Photoelectron Diffraction Intensities Research Articles

Effects of epitaxial films of nanometric thickness on the X-ray photoelectron diffraction intensities from substrates

The effects on the X-ray photoelectron diffraction intensities from the substrate produced by epitaxial NiO(001) films of various thickness deposited on Ag(001) were investigated. The variations in the Ag XPD curves induced by the NiO films can be explained in terms of multiple scattering of the electrons emitted by the substrate atoms along the close-packed rows of the overlayer. Intensity minima in the XPD curves from the substrate in correspondence to intensity maxima in the XPD curves from the overlayer are observed when the thin film is commensurate with the substrate. For films of suitable thickness, the analysis of XPD curves from the substrate allows one to get information about the structure of the film and of the film–substrate interface.

Global search in photoelectron diffraction structure determination using geneticalgorithms

Photoelectron diffraction (PED) is an experimental technique widely used to performstructural determinations of solid surfaces. Similarly to low-energy electron diffraction(LEED), structural determination by PED requires a fitting procedure between theexperimental intensities and theoretical results obtained through simulations. Multiplescattering has been shown to be an effective approach for making such simulations. Thequality of the fit can be quantified through the so-called R-factor. Therefore, the fittingprocedure is, indeed, an R-factor minimization problem. However, the topography of theR-factor as a function of the structural and non-structural surface parameters to bedetermined is complex, and the task of finding the global minimum becomes tough,particularly for complex structures in which many parameters have to be adjusted. In thiswork we investigate the applicability of the genetic algorithm (GA) global optimizationmethod to this problem. The GA is based on the evolution of species, and makes use ofconcepts such as crossover, elitism and mutation to perform the search. We showresults of its application in the structural determination of three different systems:the Cu(111) surface through the use of energy-scanned experimental curves; theAg(110)–c(2 × 2)-Sb system, in which a theory–theory fit was performed; and the Ag(111) surface for whichangle-scanned experimental curves were used. We conclude that the GA is a highly efficientmethod to search for global minima in the optimization of the parameters that best fitthe experimental photoelectron diffraction intensities to the theoretical ones.

Adsorption of oxygen onPt3Sn(111)studied by scanning tunneling microscopy and x-ray photoelectron diffraction

The adsorption of oxygen on the (111) surface of the ${\mathrm{Pt}}_{3}\mathrm{Sn}$ alloy was studied by means of scanning tunneling microscopy (STM) and x-ray photoelectron diffraction (XPD). As Auger electron spectroscopy, low-energy ion scattering, and x-ray photoelectron spectroscopy indicate, upon oxygen exposure (range 10000 L, with the sample at circa 770 K) Sn segregates to the surface and forms a two-dimensional tin-oxygen layer. In the x-ray photoemission spectra no pronounced chemical shift is visible that would indicate a thicker tin oxide layer. After exposure to 10000 L ${\mathrm{O}}_{2}$ at 770 K low-energy electron diffraction shows a streaky2\ifmmode\times\else\texttimes\fi{}2 pattern with additional spots. Scanning tunneling microscopy images show a surface with a 2\ifmmode\times\else\texttimes\fi{}2periodicity characterized by an high defect density. The corrugation of this surface is substantially higher than that of the clean surface. After annealing in vacuum at temperatures ranging from 600 to 800 K, a sharp 4\ifmmode\times\else\texttimes\fi{}4 low-energy electron diffraction pattern can be observed. STM then reveals a superlattice of depressions, the remaining protrusions are slightly laterally displaced from their 2\ifmmode\times\else\texttimes\fi{}2 positions. X-ray photoelectron diffraction intensities of the 4\ifmmode\times\else\texttimes\fi{}4 phase show hardly any change for Pt $4f,$ whereas $\mathrm{Sn}3d$ azimuthal curves measured at higher polar angles are substantially modified after oxygen exposure. In order to understand the nature of the features observed in the STM images, the experimental XPD curves were compared with single and multiple scattering cluster calculations performed for various structural models. On the basis of these results we propose a model involving the reconstruction of the substrate surface.

Computation of photoelectron and Auger-electron diffraction III. Evaluation of angle-resolved intensities PAD3

In this paper, we describe the third part of a suite of computer programs for the computation of Auger- and photoelectron diffraction intensities, using the so-called concentric shell algorithm (CSA). The function of the present program is to calculate the diffraction intensities capable of being measured in a variety of different experimental configurations. The present program takes as its input the cluster transmission matrix calculated in the second part of this sequence of programs, as well as the spherical wave amplitudes of the atomic photoemission process, and other input specifying the type of experiment. The output of this program is a file containing the diffraction intensities in a form for direct comparison with experimental results.

Angle-resolved PED and AED calculations for different structures of the diamond C(111) surface

Angle-resolved (AR) photoelectron diffraction (PED) spectra for electrons excited from the C 1s core state and angle-resolved KVV Auger electron diffraction (AED) spectra are calculated for the Pandey and the Tsai stucture models of diamond C(111) which extend previous investigations of the ideal structure. It is shown how to decide on the structure model by comparing PE spectra for different directions and by comparing PED and AED spectra. Calculations have been performed by evaluating the scattering path operator for a finite cluster in a curved-wave approximation. The different matrix elements for the photoelectron excitation and for the Auger process, respectively, are included. It is shown that the PED intensities are very sensitive to the surface reconstruction for polar angles in the range of 80°. In the AED intensities, polar scans in the plane perpendicular to the chain direction can be considered.

Manifestation of Quantum Chaos in Scattering Techniques: Application to Low-Energy and Photoelectron Diffraction Intensities

Intensities of low-energy electron diffraction and photoelectron diffraction are analyzed from a statistical point of view. The probability distribution is compared with a Porter-Thomas law, characteristic of a chaotic quantum system. The agreement obtained is understood in terms of analogies between simple models and Berry's conjecture for a typical wave function of a chaotic system. The consequences of this behavior on surface structural analysis are qualitatively discussed by looking at the behavior of standard correlation factors.

Influence of surface oxidation on the photoelectron diffraction intensities from InP single crystals

Abstract The Monte Carlo method was used to calculate the influence of amorphous overlayers on the angle-dependent intensity distribution due to photoelectron diffraction (XPD) in monocrystalline substrates. The influences of the overlayer thickness, the elastic and inelastic mean free paths in the amorphous overlayer, the shape of the initial intensity distribution, as well as the overlayer chemical composition, on XPD is discussed and some trends are formulated. The simulated P 2p intensities of oxidized InP agree with the experimental data. The effect of elastic scattering on the intensity minimum around the detection angle 18° is stronger than that of inelastic scattering. A new method for the thickness determination of thin overlayers on single ecrystals is discussed.

Chemical-state-specific low-energy photoelectron diffraction on III-V semiconductors

A theoretical photoelectron diffraction (PD) study was performed on the (110) surface of III–V semiconductors like InSb, GaSb, InP, GaP, InAs and GaAs at low photon energies (40–90 eV) in order to probe the influence of the scattering properties of the different atoms in these semiconductors as well as that of the geometric structure. The atoms at the surface are chemically different from their bulk counterparts and by achieving the photoelectron diffraction intensities from each one, it is possible to probe independently the structural environment around each atomic species. In a photoemission experiment as a consequence of high surface sensitivity and overall experimental resolution the core-level peaks can be decomposed into bulk and surface components. As an example experimental and theoretical photoelectron diffraction data for InP(110) are compared to find out the sensitivity of the PD pattern to the structural parameters.

A study on the holographic transfrom for electron diffraction from surfaces

We break down photoelectron diffraction intensities into four terms in analogy to optical holography and discuss the effect of each term on reconstructed images. The second term involving products of scattered waves ΣΣ O i O* j , is in this case not structure-less. Theoreticl analysis and simulations demonstrate that this term may lead to spurious features in real space images in holographic transforms of medium energy electron diffraction patterns. If it is small enough the problem may be overcome by an iterative correction process.

The role of core orbital angular momentum in the calculation of X-ray photoelectron diffraction intensities

An expression for the photoemission amplitude from an s core orbital of an atom in a crystalline environment, developed within a plane wave approximation and frequently used to predict the angular variation in intensity observed in X-ray photoelectron diffraction, is extended straight-forwardly so as to take general account of core subshell orbital angular momentum l c. It is found that the main structure observed in XPD, due to strong forward scattering of the photoelectron by atomic centres, is insensitive to l c but that secondary features associated with interference effects should be progressively suppressed for higher values of l c.

Photoelectron diffraction from Te on Ni(100) and Cu(100)

Angle resolved measurements of the azimuthai anisotropy in the photoelectron diffraction intensities from the Te 4d level in Ni(100)p(2 × 2)Te, Ni(100)c(2 × 2)Te and Cu(100)p(2 × 2)Te surface structures have been made for a variety of photon energies and polar angles. Strong similarities are observed in the major features of the data from all three structures indicating that the environment in the immediate vicinity of the adsorbate is essentially identical for all three surface structures in agreement with existing LEED calculations. The relative insensitivity of the data to the number of nearest neighbors within the adsorbate overlayer itself is taken as evidence of the importance of interlayer scattering under these experimental conditions.

Photoelectron diffraction from Te on Ni(100) and Cu(100)

Abstract Angle resolved measurements of the azimuthai anisotropy in the photoelectron diffraction intensities from the Te 4d level in Ni(100)p(2 × 2)Te, Ni(100)c(2 × 2)Te and Cu(100)p(2 × 2)Te surface structures have been made for a variety of photon energies and polar angles. Strong similarities are observed in the major features of the data from all three structures indicating that the environment in the immediate vicinity of the adsorbate is essentially identical for all three surface structures in agreement with existing LEED calculations. The relative insensitivity of the data to the number of nearest neighbors within the adsorbate overlayer itself is taken as evidence of the importance of interlayer scattering under these experimental conditions.