Papers

AVS1996 Session SS2-MoA: Electronic Properties of Surfaces

Monday, October 14, 1996 1:30 PM in Room 203B

Monday Afternoon

Start	Invited?	Item
1:30 PM		SS2-MoA-1 Giant Surface Friedel Oscillations: STM Study of Be(0001) P. Sprunger (Louisiana State University); L. Petersen (University of Aarhus, Denmark); E. Plummer (University of Tennessee and Oak Ridge National Laboratory); F. Besenbacher (University of Aarhus, Denmark) Large-amplitude surface Friedel oscillations are observed on Be(0001) with low-temperature scanning tunneling microscopy. The interference pattern of these charge-density oscillations, having a relatively short wavelength of 3.2 \Ao\ (2k\sub F\), dominates the surface topography in normal constant-current tunneling mode due to the unique electronic properties of Be(0001). It is demonstrated how two-dimensional Fourier transforms of the STM images yield a direct picture of the surface Brillouin zone, and lead to a determination of the surface Fermi wave vector, being k\sub F\ = 0.95\+-\0.02 \Ao\\super -1\, in excellent agreement with photoemission studies. The decay length of the waves will be discussed in light of both past and present experimental and theoretical investigations.*present address: Louisianna State Univ./CAMD, Baton Rouge, LA
1:50 PM		SS2-MoA-2 Core Hole Decay Mechanisms in Ru Studied by Auger-Photoelectron Coincidence Spectroscopy R. Gotter (Laboratorio TASC-INFM, Italy); R. Bartynski (Rutgers University); X. Wu, S. Hulbert (Brookhaven National Laboratory); H. Nozoye (National Institute of Materials and Chemical Research, Japan) The 4\italics p\ core levels of the late 4\italics d\ transition metals are particularly interesting as they are unusually broad, with the 1/2 spin-orbit split component typically having a larger width than the 3/2 component. To explore the photoexcitation and decay mechanisms in these metals, the N\sub 23\VV Auger spectrum of Ru(0001) has been measured in coincidence with 4\italics p\\sub 3/2\ and 4\italics p\\sub 1/2\ core level photoelectrons. The Auger spectrum in coincidence with Ru 4\italics p\\sub 3/2\ photoelectron exhibits a well defined threshold at a kinetic energy of 44 eV (with respect to the Fermi level), consistent with independent electron theory. However, the lineswidth is larger than the self convolution of the valence band density of states and the lineshape exhibits structure indicative of significant electron-electron interaction. The Auger spectrum in coincidence with 4\italics p\\sub 1/2\ photoelectrons has a markedly different lineshape with spectral weight shifted away from threshold to lower kinetic energy. These different N\sub 2\VV and N\sub 3\VV Auger lineshapes can be understood in terms of a correlated two-hole Auger final state where quasiatomic multiplet components have different intensities for core holes of different total angular momentum. In addition, we have measured the N\sub 23\VV Auger spectrum in coincidence with photoelectrons in a satellite feature shifted 10 eV below the primary core level. This spectrum has significant weight well above the independent electron threshold for Auger transitions, demonstrating direct exchange of energy from photoelectrons in the satellite feature to Auger electrons involved in the decay. \*\ Supported by the NSF under grant number NSF-DMR 9411610
2:10 PM		SS2-MoA-3 Surface Plasmon Dispersion and Damping on Ag Surfaces Revisited with EELS-LEED F. Moresco, M. Rocca (CNR and INFM, Italy); V. Zielasek, M. Henzler (Universit\um a\t Von Hannover, Germany) Collective surface electronic exitations on silver surfaces are revisited by Energy Loss Spectroscopy Low Energy Electron Diffraction (EELS-LEED). The data allow for a better k-resolution than with conventional HREELS. For the first time the contributions to the loss intensity due to surface plasmon and to multipole surface plasmon of Ag(111) are observed, demonstrating that the latter mode can be supported also by silver surfaces in spite of the closeness of surface and bulk plasmon frequency. Surface plasmon dispersion on Ag(111) comes out to be initially linear and positive. In accord with previous investigations, the initial slope is smaller than for Ag(001) and comparable to Ag(110) <0-11>. The surface plasmon shows a smaller linewidth (70 meV at q\sub\|\|\=0) than reported so far, which is nearly independent of q\sub\|\|\ for small q\sub\|\|\, contrary to expectation. We observe a linear shift in surface plasmon frequency with crystal temperature, caused by the thermal expansion, and a quadratic growth of the linewidth with T, which we ascribe to electronic damping. The reinvestigation of the anisotropic Ag(110) surface by ELS-LEED is currently in progress. Results will be presented for collective electronic exitations both for the bare surface and for the reconstructed phases induced by oxygen (nx1) and alkali metals adsorption (1x2).
2:30 PM		SS2-MoA-4 Surface Investigations of an AlPdMn Quasicrystal and a Related Crystalline Alloy C. Jenks (Iowa State University); S. Chang (FSI International); J. Anderegg, D. Lynch, P. Thiel (Iowa State University) Quasicrystals are structures which exhibit long-range aperiodic order and crystallographically forbidden rotational symmetries. To help understand what surface properties are unique to quasicrystals, we compare X-ray photoelectron spectroscopy studies of a single-grain of Al\sub 70\Pd\sub 21\Mn\sub 9\ to studies of a single-crystal of a related cubic alloy, Al\sub 60\Pd\sub 25\Mn\sub 15\, and elemental Al, Pd, and Mn. We find that the quasicrystal Pd 3d lines are 2.2 eV higher than for pure Pd. This is not unique to the AlPdMn quasicrystal; a similar shift has been reported for other Pd alloys and is found in the cubic alloy studied. We also find that the Mn 2p\sub 3/2\ lineshape is very narrow for the quasicrystal. Doniach-Sunjic lineshape fits to the alloy spectra, as well as pure Mn and Pd, suggest that the Mn peak positions and density of states near the Fermi level are very sensitive to the structural and/or chemical environment of Mn. Our results are consistent with previous work that shows a pseudogap at the Fermi level, and suggests that the density of Mn states at the Fermi level varies more from the pure metal to the quasicrystal than Pd. Furthermore, the FWHM suggest that the quasicrystal Mn 2p\sub 3/2\ photohole has a lifetime longer than that expected for the free atom. Finally, our results suggest that the shape and width of the Mn 2p\sub 3/2\ line may be a fingerprint for the quasicrystalline phase in AlPdMn alloys.
2:50 PM		SS2-MoA-5 Synchrotron Radiation Photoemission from Photoionization to Electron Holography G. Lapeyre (Montana State University) The presentation summarizes the development of synchrotron radiation photoemission from the perspective of events at the Wisconsin Synchrotron Radiation Center, a facility which developed the first electron storage ring dedicated to radiation experiments. The early photoemission spectroscopy (PES) experiments as well as the instrumentation is noted. The presentation covers the utilization of the polarization properties of synchrotron radiation and the spectral continuum property which lead to the three modes of PES: energy distribution curve (EDC), constant final energy spectroscopy (CFS), and constant initial-state-energy spectroscopy (CIS). The merging of synchrotron radiation photoemission and angle-resolved photoemission is reviewed. The new development of using photoelectron diffraction data to obtain images of individual atoms is presented (1). The inversion of the spectra is based on the holographic principle. Several applications of photoelectron holographic imaging (PHI) are presented. * AFOSR, NSF, ONR/DEPSCOR 1. Huasheng Wu and G.J. Lapeyre, Phys. Rev. B51, 14549 (1995).
3:30 PM		SS2-MoA-7 The Interfaces of Model Bimetallic Systems: A Core-level Photoemission Study H. Tao, T. Madey (Rutgers University); J. Rowe (U.S. Army Research Office) In a search for insights into the morphological instability of W(111) surfaces covered by ultrathin metal films, we have used high resolution core-level photoelectron spectroscopy based on synchrotron radiation to study the 4f\sub7/2\ core level of W atoms at the interfaces between ultrathin films (K, Ag, Cu, Au, Ni, Pd, Pt, and Co) and the W(111) surface onto which they are deposited. The interfaces of Au, Pd, Pt/W(111) are morphologically unstable upon heating above 750K, where facets with (211) interfaces develop [1]. On the other hand, interfaces of K, Ag, Cu, Ni, and Co/W(111) are stable without faceting upon heating up to the desorption temperature of the films. The 4f\sub7/2\ core level shifts of W atoms at the interface exhibit a wide range of behavior. For example, three different interfacial W 4f\sub7/2\ core levels are observed for Ag/W(111), while only one interfacial W 4f\sub7/2\ core level is observed for Ni/W(111) and Pd/W(111). The average binding energy of the interfacial W 4f\sub7/2\ core level appears to scale with the heat of adsorption of the ultrathin metal film.[1] J. Guan, R. Campbell, T. E. Madey, Surf. Sci. 341 (1995) 317.
3:50 PM		SS2-MoA-8 A Symmetry Selective View of the Chemical Bonding of Organic Acids on Cu(110) K. Karis, A. Nilsson (Uppsala University, Sweden); M. Nyberg, L. Pettersson (Stockholm University, Sweden); J. Hasselstr\um o\m, N. Wassdahl (Uppsala University, Sweden) We present a study of the bonding of HCO\sub 2\H (formic acid) CH\sub 3\CO\sub 2\H (acetic acid) and NH\sub 2\CH\sub 2\CO\sub 2\H (glycine) monolayers on Cu(110) using core level spectroscopies. For the first time we apply x-ray emission spectroscopy (XES) to these systems. When XES is used in conjunction with x-ray absorption spectroscopy (XAS) and state-of-the-art ab initio calculations, within the static exchange approximation\sup 1\, new information about the electronic interaction in the adsorbate - substrate system is provided. The local character of the core excitation provides an element specific view of the bonding interaction, free of strong emission from overlapping substrate bands. Such backgrounds often make the analysis of valence-band photoemission spectra more difficult. In addition, the symmetry selective elements introduced in the dipole transitions, dominating the XES and XAS processes, provide tools for a symmetry resolved look at the surface chemical bond\sup 2\. In particular, we have here used the azimuthal orientation of the COO - surface bond on the (110) surface, making a complete partition into x, y and z orbital contributions possible, thereby, providing a unique view of the local electronic structure of the surface - adsorbate system.[1] H. \Ao\gren, V. Carravetta, O. Vahtras and L. G. M. Pettersson, Chem. Phys. Letters 222, 75-81 (1994) .[2] A. Nilsson et al., Phys. Rev. B, 51, 10244 (1995)
4:10 PM		SS2-MoA-9 Surface and Layer Electronic Structure of the Organic Superconductor \kappa\(BEDT-TTF)\sub 2\Cu[N(CN)\sub 2\]Br C. Stagarescu, S. Dhesi, K. Breuer, M. Bunea, K. Smith (Boston University); R. Haddon (Bell Laboratories); J. Brooks (Florida State University) The organic solid \kappa\-(BEDT-TTF)\sub 2\Cu[N(CN)\sub 2\]Br, commonly referred to as ET-Br, is a quasi-two dimensional conductor, with layers of BEDT-TTF stacks separated from each other by insulating anion sheets. We report new results of a study of the electronic structure of ET-Br using angle resolved photoemission (ARP) spectroscopy and soft x-ray emission (SXE) spectroscopy. The electronic structure, as measured by ARP, of the clean, highly defective, O\sub 2\ exposed and Cs-dosed surfaces will be discussed. A significant problem associated with ARP measurements of organic conductors is the difficulty in preparing and characterizing clean, well ordered surfaces. We have also studied the electronic structure of ET-Br using synchrotron radiation excited SXE. The chemical specificity of SXE provides the opportunity of studying a layer-specific electronic structure, since certain elements only appear in specific layers in ET, e.g., N atoms reside only in the anion layers. The valence band partial density of states for transitions into the N1s states was measured. By tuning the incident photon energy through the threshold for valence band-to-N1s emission, dispersion effects in the valence band were measured. These can be related to dispersion within the anion layers. Work supported in part by NSF CAREER award DMR 95-01174. SXE performed on beamline BW3/HASYLAB, Hamburg, Germany; ARP performed on the Boston University/NSLS beamline U4A. Coauthors: L. Duda, J. Guo and J. Nordgren, Department of Physics, Uppsala University, Uppsala, Sweden. \super \Current address: Universite de Neuchatel, Institut de Physique, CH-2000, Neuchatel, Switzerland.
4:30 PM		SS2-MoA-10 The Electronic Structure of Oxidised Aluminium Surfaces Studied by (e,2e) Spectroscopy M. Vos, S. Canney, X. Guo, A. Kheifets (Flinders University of South Australia) Traditionally one measures dispersion in solids by angle resolved photoemission. This requires single crystals. Unfortunately there are many technologically important surfaces that are not available in this form. (e,2e) spectroscopy is an electron scattering approach to the problem of the measurement of dispersion, that differs in many ways from conventional photoemission techniques. It measures the relation between real momentum (not crystal momentum) and binding energy. This makes a whole new class of measurements possible. Here we present the results for clean polycrystalline Al films after evaporation and after controlled oxidation. The (e,2e) results of clean Al films display a simple free electron parabola, whereas fully oxidised Al films have a valence band with two main features, one related mainly to the O 2s level and one mainly to the O 2p level. Both levels show clear dispersion. For the clean Al surface and the fully oxidised one we compare the measured dispersion (and intensity!) with calculations using LMTO representation of the density functional theory. An important question is how does this dispersion developes with oxygen exposure? The Al 2p core level measured by the same technique displays oxygen induced satellites and thus its shape can be used to gauge to what extent the oxidised layer contributes to the measured intensity. For the intermediate oxygen coverages we subtract the Al metal related intensity and compare the remaining intensity with that of fully oxidised aluminum. These results, and other possible applications of (e,2e) spectroscopy on surface science are discussed.