xe5 p 3 2 , 1 2 electron binding energies of 5.40 and 6.65 eV, an adsorption energy of E ad≈ 5 kcal/mole and dipole moment of μ' T ≈ 0.25 D. On a stepped-kinked Ru(0001) surface, the terrace-width, the step-height and step-orientation of which are well characterized with LEED, however, two coexisting xenon adsorption states are distinguishable by an unprecedented separation in Xe 5 p 3 2 , 1 2 electron binding energies of 800 meV, by their different UPS intensities and line shapes, by their difference in adsorption energy of ΔE ad ≈ 3 kcal/mole and finally by their strongly deviating dipole moments of μ S = 1.0 D and μ T = 0.34 D. The two xenon states (which are also observed on a slightly sputtered surface) are identified as corresponding to xenon atoms being adsorbed at step and terrace sites, respectively. Their relative concentrations as deduced from the UPS intensities quantitatively correlate with the abundance of step and terrace sites of the ideal TLK surface structure model as derived from LEED. Furthermore, ledge-sites and kink-sites are distinguishable via E ad. The E ad heterogeneity on the stepped-kinked Ru(0001) surface is interpreted in terms of different coordination and/or different charge-transfer-bonding at the various surface sites. The enormous increase in Xe 5p electron binding energy of 0.8 eV for Xe atoms at step sites is interpreted as a pure surface dipole potential shift. —The observed effects suggest selective xenon adsorption as a tool for local surface structure determination.

Surface structure determinations with ion beams

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTSurface structure determinations with ion beamsNicholas Winograd and Barbara J. GarrisonCite this: Acc. Chem. Res. 1980, 13, 11, 406–412Publication Date (Print):November 1, 1980Publication History Published online1 May 2002Published inissue 1 November 1980https://doi.org/10.1021/ar50155a004RIGHTS & PERMISSIONSArticle Views50Altmetric-Citations40LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (978 KB) Get e-Alerts Get e-Alerts

Determination of adsorbate geometries from final state scattering in X-ray photoemission: Carbon monoxide and oxygen on (001) Ni

Azimuthal anisotropies in deep core level X-ray photoemission from atoms adsorbed on single-crystal surfaces have recently been observed and been found to be well described by a single-scattering model, thereby providing a new method for determining surface atomic geometries. We here apply this technique to a molecular chemisorption system, specifically c(2 × 2) CO on (001) Ni. When the C 1s intensity is measured as a function of the polar angle, a peak in intensity with magnitude I max /I back = 1.5 and a full width at half maximum intensity of 25° is found to be centered on the surface normal. The average orientation of CO is thus determined to be within 12° of normal. Also oxygen atoms chemisorbed on (001) Ni were studied as a function of exposure to O 2. The azimuthal variation of the O 1s intensity is strongly dependent on the oxygen coverage. For voverages greater than 0.5 monolayer the best agreement between theory and experiment is obtained if the oxygen atoms are assumed to lie approximately coplanar with the surface nickel atoms. For lower coverages (⪅0.3) an adsorbate vertical position of about 0.8 Å above the first nickel layer gives the best fit. These results are in qualitative agreement with prior experimental studies of this system using other methods and thus suggest a potentially wide applicability of similar measurements for surface structure determinations.

Aims and recent accomplishments of surface theory

The goals of surface science include the understanding of surface bonding, structure, transport, and chemistry. However, our present activities are mainly focused on the most basic problem, viz., what are the constituents of a given surface, where are they located and how are they bonded? Theorists are approaching this surface characterization problem from two sides. To make better use of the data available from the many experimental surface probes, we are developing an understanding of the force laws which govern the motion of probe particles near surfaces, and we are studying intensively the relation between the excitation and ground state spectra of a solid. In order to develop a predictive capability regarding surface structure, we are studying a variety of intrinsic surface properties including the nature of bonds at transition metal surfaces, the meaning of bond locality in extended systems, and generally the electronic factors underlying the forces which govern surface geometry. These studies often raise technical issues such as the validity of using local exchange correlation potentials and the applicability of cluster calculations to the analysis of extended surface situations, which, however, should not obscure the main thrust of current work — empirical and predictive surface structure determination. These points will be illustrated with examples from recent research and ways in which experimental surface work might assist theoretical efforts will be indicated.

Reliability of surface structure determinations by LEED

The search for a solution in the problem of surface structure determination by analysis of low-energy electron diffraction (LEED) intensities is made by comparing a number of calculated and observed intensity spectra with one another. The test for the correctness of a given structural model consists often in the subjective judgment of the quality of the fit between calculated and observed curves, a judgment based merely on visual comparison between the two sets of data. The subjectivity and arbitrariness of this procedure have long been deplored, and have often led to controversies, inaccurate reports, and sometimes blunders. The subsequent need for an objective criterion of evaluation seem to be filled by the reliability factor proposed by Zanazzi and Jona. Definition and properties of this r factor are reviewed, and examples and suggestions of its use for both the evaluation and the refinement of surface structural models are given.

Surface structure effects via photoexcited core electron diffraction for Na on Ni

Photoemission from the Na 2p core level has been measured in the range ℏ ω = 40–120 eV for a c(2×2) Na overlayer on a Ni(001). surface. Weak oscillations in the Na 2p emission intensity vs. wavelength were observed and are interpreted as due to photoelectron scattering. The observed peaks closely match, in energy, those derived in recent calculations which assume the Na occupies a hollow site 2.23Å above the Ni surface layer. The data therefore indicate that observation and calculation of such effects may allow surface structure determinations.

Surface structure determination by leed rotation diagrams; Application to the surface relaxation of W(001)

We examine the potentialities of LEED rotation diagrams at large angles of incidence for surface structure determination. From a comparison of relativistic model calculations for the (00) beam from unreconstructed W(001) to experimental results we find that the structure of the rotation diagrams is essentially determined by strong intralayer scattering within the first atomic layer, whereas peak intensities in rotation diagrams depend strongly on the distance to the adjacent layers. By means of a simple criterion based on the comparison of theoretical and experimental peak intensity ratios, the top layer contraction is determined to be 5.5% ± 1.5%. The influence of the parameters entering the calculations and of the experimental uncertainties on the validity of this result are discussed.

Xenon adsorption as a tool for local surface structure determination at Ir(100) surfaces

The adsorption of Xe at reconstructed (5 × 1), unreconstructed (1 × 1) and partially reconstructed Ir(100) surfaces has been studied using UV photoemission. It is found that the Xe induced photoemission peaks, due to emission from Xe 5 p 1 2 and 5 p 3 2 orbitals, are displaced by 0.3 eV comparing spectra obtained at (1 × 1) and (5 × 1) Ir(100) surfaces at saturation Xe coverage. In addition, with both surface structures the Xe induced 5p photoemission features shift to higher binding energies with increasing coverage in accord with various other Xe/metal systems. Using the Xe 5p emission as a fingerprint of the surface structure, spectra obtained at partially reconstructed Ir(100) surfaces clearly are composed of weighted contributions from Xe adsorbed at (1 × 1) and (5 × 1) surfaces, respectively. From this it can be concluded that adsorbed Xe senses the local surface structure present in patches of (1 × 1) and (5 × 1) surface geometry.

Diffraction of Photoelectrons Emitted from Core Levels of Te and Na Atoms Adsorbed on Ni(001)

Azimuthal anisotropies in the photoemission from the Te $4d$ and Na $2p$ core levels have been measured for these atoms adsorbed in the $c(2\ifmmode\times\else\texttimes\fi{}2)$ configuration on Ni(001). The observed variations with photoelectron kinetic energy, polar angle of emission, and adsorbate species indicate that this is a very promising technique for surface-structure determination. Preliminary multiple-scattering calculations show encouraging agreement with experiments.

Calculation of low-energy-electron-diffraction intensities fromZnO(101¯0). II. Influence of calculational procedure, model potential, and second-layer structural distortions

Dynamical calculations of the intensities of normally-incident low-energy electrons diffracted from $\mathrm{ZnO}(10\overline{1}0)$, performed using an "exact" matrix-inversion method, are compared both with earlier calculations based on the renormalized-forward-scattering (RFS) method and with measured intensities. The sensitivity of the calculated intensities to the choice of model potential and the magnitude of thermal atomic vibrations is displayed within the context of examining the implications of uncertainties in nonstructural model parameters on surface-structure determinations via elastic low-energy-electron diffraction (ELEED) intensity analyses. We extend our earlier analysis of the surface structure of $\mathrm{ZnO}(10\overline{1}0)$ by utilizing the matrix inversion rather than RFS method, a recently revised bulk geometry for ZnO, an improved model potential, and a consideration of second-as well as top-layer structural distortions. The combination of these four improvements lead to the selection of the most probable surface structure for $\mathrm{ZnO}(10\overline{1}0)$ as one in which the top-layer oxygen is displaced vertically downward by $\ensuremath{\Delta}{d}_{\ensuremath{\perp}}(\mathrm{O})=\ensuremath{-}0.05\ifmmode\pm\else\textpm\fi{}0.1$ \AA{} and the top-layer zinc likewise by $\ensuremath{\Delta}{d}_{\ensuremath{\perp}}(\mathrm{Zn})=\ensuremath{-}0.45\ifmmode\pm\else\textpm\fi{}0.1$ \AA{}. No compelling evidence either for lateral distortions within the top layer or for second-layer distortions is obtained, although small improvements in the agreement between the calculated and observed intensities can be achieved by considering them. Our major conclusion is that given the limitations in the available ELEED intensity data and the uncertainties in the model potential and surface atomic vibrations, the vertical distortion cited above constitute the maximum structural information that can be extracted unambiguously via ELEED intensity analysis at the present time.