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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

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

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

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

Shuttering efficiencies of nanosecond-gated photoemissive shutter tubes

Description: Recent studies show that effective shuttering of photoemissive tubes, such as Silicon-Intensified-Target Vidicons (SITVs) and Microchannel-plate Image Intensifier Tubes (MCPTs), can vary widely depending upon the extent of their opacity to an input flux of photons. Optical feedthrough signals from photon transmission through the photocathode to the target or phosphor ranging from 10/sup -4/ to 10/sup -9/ (when compared with gated signals) were measured for a large sampling of commercially available units. Effective shutter ratios of 10/sup 5/ to 10/sup 8/ measured for units operated in quiescently dark environments can be substantially reduced by optical feedthrough. Furthermore, ineffective suppression of photoemission can cause further reductions in shutter ratio. Reductions are roughly correlated with the ratio of optical gate duration to light pulse duration. Experimentation with various thicknesses of aluminum depositions on MCPT phosphors and chromium layering on SITV silicon targets indicate substantial reductions (2x to 15x) in transmission with minimal increases in threshold voltages required for gain. These results, together with exploratory studies of external coating of output fiber optics with transmission filters spectrally matched to minimize feedthrough to P-20 phosphors are reported. 4 refs., 13 figs., 3 tabs.
Date: January 1, 1985
Creator: Yates, G.J.; Jaramillo, S.A.; Pagano, T.S. & Black, J.P.
Partner: UNT Libraries Government Documents Department

High-brightness photoemitter development for electron accelerator injectors

Description: Free-electron-laser (FEL) oscillators require a train of high-brightness bunches. Conventional subharmonic bunchers are currently used with rf linacs to generate pulse trains, but the resulting dilution of the transverse phase space and lower beam brightness are unacceptable for high-performance FELs. Recent developments suggest that photoemitters of high quantum efficiency combined with rapid acceleration can produce pulse trains of higher brightness than has been achieved before.
Date: January 1, 1985
Creator: Fraser, J.S.; Sheffield, R.L. & Gray, E.R.
Partner: UNT Libraries Government Documents Department

Band structure of UO/sub 2/: an angle resolved and resonant photoemission study

Description: A detailed study of angle resolved photoemission of UO/sub 2/ <100> in the normal emission configuration is presented. The results are compared to a semirelativistic LAPW band calculation with the actual calculated empty bands used in the data reduction. Excellent agreement is found with calculations both for filled states as well as critical points in the empty states with no adjustment of bands. f-p hybridization is found at the leading edge of the valence band and not at the bottom of the valence band. p-d hybridization is found throughout the valence bands.
Date: October 1, 1985
Creator: Arko, A.J.; Koelling, D.D.; Boring, A.M.; Ellis, W.P. & Cox, L.E.
Partner: UNT Libraries Government Documents Department

5f-band narrowing in UX/sub 3/B compounds: evidence from resonant photoemission. [X = Ru or Rh]

Description: Resonant photoemission at the uranium 5d disorption edge has been used to determine the 5f emission in URu/sub 3/, URh/sub 3/, URu/sub 3/B, and URh/sub 3/B polycrystalline samples. The observed 5f band narrowing (from 2.0 to 1.5 eV as one goes from URu/sub 3/ to URu/sub 3/B, and from 2.8 to 1.1 eV for URh/sub 3/ and URh/sub 3/B respectively) is explained as due to a decrease in the f-d hybridization between uranium and rhodium (ruthenium) atoms.
Date: October 1985
Creator: Zolnierek, Z.; Arko, A. J. & Koelling, D. D.
Partner: UNT Libraries Government Documents Department