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FOURIER ANALYSIS OF EXTENDED FINE STRUCTURE WITH AUTOREGRESSIVE PREDICTION

Description: Autoregressive prediction is adapted to double the resolution of Angle-Resolved Photoemission Extended Fine Structure (ARPEFS) Fourier transforms. Even with the optimal taper (weighting function), the commonly used taper-and-transform Fourier method has limited resolution: it assumes the signal is zero beyond the limits of the measurement. By seeking the Fourier spectrum of an infinite extent oscillation consistent with the measurements but otherwise having maximum entropy, the errors caused by finite data range can be reduced. Our procedure developed to implement this concept applies autoregressive prediction to extrapolate the signal to an extent controlled by a taper width. Difficulties encountered when processing actual ARPEFS data are discussed. A key feature of this approach is the ability to convert improved measurements (signal-to-noise or point density) into improved Fourier resolution.
Date: January 1, 1985
Creator: Barton, J. & Shirley, D.A.
Partner: UNT Libraries Government Documents Department

THE TWO-DIMENSIONAL VALENCE ELECTRONIC STRUCTURE OF A MONOLYAER OF Ag ON Cu(00l)

Description: The metal overlayer system c(10x2)Ag/Cu(001) was studied at coverages near one monolayer with angle-resolved photoemission. The observed spectroscopic features indicate a two-dimensional d-band electronic structure that can be interpreted using a model with planar, hexagonal symmetry in which crystal field effects dominate over spin-orbit effects.
Date: May 1, 1985
Creator: Tobin, J. G.; Robey, S. W. & Shirley, D. A.
Partner: UNT Libraries Government Documents Department

HIGH RESOLUTION FOURIER ANALYSIS WITH AUTO-REGRESSIVE LINEAR PREDICTION

Description: Auto-regressive linear prediction is adapted to double the resolution of Angle-Resolved Photoemission Extended Fine Structure (ARPEFS) Fourier transforms. Even with the optimal taper (weighting function), the commonly used taper-and-transform Fourier method has limited resolution: it assumes the signal is zero beyond the limits of the measurement. By seeking the Fourier spectrum of an infinite extent oscillation consistent with the measurements but otherwise having maximum entropy, the errors caused by finite data range can be reduced. Our procedure developed to implement this concept adapts auto-regressive linear prediction to extrapolate the signal in an effective and controllable manner. Difficulties encountered when processing actual ARPEFS data are discussed. A key feature of this approach is the ability to convert improved measurements (signal-to-noise or point density) into improved Fourier resolution.
Date: April 1, 1984
Creator: Barton, J. & Shirley, D.A.
Partner: UNT Libraries Government Documents Department

ANGLE-RESOLVED PHOTOEMISSION STUDIES OF Ag, Au, AND Pt

Description: An important question regarding the technique of angle-resolved photoemission (ARP) is the extent to which it can be used to determine experimental valence-band dispersion relations E{sub i}({rvec k}) for single crystalline solids. In the case of the 3d and 4d transition metals, studies of copper, nickel, palladium, and silver, show that a model based on the assumption of direct interband transitions (direct-transition model) may be used, in conjunction with an appropriate final-state dispersion relation E{sub f}({rvec k}), to elucidate E{sub i}({rvec k}) for these materials along several high symmetry lines (primarily {Gamma}{Lambda}L) in k-space. To answer this question more generally, we have undertaken an extensive study of the valence band structures of other transition metals along various k-space lines. To date, studies have been extended to the (111) faces of the 5d metals Pt and Au along with the Pt(100) ((5 x 20) surface structure) face, and the (110) and (100) faces of Ag. The experiments were all conducted at SSRL, using synchrotron radiation in the range 6 eV < h{nu} < 34 eV. The results of these studies, combined with our previous Ag(111) work at these energies, allow us to invoke important conclusions concerning the relationships between ARP data, E{sub i}({rvec k}) and E{sub f}({rvec k}) for these materials. Several are summarized. For each crystal face investigated, the direct-transition model, along with a simple quasi-free-electron E{sub f}({rvec k}), was sufficient to determine experimental E{sub i}({rvec k}) relations along the appropriate k-space line that were in general agreement with theoretical RAPW band structure calculations. Essentially, we required E{sub f}({rvec k}) to be of the form (h{sup 2}/2m*)|{rvec k} + {rvec G}|{sup 2} + V{sub o}, where {rvec G} is a reciprocal lattice vector, fitting this relation to the appropriate calculated bulk conduction band near the center of the line under ...
Date: June 1, 1980
Creator: Davis, R.F.; Mills, K.A.; Thornton, G.; Kevan, S.D. & Shirley, D.A.
Partner: UNT Libraries Government Documents Department

Electron spectroscopy study of the heavy fermion compound U/sub 2/Zn/sub 17/

Description: We report x-ray photoemission, resonant photoemission and Bremsstrahlung isochromat spectra of the 4f core levels, the valence band and the conduction band, respectively, of the heavy fermion compound U/sub 2/Zn/sub 17/ and the reference compound Th/sub 2/Zn/sub 17/, and we assess current efforts to interpret such spectra. 19 refs., 4 figs.
Date: January 1, 1986
Creator: Lassailly, Y.; Allen, J.W.; Ellis, W.; Cox, L.; Pate, B.; Fisk, Z. et al.
Partner: UNT Libraries Government Documents Department

Electronic band structure and photoemission: A review and projection

Description: A brief review of electronic-structure calculations in solids, as a means of interpreting photoemission spectra, is presented. The calculations are, in general, of three types: ordinary one-electron-like band structures, which apply to bulk solids and are the basis of all other calculations; surface modified calculations, which take into account, self-consistently if at all possible, the presence of a vacuum-solid interface and of the electronic modifications caused thereby; and many-body calculations, which go beyond average-field approximations and consider dynamic rearrangement effects caused by electron-electron correlations during the photoemission process. 44 refs.
Date: September 1, 1987
Creator: Falicov, L.M.
Partner: UNT Libraries Government Documents Department

Mechanism of current modulation by optic phonon emission in heterojunction tunneling experiments

Description: We explain recent observations by Hickmott et al. of sequential longitudinal optic phonon emission in tunneling currents of GaAs-Al/sub x/Ga/sub 1-x/As heterojunctions in terms of inhomogeneous tunneling and a magnetopolaronic mass correction. 16 refs., 13 figs.
Date: August 27, 1985
Creator: Hanna, C.B.; Hellman, E.S. & Laughlin, R.B.
Partner: UNT Libraries Government Documents Department

High intensity polarized electron sources

Description: The status of the polarized electron source development program at SLAC will be reviewed. Emission currents of 60 A, corresponding to a space charge limited current density of 180 A/cm/sup 2/, have been obtained from GaAs photocathodes. Electron beam polarization 20% greater than that obtainable from GaAs cathodes has been observed from multilayer GaAs-GaAlAs structures. Work in progress to produce high beam polarization from II-IV-V/sub 2/ chalcopyrite photocathodes will also be described.
Date: October 1, 1980
Creator: Sinclair, C.K.
Partner: UNT Libraries Government Documents Department

Photoemission from solids: the transition from solid-state to atomic physics

Description: As the photon energy is increased, photoemission from solids undergoes a slow transition from solid-state to atomic behavior. However, throughout the energy range h..nu.. = 10 to 1000 eV or higher both types of phenomena are present. Thus angle-resolved photoemission can only be understood quantitatively if each experimenter recognizes the presence of band-structure, photoelectron diffraction, and photoelectron asymmetry effects. The quest for this understanding will build some interesting bridges between solid-state and atomic physics and should also yield important new insights about the phenomena associated with photoemission.
Date: August 1, 1980
Creator: Shirley, D.A.
Partner: UNT Libraries Government Documents Department

Inner-shell photoemission from atoms and molecules using synchrotron radiation

Description: Photoelectron spectroscopy, in conjunction with synchrotron radiation, has been used to study inner-shell photoemission from atoms and molecules. The time structure of the synchrotron radiation permits the measurements of time-of-flight (TOF) spectra of Auger and photoelectrons, thereby increasing the electron collection efficiency. The double-angle TOF method yielded angle-resolved photoelectron intensities, which were used to determine photoionization cross sections and photoelectron angular distributions in several cases. Comparison to theoretical calculations has been made where possible to help explain observed phenomena in terms of the electronic structure and photoionization dynamics of the systems studied. 154 references, 23 figures, 7 tables.
Date: December 1, 1983
Creator: Lindle, D.W.
Partner: UNT Libraries Government Documents Department

Gas-phase photoemission with soft x-rays: cross sections and angular distributions

Description: A summary is presented of typical gas-phase photoemission studies based on synchrotron radiation in the 50-5000 eV range, using beam lines at the Stanford Synchrotron Radiation Laboratory. Three topics are addressed: atomic inner-shell photoelectron cross sections and asymmetries, correlation peaks in rare gases, and core-level shape resonances in molecules.
Date: September 1, 1983
Creator: Shirley, D.A.; Kobrin, P.H.; Truesdale, C.M.; Lindle, D.W.; Ferrett, T.A.; Heimann, P.A. et al.
Partner: UNT Libraries Government Documents Department

Nanosecond length electron pulses from a laser-excited photocathode

Description: A photocathode made from polycrystalline lanthanum hexaboride (LaB{sub 6}) has produced nanosecond length electron pulses when excited by an excimer laser at 308nm. Peak currents in excess of 1A have been observed, with quantum yields of 4 {times} 10{sup {minus}5} being measured. A method for extracting the electrons from an emission-limited cathode, plasma extraction, has been demonstrated. This technique uses a low power continuous discharge to provide the electric field needed to extract the photoelectrons. This technique may be useful in producing high repetition rate short pulse ion sources. 10 refs., 4 figs.
Date: June 1, 1989
Creator: Young, A.T.; D'Etat, B.; Stutzin, G.C.; Leung, K.N. & Kunkel, W.B.
Partner: UNT Libraries Government Documents Department

Angle-resolved photoemission extended fine structure

Description: Core level angle-resolved photoemission intensity oscillates sinusoidally with increasing photoelectron momentum. Interference between direct and scattered photo-emission causes this angle-resolved photoemission extended fine structure (ARPEFS). We will discuss an analytic single-scattering theory which quantitatively describes the oscillations. The procedures for extracting surface geometry information from photoemission measurements will be illustrated with S(1s) ARPEFS from S on Ni(100) and Cu(100) obtained with the soft X-ray double crystal monochromator at the Stanford Synchrotron Radiation Laboratory. Building on the surface sensitivity and chemical selectivity of photoemission, ARPEFS analysis provides direct geometrical information from the oscillation frequencies (derived with auto-regressive Fourier analysis), from intensity changes with polarization and analyzer position, and from analysis of scattering phase-shift zero-crossings.
Date: September 1, 1983
Creator: Barton, J.J.; Bahr, C.C.; Hussain, Z.; Robey, S.W.; Klebanoff, L.E. & Shirley, D.A.
Partner: UNT Libraries Government Documents Department

Dimensionality and its effects upon the valence electronic structure of ordered metallic systems

Description: The system c(10x2)Ag/Cu(001) was investigated with Angle-Resolved Photoemission (ARP), Low Energy Electron Diffraction (LEED) and Auger Electron Spectroscopy (AES). LEED and AES provided the calibration of a quartz microbalance used to measure the amount of silver evaporated onto the copper single crystal and also established the monolayer geometrical structure at one monolayer exposure. An off-normal ARP bandmapping study performed with polarized HeI and NeI radiation demonstrated the electronically two-dimensional nature of the silver d-bands at coverages of near one monolayer. The states at the surface Brillouin Zone center were assigned upon the basis of their polarization dependences and a structural model of hexagonal symmetry. A normal emission ARP experiment was performed at the Stanford Synchrotron Radiation Laboratory (SSRL) over the photon energy range of 6 to 32 eV. Data from it documented the evolution of the valence electronic structure of the silver overlayer from a two-dimensional hexagonal valence to a three-dimensional behavior converging towards that of bulk Ag(111). A structural study was attempted using the ARP technique of Normal Emission Photoelectron Diffraction over the photon energy range of 3.4 to 3.7 keV at SSRL, the results of which are inconclusive.
Date: July 1, 1983
Creator: Tobin, J.G.
Partner: UNT Libraries Government Documents Department

A scanning photoelectron microscope (SPEM) at the National Synchrotron Light Source (NSLS)

Description: We are in the process of developing and commissioning a scanning photoelectron microscope (SPEM) at the X1A beamline of the National Synchrotron Light Source (NSLS). It is designed to make use of the Soft X-ray Undulator (SXU) at the NSLS. This high brightness source illuminates a Fresnel zone plate, which forms a focused probe, {<=} 0.2{mu}m in size, on the specimen surface. A grating monochromator selects the photon energy in the 400-800 eV range with an energy resolution of better than 1 eV. The expected flux in the focus is in the 5 {times} 10{sup 7} {minus} 10{sup 9} photons/s range. A single pass Cylindrical Mirror Analyzer (CMA) is used to record photoemission spectra, or to form an image within a fixed electron energy bandwidth as the specimen is mechanically scanned. As a first test, a 1000 mesh Au grid was successfully imaged with a resolution of about 1{mu}m and the CMA tuned to the Au 4 f photoelectron peak. Once it is commissioned, a program is planned which will utilize the microscope to study beam sensitive systems, such as thin oxide/sub-oxide films of alumina and silica, and ultimately various adsorbates on these films. 14 refs., 4 figs.
Date: January 1, 1989
Creator: Ade, H.; Kirz, J.; Hulbert, S.; Johnson, E.; Anderson, E.; Kern, D. (State Univ. of New York, Stony Brook, NY (USA). Dept. of Physics et al.
Partner: UNT Libraries Government Documents Department

Electron spectroscopy studies in heavy fermions

Description: Photoemission experiments (whereby an electron absorbs a packet of light energy and is able to escape from the host material due to its increased energy) can measure directly the energy distribution of electrons in various materials. Our measurements on a recently-discovered class of metallic materials called ''heavy fermions'' show that the electrons that actually carry the electric current in these metals exist only within an extremely narrow range of energies. This range, which we will call the bandwidth, is narrower than that found in ordinary metals like copper by at least a factor of 10. Indeed it is surprising that they can carry electric current at all since such narrow energy ranges (or band widths) are characteristic of electrons confined to their host atoms, as in a non-metal, rather than of electrons that are free to wander through a metal. 8 refs.
Date: February 1, 1986
Creator: Arko, A.J.
Partner: UNT Libraries Government Documents Department

Study of electrons photoemitted from field-emission tips. Progress report, July 1, 1982-June 30, 1983

Description: Photo-induced field emission is a technique which studies electrons that have been photoemitted from a field emission tip. This experimental method promises to combine the proven utility of both field emission and photoemission for investigating the electronic states near a metal surface. The primary objective of the reseach decribed in this proposal is to measure photo-induced field emitted electrons as a function of both their final state energy and the applied electric field. An Argon-ion laser in conjunction with a tuneable cw dye laser will be used to photo-excite the electrons. A differential energy analyzer will allow energy resolved measurements of the photo-field emitted electrons. Other experiments, directly related to the unique effects associated with the illumination of metallic tips by laser light, are also proposed. Experiments that exploit the large tempeature rise of a field emission tip when illuminated by a focussed laser beam are proposed to study thermally activated surface diffusion of alkali adatoms on a field emission tip. These experiments may eventually provide information about thermally-induced or laser-induced chemical reactions on small metallic surfaces.
Date: January 1, 1982
Creator: Reifenberger, R.
Partner: UNT Libraries Government Documents Department

Mechanism of the optogalvanic effect in a hollow-cathode discharge

Description: There are two significantly different mechanisms proposed for the origin of the optogalvanic effect in a hollow-cathode discharge: (1) laser excitation of atoms to higher electronic states leads to an increased cross section for electron impact ionization, with the result that the excited atom becomes ionized and the conductivity of the discharge increases; and (2) laser excitation of atoms to higher electronic states perturbs the equilibrium established between the electron temperature and the atomic excitation temperature. Superelastic collisions between the electrons and the laser-excited atoms restore the equilibrium, with the excess energy ending up in an increased electron temperature and therefore an increased conductivity of the discharge. Both mechanisms undoubtedly proceed simultaneously and what needs to be determined is their relative importance at different discharge conditions and different excitation conditions. This is important because laser isotope enrichment schemes have been proposed using selective excitation in a hollow-cathode discharge. In order for these schemes to work, (1) must be the predominant mechanism. We have measured the optogalvanic signal, concentration of uranium atoms, impedance of the discharge, and electron temperature as a function of the discharge current in a neon-filled uranium hollow-cathode discharge. The hollow cathode operating characteristics are used as input parameters in a simple discharge model. Predictions of electron density, changes in electron temperature, and discharge impedance compare well with experimental observations. Our model and experimental observations yield a qualitative understanding of the optogalvanic effect in a hollow-cathode discharge and estimate the relative importance of the two optogalvanic mechanisms.
Date: June 17, 1983
Creator: Keller, R.A.; Warner, B.E.; Zalewski, E.F.; Dyer, P.; Engleman, R. Jr. & Palmer, B.A.
Partner: UNT Libraries Government Documents Department

Scanning Photoelectron Microscope (SPEM) with a zone plate generated microprobe

Description: We describe instrumentation of a scanning photoelectron microscope (SPEM), which we are presently developing and commissioning at the X1A beamline of the National Synchrotron Light Source (NSLS). This instrument is designed to use the Soft X-ray Undulator (SXU) at the NSLS as a high brightness source to illuminate a Fresnel zone plate, thus forming a finely focused probe, {le} 0.2{mu}m in size, on the specimen surface. A grating monochromator selects the photon energy in the 400-800 eV range with an energy resolution better than 1 eV. The expected flux in the focus is in the 5 {times} 10{sup 7} {minus} 10{sup 9} photons/s range. A single pass Cylindrical Mirror Analyzer (CMA) is used to record photoemission spectra, or to form an image within a fixed electron energy bandwidth as the specimen is mechanically scanned. As a first test, a 1000 mesh Au grid was successfully imaged with Au 4 f primary photoelectrons, achieving a resolution of about 1{mu}m. 10 refs., 5 figs., 1 tab.
Date: January 1, 1989
Creator: Ade, H.; Kirz, J.; Hulbert, S.; Johnson, E.; Anderson, E.; Kern, D. (State Univ. of New York, Stony Brook, NY (USA). Dept. of Physics et al.
Partner: UNT Libraries Government Documents Department

UV photoemission from metal cathodes for picosecond power switches

Description: Results are reported of photoemission studies using laser pulses of 10 ps duration and 4.66 eV photon energy on metal cathodes. These included thin wires, flat surfaces and an yttrium cathode with a grainy surface. The measurements of current density and quantum efficiency under low and high surface fields indicate that field assisted efficiencies exceeding 0.1% and current densities exceeding 10/sup 5/ A/cm/sup 2/ are obtainable. The results are compared to the requirements of switch power applications. 24 refs., 13 figs., 1 tab.
Date: January 1, 1989
Creator: Fischer, J.; Srinivasan-RAo, T. & Tsang, T.
Partner: UNT Libraries Government Documents Department

Ultra-high-vacuum double-axis goniometer for use with an electron spectrometer

Description: A double-axis goniometer designed for moving and indexing an electon spectrometer for angle-resolved photoemission studies is described. A feature of the design is that the two rotations operate independently and either can be carried out with the analyzer at any position. The goniometer is designed to be installed in a 14'' spool piece which makes it possible to add it to an existing 14'' uhv system.
Date: January 1, 1981
Creator: Shleifer, M. & Williams, G.P.
Partner: UNT Libraries Government Documents Department

Magnetism and chemisorption

Description: Spin polarized photoemission studies of different adsorbates on an Fe (001) substrate show the interaction between the adsorbate and the substrate d bands. Studies of the adsorption of oxygen reveal exchange split adsorbate bands indicative of both a magnetic moment on the adsorbate atom and also a strong adsorbate p{sub z} substrate d{sub z}{sup 2} bond. The oxygen p{sub z} bands show a large variation in exchange splitting across the surface zone, an observation that is not predicted in first principles calculations of this system. Spin Polarized Auger Electron Spectroscopy studies of sulfur adsorbed on the same surface provide some indication of the magnetic moment on the adsorbate. It is found to be of the order of 0.1 {mu}B. Studies of carbon monoxide adsorption in two different configurations, vertical and tilted, reveal little or no exchange splitting in the molecular 5{sigma} orbital suggesting that the interaction of this orbital with the substrate d bands is much weaker. 38 refs., 11 figs.
Date: January 1, 1989
Creator: Johnson, P.D.
Partner: UNT Libraries Government Documents Department

Characterization of Rh films on Ta(110)

Description: The surface and electronic structure of Rh films on Ta(110) up to several monolayers thick on Ta(110) are characterized by photoemission, Auger emission, low energy electron diffraction and low energy ion scattering. From the variation of the Rh Auger peak-to-peak intensity as a function of evaporation time, Rh/Ta(110) appears to grow in the Stranski-Krastanov mode at room temperature. However, the LEIS data show that the Rh adatoms begin to cluster on Ta(110) before growth of the monolayer is completed. Diffuse LEED scattering suggests that the Rh films are disordered. Photoemission shows that Rh chemisorption on Ta(110) generates two peaks located at 1.2 and 2. 5 eV binding energy during the initial phase of thin film growth (0 < {Theta} < 0.5 ML). By 0.75 ML Rh coverage, those states merge into a broad structure centered near 2 eV binding energy. Photoemission peaks typical of a Rh(111) surface are seen at higher coverages ({Theta} > 3.7 ML). Photoemission data for CO covered surfaces show that CO dissociates on the Rh/Ta(110) surface for Rh coverages less than 2.5 ML and also show that the Rh clusters develop at least one site capable of molecular CO adsorption above 0.3 ML Rh coverage. 38 refs., 5 figs.
Date: January 1, 1989
Creator: Jiang, L.Q.; Ruckman, M.W. & Strongin, M.
Partner: UNT Libraries Government Documents Department