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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

Simple laser-driven, metal photocathodes as cold, high-current electron sources

Description: Recent developments in excimer laser design have made near ultraviolet light intensities of several MW/cm/sup 2/ possible in unfocused beams. These advances and recent experiments indicate that high-current, simple-metal photoemissive electron guns are now feasible. Producing more than 50 A/cm/sup 2/ of illuminated cathode surface, the guns could operate at vacuums of 10/sup -6/ torr with no complicated system components inside the vacuum enclosure. The electron beam produced by such photoemission guns would have very low emittance and high brightness. This beam would also closely follow the temporal characteristics of the laser pulse, making fast risetime, ultrashort electron beam pulses possible.
Date: January 1, 1987
Creator: Saunders, J.D.; Ringler, T.J.; Builta, L.A.; Kauppila, T.J.; Moir, D.C. & Downey, S.W.
Partner: UNT Libraries Government Documents Department

UV photoemission studies of metal photocathodes for particle accelerators

Description: Photoemission from several metals was studied with 10 ps laser pulses at 266 nm. The yield was linear with energy and with area. Quantum efficiencies (/eta/) were determined (up to 10/sup /minus/3/ e/photons for samarium), and found to vary as (h..nu..-/phi/)/sup 2/. /eta/ also increased with the field. The field assisted efficiencies were calculated for some metals and confirmed by experiment for gold, up to surface fields of /approximately/3/times/10/sup 8/ V/m. High charge and current densities, close to 10/sup 5/ A/cm/sup 2/ from macroscopic areas, were measured or indicated. Results are then related to applications in accelerators. 18 refs., 15 figs., 4 tabs.
Date: September 1, 1988
Creator: Fischer, J. & Srinivasan-Rao, T.
Partner: UNT Libraries Government Documents Department

Electronic structure of monoclinic BaBiO sub 3

Description: We present the results of photoemission studies of valence disordered'' monoclinic BaBiO{sub 3} in the photon energy range 15--120 eV. The line-shapes of the valence band photoemission spectra and the Ba contributions to the valence band are very similar to the line shapes of the total density of states and Ba partial density of states, respectively. Oxygen resonance is observed, demonstrating the existence of empty O 2p states. These results support a more covalent rather than a simple ionic picture for the electronic states of BaBiO{sub 3}. 10 refs., 2 figs.
Date: January 1, 1989
Creator: Shen, Z.X.; Lindberg, P.A.P.; Wells, B.O.; Dessau, D.S.; Borg, A.; Lindau, I. et al.
Partner: UNT Libraries Government Documents Department

Energy distributions and yields of secondary ions and electrons emitted by GIOTTO during Halley encounter

Description: The extreme ram velocity of the GIOTTO space-craft with respect to the Halley environment will produce a host of surface impact phenomena not previously encountered by planetary spacecraft. This paper extends earlier efforts to model impact produced plasma at GIOTTO by investigating published data on secondary electron and ion energy distributions, including the effect of possible surface contamination. One new result, is the finding that the threshold velocity for kinetic emission of secondary electrons by neutral gas is very nearly equal to the GIOTTO ram velocity. This makes it difficult to predict electron yields due to neutral gas impact.
Date: January 1, 1983
Creator: Young, D.T.
Partner: UNT Libraries Government Documents Department

Gallium arsenide photocathode for the free electron laser

Description: The efforts of the FEL source have been concentrated on cesiated GaAs(100) wafers. These crystals have shown photoyield of <.1 to 9% quantum efficiency with the cesium and oxygen treatment. The work function and coverage curves exhibit the same properties as measured in the literature. The use of Auger Electron Spectroscopy and X-ray Photoelectron Spectroscopy have been instrumental in determining the surface cleanliness and the surface oxidation states. The sputtered surfaces have been investigated as a function of rare gas mass and sputter ion voltage, giving similar results to earlier literature values. Temperature annealing appears to be critical after sputter cleaning in achieving any significant photoyield. Contacts of Ag-Mn and Ni-Si have been deposited, heated, and analyzed using Auger Depth Profiling techniques. 16 refs., 9 figs.
Date: January 1, 1985
Creator: Stotlar, S.C.; Springer, R.W.; Sherwood, B. & Cordi, R.
Partner: UNT Libraries Government Documents Department

Study of electrons photoemitted from field emission tips. Progress report, July 1, 1979-March 1, 1980

Description: Photo-induced field emission is a technique which studies electrons that have been photoemitted from a field emission tip. This new 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 research being performed is to investigate photo-induced field emitted electrons using a tuneable cw dye laser. To fully exploit this continuously tuneable photon source, a differential energy analyzer is being constructed to allow energy resolved measurements of the photo-field emitted electrons. This report describes the progress made in implementing experiments on photo-induced field emission from July 1979 to March 1980.
Date: February 1, 1980
Creator: Reifenberger, R.
Partner: UNT Libraries Government Documents Department

Experimental program to build a multimegawatt lasertron for super linear colliders

Description: A lasertron (a microwave ''triode'' with an RF output cavity and an RF modulated laser to illuminate a photocathode) is a possible high power RF amplifier for TeV linear colliders. As the first step toward building a 35 MW, S-band lasertron for a proof of principle demonstration, a 400 kV dc diode is being designed with a GaAs photocathode, a drift-tube and a collector. After some cathode life tests are made in the diode, an RF output cavity will replace the drift tube and a mode-locked, frequency-doubled, Nd:YAG laser, modulated to produce a 1 us-long comb of 60 ps pulses at a 2856 MHz rate, will be used to illuminate the photocathode to make an RF power source out of the device. This paper discusses the plans for the project and includes some results of numerical simulation studies of the lasertron as well as some of the ultra-high vacuum and mechanical design requirements for incorporating a photocathode.
Date: April 1, 1985
Creator: Garwin, E.L.; Herrmannsfeldt, W.B.; Sinclair, C.; Weaver, J.N.; Welch, J.J. & Wilson, P.B.
Partner: UNT Libraries Government Documents Department

Normal state electronic structure and the superconducting energy gap in HTSC's as determined from photoemission spectroscopy

Description: Photoemission spectroscopy has been utilized to determine the electronic structure of high-T{sub c} materials. The observation of dispersive bands at E{sub F} suggests a Fermi surface similar to that obtained from a band calculation. The results apparently are not inconsistent with the notion of a correlated Fermi liquid consisting of hybridized p-d bands. However, it is becoming more and more difficult to distinguish between Fermi liquid behavior in the new high-T{sub c} superconductors and behavior expected on the basis of the novel new non-Fermi liquid theories. The differences are now predicted to be on an energy scale smaller than our experimental resolution. We point out that, while deviations from simple band theory certainly do exist in the form of core and valence band satellites, band narrowing, and rapid photoemission peak broadening away from E{sub F}, there are sufficient agreements with the overall DOS that it should be considered a good starting point for the electronic structure. For example, the calculated Fermi surface for both the 123 and 2212 structures is reasonably well reproduced experimentally and the bands at E{sub F} consist of p-d hybridized orbitals just as predicted by local density functional theory. Our spectra clearly show that a BCS-like DOS is obtained at the Fermi energy as a gap opens up below T{sub c}. This is just one more indication that the old conventional models should be considered more seriously. 18 refs., 6 figs.
Date: January 1, 1989
Creator: Arko, A.J.; List, R.S.; Bartlett, R.J.; Cheong, S.W.; Fisk, Z.; Thompson, J.D. (Los Alamos National Lab., NM (USA)) et al.
Partner: UNT Libraries Government Documents Department

On the photoemission spectrum and optical gap in La/sub 2/CuO/sub 4/

Description: The photoemission and optical spectrum of La/sub 2/CuO/sub 4/ are computed for a CuO/sub 6/ cluster model using the techniques of ab initio quantum chemistry. Evidence for a near degeneracy and strong electronic coupling between the configurations Cu/sup 3/plus//O/sup 2/minus// (/sup 1/Ag) and Cu/sup 2/plus//O/sup 1/minus// (/sup 1/Ag) is presented. This interaction leads to d/sup 8/ character at the Fermi energy as well as in the 12eV satellite. 17 refs., 1 fig., 2 tabs.
Date: January 1, 1989
Creator: Martin, R.L.
Partner: UNT Libraries Government Documents Department

Status of Switched-Power Linac studies at BNL (Brookhaven National Laboratory) and CERN (European Organization for Nuclear Research)

Description: The switched-power linac (SPL) concepts are reviewed briefly, and recent work on computer-modelling of the photoemission process at the photocathode and the experimental study of the process are discussed. Work on rf-modelling of the properties of the radial transmission line is outlined. (LEW)
Date: October 31, 1986
Creator: Aronson, S.
Partner: UNT Libraries Government Documents Department

Study of electrons photoemitted from field-emission tips. Progress report, July 1, 1980-January 1, 1981

Description: Photo-induced field emission is a technique which studies electrons that have been photoemitted from a field emission tip. This new 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 research being performed is to investigate photo-induced field emitted electrons using a tuneable cw dye laser. To fully exploit this continuously tuneable photon source, a differential energy analyzer has been constructed to allow energy resolved measurements of the photo-field emitted electrons. This report describes the progress made in implementing experiments on photo-induced field emission from July 1980 to January 1981.
Date: January 1, 1981
Creator: Reifenberger, R.
Partner: UNT Libraries Government Documents Department

The SLAC lasertron project

Description: SLAC has a program underway to develop an efficient high power microwave source for accelerator applications employing a photoemission cathode, the so-called lasertron. After a brief discussion of the lasertron idea, the various elements of the current program are reviewed. A proof-of-principle experiment to build a 35 MW, 70% efficiency, S-band microwave source is nearing completion. Actual RF power testing is expected to begin in late 1986 or early 1987.
Date: October 1, 1986
Creator: Sinclair, C.K.
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