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D.p. Woodruff - One of the best experts on this subject based on the ideXlab platform.
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structural determination of bilayer graphene on sic 0001 using synchrotron radiation Photoelectron Diffraction
Scientific Reports, 2018Co-Authors: Ivy Razadocolambo, D.p. Woodruff, Jose Avila, D Vignaud, S Godey, X Wallart, Maria C AsensioAbstract:In recent years there has been growing interest in the electronic properties of 'few layer' graphene films. Twisted layers, different stacking and register with the substrate result in remarkable unconventional couplings. These distinctive electronic behaviours have been attributed to structural differences, even if only a few structural determinations are available. Here we report the results of a structural study of bilayer graphene on the Si-terminated SiC(0001) surface, investigated using synchrotron radiation-based Photoelectron Diffraction and complemented by angle-resolved photoemission mapping of the electronic valence bands. Photoelectron Diffraction angular distributions of the graphene C 1s component have been measured at different kinetic energies and compared with the results of multiple scattering simulations for model structures. The results confirm that bilayer graphene on SiC(0001) has a layer spacing of 3.48 A and an AB (Bernal) stacking, with a distance between the C buffer layer and the first graphene layer of 3.24 A. Our work generalises the use of a versatile and precise Diffraction method capable to shed light on the structure of low-dimensional materials.
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v doped tio2 110 quantitative structure determination using energy scanned Photoelectron Diffraction
Surface Science, 2014Co-Authors: David A Duncan, Werner Unterberger, Dagmar Kreikemeyerlorenzo, Elena Primorac, Osman Karslioglu, Matthias Naschitzki, Helmut Kuhlenbeck, D.p. WoodruffAbstract:The surface structure of a novel vanadium-titanium dioxide epitaxial film (Ti1-xVxO2, x ~ 0.2) has been explored using V4+ 2p and Ti4+ 2p energy-scanned Photoelectron Diffraction (PhD). The determined structure is a rutile TiO2(110)-like surface, with V atoms substitutionally replacing some Ti atoms. The results show no evidence for significant preferential occupation by V atoms of any specific surface or sub-surface sites. LEED shows a (1x2) reconstruction to be present on the surface, and the PhD simulations do favour this being the dominant surface termination, although the reliability factor for simulations for a (1x1) termination falls just within the variance of the value for the preferred (1x2) structure. The V3+ and Ti3+ species were observed to occupy the same sites as the V4+ and Ti4+ species; V5+ species do not appear to occupy a single well-defined structural site.
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global search algorithms in surface structure determination using Photoelectron Diffraction
Surface Science, 2012Co-Authors: David A Duncan, Joong Il Jake Choi, D.p. WoodruffAbstract:Three different algorithms to effect global searches of the variable-parameter hyperspace are compared for application to the determination of surface structure using the technique of scanned-energy mode Photoelectron Diffraction (PhD). Specifically, a new method not previously used in any surface science methods, the swarm-intelligence-based particle swarm optimisation (PSO) method, is presented and its results compared with implementations of fast simulated annealing (FSA) and a genetic algorithm (GA). These three techniques have been applied to experimental data from three adsorption structures that had previously been solved by standard trial-and-error methods, namely H2O on TiO2(110), SO2 on Ni(111) and CN on Cu(111). The performance of the three algorithms is compared to the results of a purely random sampling of the structural parameter hyperspace. For all three adsorbate systems, the PSO out-performs the other techniques as a fitting routine, although for two of the three systems studied the advantage relative to the GA and random sampling approaches is modest. The implementation of FSA failed to achieve acceptable fits in these tests.
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uracil on cu 110 a quantitative structure determination by energy scanned Photoelectron Diffraction
Journal of Chemical Physics, 2011Co-Authors: David A Duncan, Werner Unterberger, Dagmar Kreikemeyerlorenzo, D.p. WoodruffAbstract:The local adsorption site of the nucleobase uracil on Cu(110) has been determined quantitatively by energy-scanned Photoelectron Diffraction (PhD). Qualitative inspection of the O 1s and N 1s soft x-ray Photoelectron spectra, PhD modulation spectra, and O K-edge near-edge x-ray adsorption fine structure indicate that uracil bonds to the surface through its nitrogen and oxygen constituent atoms, each in near atop sites, with the molecular plane essentially perpendicular to surface and aligned along the close packed [1 (1) over bar0] azimuth. Multiple scattering simulations of the PhD spectra confirm and refine this geometry. The Cu-N bondlength is 1.96 +/- 0.04 angstrom, while the Cu-O bondlengths of the two inequivalent O atoms are 1.93 +/- 0.04 angstrom and 1.96 +/- 0.04 angstrom, respectively. The molecule is twisted out of the [1 (1) over bar0] direction by 11 +/- 5 degrees.
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surface structural information from Photoelectron Diffraction
Journal of Electron Spectroscopy and Related Phenomena, 2010Co-Authors: D.p. WoodruffAbstract:Abstract The coherent elastic scattering by surrounding atoms of a Photoelectron wave field emitted from an atomic core level provides structural information on the environment of the emitter. This basic phenomenon leads to a range of interrelated methods, the information content of which depends on the Photoelectron energy and the mode of detection (angle-scan or energy-scan). A short review is presented of these methods and their application to the investigation of atomic and molecular adsorbates and epitaxial films and particles, using both laboratory-based instrumentation and synchrotron radiation.
C S Fadley - One of the best experts on this subject based on the ideXlab platform.
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high energy Photoelectron Diffraction model calculations and future possibilities
New Journal of Physics, 2008Co-Authors: Aimo Winkelmann, C S Fadley, Javier Garcia F De AbajoAbstract:We discuss the theoretical modeling of x-ray Photoelectron Diffraction (XPD) with hard x-ray excitation at up to 20?keV, using the dynamical theory of electron Diffraction to illustrate the characteristic aspects of the Diffraction patterns resulting from such localized emission sources in a multilayer crystal. We show via dynamical calculations for diamond, Si and Fe that the dynamical theory predicts well the available current data for lower energies around 1?keV, and that the patterns for energies above about 1?keV are dominated by Kikuchi bands, which are created by the dynamical scattering of electrons from lattice planes. The origin of the fine structure in such bands is discussed from the point of view of atomic positions in the unit cell. The profiles and positions of the element-specific Photoelectron Kikuchi bands are found to be sensitive to lattice distortions (e.g. a 1% tetragonal distortion) and the position of impurities or dopants with respect to lattice sites. We also compare the dynamical calculations with results from a cluster model that is more often used to describe lower energy XPD. We conclude that hard XPD (HXPD) should be capable of providing unique bulk-sensitive structural information for a wide variety of complex materials in future experiments.
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high energy Photoelectron Diffraction model calculations and future possibilities
arXiv: Materials Science, 2008Co-Authors: Aimo Winkelmann, C S Fadley, Javier Garcia F De AbajoAbstract:We discuss the theoretical modelling of x-ray Photoelectron Diffraction (XPD) with hard x-ray excitation at up to 20 keV, using the dynamical theory of electron Diffraction to illustrate the characteristic aspects of Diffraction patterns resulting from such localized emission sources in a multi-layer crystal. We show via dynamical calculations for diamond, Si, and Fe that the dynamical theory well predicts available current data for lower energies around 1 keV, and that the patterns for energies above about 1 keV are dominated by Kikuchi bands which are created by the dynamical scattering of electrons from lattice planes. The origin of the fine structure in such bands is discussed from the point of view of atomic positions in the unit cell. The profiles and positions of the element-specific Photoelectron Kikuchi bands are found to be sensitive to lattice distortions (e.g. a 1% tetragonal distortion) and the position of impurities or dopants with respect to lattice sites. We also compare the dynamical calculations to results from a cluster model that is more often used to describe lower-energy XPD. We conclude that hard XPD (HXPD) should be capable of providing unique bulk-sensitive structural information for a wide variety of complex materials in future experiments.
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x ray Photoelectron spectroscopy and Diffraction in the hard x ray regime fundamental considerations and future possibilities
Nuclear Instruments & Methods in Physics Research Section A-accelerators Spectrometers Detectors and Associated Equipment, 2005Co-Authors: C S FadleyAbstract:Abstract The prospects for extending X-ray Photoelectron spectroscopy (XPS) and X-ray Photoelectron Diffraction (XPD) measurements into the hard X-ray regime of 5–15 keV excitation energies are discussed from a fundamental point of view, in some cases using prior results obtained in the 1–2 keV range as starting points of discussion, together with theoretical estimates of behavior at higher energies. Subjects treated are: the instrumentation improvements needed to optimize peak intensities; the tuning of experimental conditions to achieve bulk or surface sensitivity; the use of grazing incidence to suppress spectral backgrounds; the use of standing waves created by Bragg reflection from crystal planes or synthetic multilayers to achieve position-sensitive densities of states, compositions, and magnetizations; Photoelectron Diffraction and Kikuchi-band effects as element-specific local structure probes; and valence-level measurements, including the role of non-dipole effects and mechanisms leading to complete Brillouin zone averaging and density-of-states like spectra. Several distinct advantages are found for such high-energy extensions of the XPS and XPD techniques.
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atomic scale structure of the fivefold surface of an alpdmn quasicrystal a quantitative x ray Photoelectron Diffraction analysis
Physical Review B, 2004Co-Authors: Jincheng Zheng, C S Fadley, M A Van Hove, Eli Rotenberg, C H A Huan, A T S Wee, F Shi, S R BarmanAbstract:The atomic scale structure of the 5-fold symmetric surface of an AlPdMn quasicrystal is investigated quantitatively by comparing x-ray Photoelectron Diffraction (XPD) simulations to experiment. The observed 5-fold symmetry of the Diffraction patterns indicates that the surface is quasicrystalline with no hint of a reconstruction from the bulk structure. In analyzing the experimental data, many possible bulk terminations have been tested. Those few that fit best to the data have in common that they contain an Al-rich surface layer followed by a dense mixed Al/Pd/Mn layer. These best terminations, while not identical to each other, are suggested to form terraces coexisting on a real surface. Structural relaxations of the quasicrystal surface are also analyzed: mixing several best-fit terminations gives average best-fit interlayer spacing changes of Dd12 = -0.057 Angstrom, Dd24 = +0.159 Angstrom. These results are in good agreement with a prior structure determination by LEED on a sample that was prepared in a different manner.
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surface structure of mbe grown α fe 2 o 3 0001 by intermediate energy x ray Photoelectron Diffraction
Surface Science, 1999Co-Authors: Suntharampillai Thevuthasan, C S Fadley, Scott A Chambers, Y J Kim, Jonder Morais, R Denecke, P Liu, T Kendelewicz, Gordon E BrownAbstract:Abstract We have used intermediate-energy X-ray Photoelectron Diffraction to determine the surface structure of epitaxial α-Fe 2 O 3 (0001) grown on α-Al 2 O 3 (0001). Comparison of experiment with quantum mechanical scattering theory reveals that the surface is Fe-terminated, and that the first four layer spacings are −41, +18, −8, and 47% of the associated bulk values, respectively. These results agree reasonably well with the predictions of molecular mechanics and spin-density functional theory previously reported in the literature for the Fe-terminated surface. However, we find no evidence for an O-terminated surface predicted to be stable by spin-density functional theory.
Rebecca Boll - One of the best experts on this subject based on the ideXlab platform.
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Photoelectron Diffraction imaging of a molecular breakup using an x ray free electron laser
Physical Review X, 2020Co-Authors: G Kastirke, Rebecca Boll, M Schoffler, M Weller, J Rist, Nils Anders, Thomas M Baumann, Sebastian Eckart, Benjamin Erk, Alberto De FanisAbstract:© 2020 authors. Published by the American Physical Society. A central motivation for the development of x-ray free-electron lasers has been the prospect of time-resolved single-molecule imaging with atomic resolution. Here, we show that x-ray Photoelectron Diffraction - where a Photoelectron emitted after x-ray absorption illuminates the molecular structure from within - can be used to image the increase of the internuclear distance during the x-ray-induced fragmentation of an O2 molecule. By measuring the molecular-frame Photoelectron emission patterns for a two-photon sequential K-shell ionization in coincidence with the fragment ions, and by sorting the data as a function of the measured kinetic energy release, we can resolve the elongation of the molecular bond by approximately 1.2 a.u. within the duration of the x-ray pulse. The experiment paves the road toward time-resolved pump-probe Photoelectron Diffraction imaging at high-repetition-rate x-ray free-electron lasers.
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Photoelectron Diffraction imaging of a molecular breakup using an x ray free electron laser
Physical Review X, 2020Co-Authors: G Kastirke, Rebecca Boll, M Schoffler, M Weller, J Rist, Nils Anders, Thomas M Baumann, Sebastian Eckart, Benjamin Erk, Alberto De FanisAbstract:Author(s): Kastirke, G; Schoffler, MS; Weller, M; Rist, J; Boll, R; Anders, N; Baumann, TM; Eckart, S; Erk, B; De Fanis, A; Fehre, K; Gatton, A; Grundmann, S; Grychtol, P; Hartung, A; Hofmann, M; Ilchen, M; Janke, C; Kircher, M; Kunitski, M; Li, X; Mazza, T; Melzer, N; Montano, J; Music, V; Nalin, G; Ovcharenko, Y; Pier, A; Rennhack, N; Rivas, DE; Dorner, R; Rolles, D; Rudenko, A; Schmidt, P; Siebert, J; Strenger, N; Trabert, D; Vela-Perez, I; Wagner, R; Weber, T; Williams, JB; Ziolkowski, P; Schmidt, LPH; Czasch, A; Trinter, F; Meyer, M; Ueda, K; Demekhin, PV; Jahnke, T | Abstract: © 2020 authors. Published by the American Physical Society. A central motivation for the development of x-ray free-electron lasers has been the prospect of time-resolved single-molecule imaging with atomic resolution. Here, we show that x-ray Photoelectron Diffraction - where a Photoelectron emitted after x-ray absorption illuminates the molecular structure from within - can be used to image the increase of the internuclear distance during the x-ray-induced fragmentation of an O2 molecule. By measuring the molecular-frame Photoelectron emission patterns for a two-photon sequential K-shell ionization in coincidence with the fragment ions, and by sorting the data as a function of the measured kinetic energy release, we can resolve the elongation of the molecular bond by approximately 1.2 a.u. within the duration of the x-ray pulse. The experiment paves the road toward time-resolved pump-probe Photoelectron Diffraction imaging at high-repetition-rate x-ray free-electron lasers.
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femtosecond Photoelectron Diffraction a new approach to image molecular structure during photochemical reactions
Proceedings of SPIE, 2014Co-Authors: Daniel Rolles, Rebecca Boll, Denis Anielski, Samyak Tamrakar, Cedric BommeAbstract:Continuing technical advances in the creation of (sub-) femtosecond VUV and X-ray pulses with Free-Electron Lasers and laser-based high-harmonic-generation sources have created new opportunities for studying ultrafast dynamics during chemical reactions. Here, we present an approach to image the geometric structure of gas-phase molecules with fewfemtosecond temporal and sub-Angstrom spatial resolution using femtosecond Photoelectron Diffraction. This technique allows imaging the molecules “from within” by analyzing the Diffraction of inner-shell Photoelectrons that are created by femtosecond VUV and X-ray pulses. Using pump-probe schemes, ultrafast structural changes during photochemical reactions can thus be directly visualized with a temporal resolution that is only limited by the pulse durations of the pump and the probe pulse and the synchronization of the two light pulses. Here, we illustrate the principle of Photoelectron Diffraction using a simple, geometric scattering model and present results from Photoelectron Diffraction experiments on laser-aligned molecules using X-ray pulses from a Free-Electron Laser.
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Femtosecond x-ray Photoelectron Diffraction on gas-phase dibromobenzene molecules
Journal of Physics B: Atomic Molecular and Optical Physics, 2014Co-Authors: Daniel Rolles, John D. Bozek, Rebecca Boll, M. Adolph, Andrew Aquila, Christoph Bostedt, Henry N. Chapman, Ryan Coffee, Nicola Coppola, Piero DeclevaAbstract:We present time-resolved femtosecond Photoelectron momentum images and angular distributions of dissociating, laser-aligned 1,4-dibromobenzene (C6H4Br2) molecules measured in a near-infrared pump, soft-x-ray probe experiment performed at an x-ray free-electron laser. The observed alignment dependence of the bromine 2p Photoelectron angular distributions is compared to density functional theory calculations and interpreted in terms of Photoelectron Diffraction. While no clear time-dependent effects are observed in the angular distribution of the Br(2p) Photoelectrons, other, low-energy electrons show a pronounced dependence on the time delay between the near-infrared laser and the x-ray pulse.
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femtosecond Photoelectron Diffraction on laser aligned molecules towards time resolved imaging of molecular structure
Physical Review A, 2013Co-Authors: Rebecca Boll, John D. Bozek, Christoph Bostedt, Ryan Coffee, Piero Decleva, Denis Anielski, Lauge Christensen, Benjamin Erk, Sascha W Epp, L FoucarAbstract:We demonstrate an experimental method to record snapshot Diffraction images of polyatomic gas-phase molecules, which can, in a next step, be used to probe time-dependent changes in the molecular geometry during photochemical reactions with femtosecond temporal and angstrom spatial resolution. Adiabatically laser-aligned 1-ethynyl-4-fluorobenzene (C${}_{8}$H${}_{5}$F) molecules were imaged by Diffraction of Photoelectrons with kinetic energies between 31 and 62 eV, created from core ionization of the fluorine ($1s$) level by $\ensuremath{\approx}80$ fs x-ray free-electron-laser pulses. Comparison of the experimental Photoelectron angular distributions with density functional theory calculations allows relating the Diffraction images to the molecular structure.
Jurg Osterwalder - One of the best experts on this subject based on the ideXlab platform.
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sensitivity of Photoelectron Diffraction to conformational changes of adsorbed molecules tetra tert butyl azobenzene au 111
Structural Dynamics, 2017Co-Authors: Adrian Schuler, M Greif, Ari P Seitsonen, Gerson Mette, Luca Castiglioni, Jurg Osterwalder, Matthias HengsbergerAbstract:Electron Diffraction is a standard tool to investigate the atomic structure of surfaces, interfaces, and adsorbate systems. In particular, Photoelectron Diffraction is a promising candidate for real-time studies of structural dynamics combining the ultimate time resolution of optical pulses and the high scattering cross-sections for electrons. In view of future time-resolved experiments from molecular layers, we studied the sensitivity of Photoelectron Diffraction to conformational changes of only a small fraction of molecules in a monolayer adsorbed on a metallic substrate. 3,3',5,5'-tetra-tert-butyl-azobenzene served as test case. This molecule can be switched between two isomers, trans and cis, by absorption of ultraviolet light. X-ray Photoelectron Diffraction patterns were recorded from tetra-tert-butyl-azobenzene/Au(111) in thermal equilibrium at room temperature and compared to patterns taken in the photostationary state obtained by exposing the surface to radiation from a high-intensity helium discharge lamp. Difference patterns were simulated by means of multiple-scattering calculations, which allowed us to determine the fraction of molecules that underwent isomerization.
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access to phases of coherent phonon excitations by femtosecond ultraviolet Photoelectron Diffraction
Physical Review B, 2016Co-Authors: M Greif, Luca Castiglioni, Jurg Osterwalder, Lamia Kasmi, Matteo Lucchini, L Gallmann, U Keller, Matthias HengsbergerAbstract:Coherent phonons are an excellent tool to investigate the interplay between electronic and structural dynamics. The displacive excitation of coherent phonons in elemental bismuth is one of the most widely studied processes for this purpose. We employ time-resolved Photoelectron Diffraction to access the structural dynamics by recording the photoemission intensity from one initial state as a function of emission angle. In comparison with tight-binding and single-scattering cluster calculations, this allows electronic and structural effects to be disentangled. Hence, the full dynamics of the hot electron gas and of coherently excited phonons can be accessed in a single experiment. As a major result the phase lag between the coherent phonons and the modulation of the electronic structure can be determined with high precision. The phonon phase lag with respect to the modulation of the electronic structure is about 2.85±0.21 rad, thus significantly smaller than π. The difference is not due to phonon decay by energy dissipation into low-energy modes, but rather caused by the very early evolution of the highly excited electron distribution.
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following the molecular motion of near resonant excited co on pt 111 a simulated x ray Photoelectron Diffraction study based on molecular dynamics calculations
Structural Dynamics, 2015Co-Authors: M Greif, Luca Castiglioni, Matthias Hengsberger, Tibor Nagy, Maksym Soloviov, Markus Meuwly, Jurg OsterwalderAbstract:A THz-pump and x-ray-probe experiment is simulated where x-ray Photoelectron Diffraction (XPD) patterns record the coherent vibrational motion of carbon monoxide molecules adsorbed on a Pt(111) surface. Using molecular dynamics simulations, the excitation of frustrated wagging-type motion of the CO molecules by a few-cycle pulse of 2 THz radiation is calculated. From the atomic coordinates, the time-resolved XPD patterns of the C 1s core level Photoelectrons are generated. Due to the direct structural information in these data provided by the forward scattering maximum along the carbon-oxygen direction, the sequence of these patterns represents the equivalent of a molecular movie.
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Photoelectron Diffraction in the x ray and ultraviolet regime sn phthalocyanine on ag 111
Physical Review B, 2013Co-Authors: M Greif, Ari P Seitsonen, Luca Castiglioni, Jurg Osterwalder, Silvan Roth, Matthias HengsbergerAbstract:The bonding geometry of tin-phthalocyanine (SnPc) on Ag(111) has been studied using x-ray and ultraviolet Photoelectron Diffraction (XPD and UPD, respectively). Experimental Diffraction patterns were compared to single-scattering-cluster calculations. XPD data could be well reproduced by the simulations and allowed for the determination of several structural parameters. At a coverage of 0.9 ML, all molecules are in a ``tin-down'' configuration and the nonplanar shuttlecock-shaped SnPc molecule undergoes flattening upon absorption on Ag(111). UPD data from the second highest occupied molecular orbital and comparison to simulations show a high sensitivity to minor structural changes, including also the vertical distance between tin atoms of the SnPc and the surface layer of the substrate, which is found to be 2.3 \AA{}. We thus demonstrate how UPD can complement the well-established XPD method and discuss remaining challenges in the theoretical description of Photoelectron Diffraction from molecular orbitals at low energies. The UPD method is particularly attractive in view of the increasing availability of ultrashort pulsed laser sources in the XUV regime, which could enable pump-probe experiments with high structural sensitivity.
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real space multiple scattering method for angle resolved photoemission and valence band Photoelectron Diffraction and its application to cu 111
Physical Review B, 2011Co-Authors: Peter Kruger, F Da Pieve, Jurg OsterwalderAbstract:A computational method is presented for angle-resolved photoemission spectra (ARPES) and Photoelectron Diffraction (PED) in the ultraviolet regime. The one-step model is employed and both initial valence and final continuum states are calculated using the finite-cluster, real-space multiple scattering method. Thereby the approach is versatile and provides a natural link to core-level PED. The method is applied to the Cu(111) valence band and good agreement with experiment is found for both ARPES spectra and PED patterns. When the PED patterns are integrated over a filled band of a single-orbital symmetry, such as Cu-3d, we show, both numerically and analytically, that the exact theory with delocalized initial states can be replaced by the much simpler, core-level-type theory where the initial states are taken as localized.
Aimo Winkelmann - One of the best experts on this subject based on the ideXlab platform.
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emitter site specificity of hard x ray Photoelectron kikuchi Diffraction
New Journal of Physics, 2020Co-Authors: O Fedchenko, Aimo Winkelmann, K Medjanik, S Babenkov, D Vasilyev, S Chernov, H J Elmers, M Hoesch, Steinn Ymir Agustsson, G SchonhenseAbstract:High-resolution full-field imaging of (k x , k y ) Photoelectron distributions (k-resolution 0.03 A−1, angular resolution 0.03° at 6.7 keV) in a large field of view (up to 16 A−1 dia.) allows to observe fine details in Kikuchi-type diffractograms. Alongside with the element specificity via core-level spectra, this method opens a new avenue to structural analysis using hard x-ray Photoelectron Diffraction (hXPD). Here we present a theoretical study of the emitter-site specificity by simulating hXPD patterns for arbitrary positions of emitter atoms in the unit cell. Using the Bloch wave approach to Photoelectron Diffraction from lattice planes, the Diffraction patterns from a number of positions in the unit cell can be obtained simultaneously exploiting the reciprocity theorem. Simulations for dopant atoms and dopant multimers (dimers, trimers, clusters) in the Si lattice at various positions in the unit cell reveal a strong site-sensitivity in terms of dramatic changes in the Diffraction patterns with emitter-atom position. The results are compared with measurements for Si hyperdoped with Te.
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influence of localized inelastic scattering on kikuchi bands in Photoelectron Diffraction patterns
Journal of Electron Spectroscopy and Related Phenomena, 2014Co-Authors: Aimo WinkelmannAbstract:Abstract In the treatment of inelastic processes in Photoelectron Diffraction calculations, the decay of the elastically scattered waves in the crystal structure is considered in an average way by the introduction of the inelastic mean free path (IMFP). The IMFP is only a single parameter that does not depend on the position or direction within a crystal and thus neglects the specific influence of the crystalline structure on inelastic scattering processes. In order to go beyond the IMFP picture, the Kikuchi band approach to Photoelectron Diffraction treats the scattering potential and its absorptive part in Fourier space and allows to incorporate localized inelastic scattering into Photoelectron Diffraction simulations. It is demonstrated that inelastic scattering processes that are localized on atomic planes (i.e. phonon scattering) can lead to contrast reversal of Kikuchi bands if the measured group of electrons dominantly originates from a crystal region which is located sufficiently far away from the surface and anomalous absorption of Bloch waves thus becomes relevant.
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observation and simulation of hard x ray Photoelectron Diffraction to determine polarity of polycrystalline zinc oxide films with rotation domains
Journal of Applied Physics, 2012Co-Authors: Jesse Robert Williams, Igor Pis, Tomohiro Matsushita, Aimo Winkelmann, Masaaki Kobata, Naoki Ohashi, Yutaka Adachi, Keisuke KobayashiAbstract:X ray Photoelectron Diffraction (XPD) patterns of polar zinc oxide (ZnO) surfaces were investigated experimentally using hard x rays and monochromatized Cr Kα radiation and theoretically using a cluster model approach and a dynamical Bloch wave approach. We focused on Photoelectrons emitted from the Zn 2p3/2 and O 1s orbitals in the analysis. The obtained XPD patterns for the (0001) and (0001¯) surfaces of a ZnO single crystal were distinct for a given emitter and polarity. Polarity determination of c-axis-textured polycrystalline ZnO thin films was also achieved with the concept of XPD, even though the in-plane orientation of the columnar ZnO grains was random.
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high energy Photoelectron Diffraction model calculations and future possibilities
New Journal of Physics, 2008Co-Authors: Aimo Winkelmann, C S Fadley, Javier Garcia F De AbajoAbstract:We discuss the theoretical modeling of x-ray Photoelectron Diffraction (XPD) with hard x-ray excitation at up to 20?keV, using the dynamical theory of electron Diffraction to illustrate the characteristic aspects of the Diffraction patterns resulting from such localized emission sources in a multilayer crystal. We show via dynamical calculations for diamond, Si and Fe that the dynamical theory predicts well the available current data for lower energies around 1?keV, and that the patterns for energies above about 1?keV are dominated by Kikuchi bands, which are created by the dynamical scattering of electrons from lattice planes. The origin of the fine structure in such bands is discussed from the point of view of atomic positions in the unit cell. The profiles and positions of the element-specific Photoelectron Kikuchi bands are found to be sensitive to lattice distortions (e.g. a 1% tetragonal distortion) and the position of impurities or dopants with respect to lattice sites. We also compare the dynamical calculations with results from a cluster model that is more often used to describe lower energy XPD. We conclude that hard XPD (HXPD) should be capable of providing unique bulk-sensitive structural information for a wide variety of complex materials in future experiments.
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high energy Photoelectron Diffraction model calculations and future possibilities
arXiv: Materials Science, 2008Co-Authors: Aimo Winkelmann, C S Fadley, Javier Garcia F De AbajoAbstract:We discuss the theoretical modelling of x-ray Photoelectron Diffraction (XPD) with hard x-ray excitation at up to 20 keV, using the dynamical theory of electron Diffraction to illustrate the characteristic aspects of Diffraction patterns resulting from such localized emission sources in a multi-layer crystal. We show via dynamical calculations for diamond, Si, and Fe that the dynamical theory well predicts available current data for lower energies around 1 keV, and that the patterns for energies above about 1 keV are dominated by Kikuchi bands which are created by the dynamical scattering of electrons from lattice planes. The origin of the fine structure in such bands is discussed from the point of view of atomic positions in the unit cell. The profiles and positions of the element-specific Photoelectron Kikuchi bands are found to be sensitive to lattice distortions (e.g. a 1% tetragonal distortion) and the position of impurities or dopants with respect to lattice sites. We also compare the dynamical calculations to results from a cluster model that is more often used to describe lower-energy XPD. We conclude that hard XPD (HXPD) should be capable of providing unique bulk-sensitive structural information for a wide variety of complex materials in future experiments.