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Element-specific magnetometry with linear dichroism in photoemission

Description: In this paper, we investigate the magnetic linear dichroism in the core-level photoemission spectra of the binary alloys Co<sub>x</sub>Ni<sub>l-x</sub> and Fe<sub>x</sub>Ni<sub>1-x</sub>/Cu(100). These epitaxial films have fee structures, but very different magnetic behavior.We show that the magnetic linear dichroism in x-ray photoemission (XMLD) signal tracks the magnetization in these alloys. Comparison with recent SQUID data provides a quantitative check and endorses the view that XMLD monitors the element-specific magnetometry.
Date: July 1, 1998
Creator: Tobin, J G
Partner: UNT Libraries Government Documents Department

Linear dichroism and resonant photoemission in Gd 011

Description: Magnetic Linear Dichroism in Angular Distributions (MLDAD) from Photoelectron Emission was used to probe the nature of Resonant Photoemission. Gd 5p and Gd 4f emission were investigated. Using novel theoretical simulations, we were able to show that temporal matching is a requirement for ``True`` Resonant Photoemission, where the Resonant Photoemission retains the characteristics of Photoelectron Emission.
Date: May 13, 1998
Creator: Mishra, S.R.; Cummins, T.R.; Gammon, W.J.; van der Laan, G.; Goodman, K.W. & Tobin, J.G.
Partner: UNT Libraries Government Documents Department

X-band photoinjector for a chirped-pulse FEL

Description: The phase noise and jitter characteristics of the laser and rf systems of a high gradient X-band photoinjector have been measured experimentally. When > 100 coherently phased 5 MeV electron bunches are produced in bursts, the photoinjector should be an ideal electron source for a pulsed, pre-bunched free-electron laser (FEL) operating at 100 GHz. The laser oscillator is a self-modelocked Titanium:Sapphire system operating at the 108th subharmonic of the rf gun. The X-band signal is produced from the laser by a phase-locked dielectric resonance oscillator, and amplified by a pulsed TWT and klystron. A comparison between the klystron and TWT amplifier phase noise and the fields excited in the rf gun demonstrates the filtering effect of the high Q structure, thus indicating that the rf gun can be used as a master oscillator, and could be energized by either a rf oscillator such as a magnetron or a compact source such as a cross-field amplifier. In particular, the rf gun can play the role of a pulsed rf clock to synchronize the photocathode laser system: direct drive of a synchronously mode-locked AlGaAs quantum well laser has been achieved using the X0-band gun rf fields. This novel, GHz repetition rate, sub-picosecond laser system is being developed to replace the more conventional femtosecond Ti: Al<sub>2</sub>O<sub>3</sub> system. Some advantages include pumping this laser with a stabilized current source instead of a costly, low efficiency pump laser. Finally, dark current measurements and initial photoelectron measurements are reported.
Date: December 15, 1998
Creator: Luhmann, Jr., N. C.; Alvis, R. M.; Baldis, H. A.; Hartemann, F. V; Heritage, J. P.; Ho, C. H. et al.
Partner: UNT Libraries Government Documents Department

Fourier transform photoelectron diffraction and its application to molecular orbitals and surface structure

Description: Photoemission intensities from the molecular orbitals of c(2x2)CO/Pt(111) over a wide photon energy range were measured and analyzed by the same methods developed for structural studies using core levels. The 4{sigma} orbital center of gravity is found to be concentrated between the C and O atoms, while that of the 5{sigma} orbital lies between the C atom and the Pt surface. The C 1s photoelectron diffraction was used to determine the adsorption geometry. The earlier ambiguity that multiple scattering is needed to correctly model a {chi} curve while single scattering is sufficient for understanding major peaks in the ARPEFS-FTS is clarified by studying the clean Ni(111) surface. In the normal emission case, several different combinations of scattering events have similar path length differences (PLDs), and can either cancel each other or enhance the corresponding FT peak. In the off-normal case the degeneracy is greatly reduced due to the lower degree of symmetry. In normal emission AR PEFS, up to third order multiple scattering is needed to describe fully both the {chi} curve and its FT spectrum. To improve the spectral resolution in the ARPEFS-FT analysis, several new spectral analysis methods are introduced. With both autocorrelation autoregression (ACAR) and autocorrelation eigenvector (ACE), we can produce a reliable power spectrum by following the order-closing procedure. The best spectra are usually obtained when the autocorrelation sequence is computed with lags up to half the data range. A simple way of determining surface adsorption sites is proposed as follows: First use a single scattering cluster for possible adsorption sites to construct the geometrical PLDs from the strong backscattering events; then compare these PLDs with those obtained from the ARPEFS-FT analysis of the experimental data. After the preferred adsorption site is determined, fine tune the interlayer distances according to the positional R-factor.
Date: November 30, 1998
Creator: Zhou, Xin
Partner: UNT Libraries Government Documents Department

Excitations and Optical Properties of Phenylene Based Polymers

Description: A complex picture of phenylene-based polymers is developed which unifies features of band and molecular exciton models. It incorporates major experimental finding in direct, and photoinduced optical absorption, stimulated photoemission and photoconductivity. The authors give new assignments for the most disputed features and identify new ones as edge states. The authors confirm a low binding energy for the principle emitting exciton and show that it dominates also in the fundamental absorption. Contradictions in the current modeling state-of-art are displayed and discussed.
Date: July 1, 1998
Creator: Kirova, N.; Brazovskii, S.; Bishop, A.R.; McBranch, D. & Klimov, V.
Partner: UNT Libraries Government Documents Department

Advances in DC photocathode electron guns

Description: At Jefferson Lab, a DC photoemission gun using GaAs and GaAs-like cathodes provides a source of polarized electrons for the main accelerator. The gun is required to produce high average current with long operational lifetimes and high system throughout. Recent work has shown that careful control of the parameters affecting cathode lifetime lead to dramatic improvements in source operation. These conditions include vacuum and the related effect of ion backbombardment, and precise control of all of the electrons emitted from the cathode. In this paper, the authors will review recent results and discuss implications for future photocathode guns.
Date: July 1, 1998
Creator: Dunham, Bruce M.; Heartmann, P.; Kazimi, Reza; Liu, Hongxiu; Poelker, B. M.; Price, J. S. et al.
Partner: UNT Libraries Government Documents Department

An investigation of resonant photoemission in Gd with x-ray linear dichroism

Description: The constructive summing of direct and indirect channels above the absorption threshold of a core level can cause a massive increase in the emission cross section, leading to a phenomenon called "resonant photoemission". Using novel magnetic linear dichroism in angular distribution photoelectron spectroscopy experiments and theoretical simulations, we have probed the nature of the resonant photoemission process in Gd metal. It now appears that temporal matching as well as energy matching is a requirement for true resonant photoemission.
Date: July 1, 1998
Creator: Tobin, J G
Partner: UNT Libraries Government Documents Department

Structural and Photoelectron Emission Properties of Chemical Vapor Deposition Grown Diamond Films

Description: The effects of methane (CH4), diborone (B2H6) and nitrogen (N2) concentrations on the structure and photoelectron emission properties of chemical vapor deposition (CVD) polycrystalline diamond films were studied. The diamond films were grown on single-crystal Si substrates using the hot-tungsten filament CVD technique. Raman spectroscopy and x-ray photoelectron spectroscopy (XPS) were used to characterize the different forms of carbon in the films, and the fraction of sp3 carbon to sp3 plus sp2 carbon at the surface of the films, respectively. Scanning electron microscopy (SEM) was used to characterize the surface morphology of the films. The photoelectron emission properties were determined by measuring the energy distributions of photoemitted electrons using ultraviolet photoelectron spectroscopy (UPS), and by measuring the photoelectric current as a function of incident photon energy.
Date: August 1998
Creator: Akwani, Ikerionwu Asiegbu
Partner: UNT Libraries