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A polarization study of strained GaAs photocathode structures

Description: The polarized electron source at SLAC has performed extremely well during recent years supplying electrons having a spin polarization of 78% (85%) for high (low) current operation with beam current limited primarily by experimental requirements. However, there is room for improvement in the electron polarization. The less-than-ideal polarizations are a result of both imperfections and depolarizing mechanisms within the photocathode. The structure of the photocathode used at SLAC in the polarized electron source is a single-strained emitting layer structure grown atop a GaAs substrate. Here, the properties of several types of strained GaAs and GaAsP photocathodes have been studied using x-ray diffraction and photoemission.
Date: October 1, 1996
Creator: Mulhollan, G.; Garwin, E.L.; Maruyama, T.; Tang, H.; Mair, R. & Prepost, R.
Partner: UNT Libraries Government Documents Department

The electronic structure of heavy fermions: Narrow temperature independent bands

Description: The electronic structure of both Ce and U heavy fermions appears to consist of extremely narrow temperature independent bands. There is no evidence from photoemission for a collective phenomenon normally referred to as the Kondo resonance. In uranium compounds a small dispersion of the bands is easily measurable.
Date: August 1, 1996
Creator: Arko, A.J.; Joyce, J.J.; Smith, J.L. & Andrews, A.B.
Partner: UNT Libraries Government Documents Department

Nondipolar photoelectron angular distributions

Description: The deviations of photoelectron angular distributions from the simple, highly symmetric shapes predicted within the electric-dipole approximation are investigated. The admixture of an electric-quadrupole component in the photon-atom interaction causes an asymmetry in the angular distribution with respect to the direction of photon propagation. The reported measurement of the angular distributions of argon {ital 1s}, krypton {ital 2s}, and krypton {ital 2p} photoemission within 2-3 keV above their respective thresholds reveal pronounced asymmetries which are present even at low electron kinetic energies. The measured asymmetry parameters are in good agreement with recent predictions from nonrelativistic calculations.
Date: December 31, 1996
Creator: Kraessig, B.; Jung, M.; Gemmell, D.S.; Kanter, E.P.; LeBrun, T.; Southworth, S.H. et al.
Partner: UNT Libraries Government Documents Department

Recent progress in photo-injectors

Description: In photoinjector electron guns, electrons are emitted from a photocathode by a short laser pulse and then accelerated by intense RF fields in a resonant cavity. Photoinjectors are very versatile tools. Normally we think of them in terms of the production of high electron density in 6-D phase space, for reasons such as injection to laser accelerators, generation of x-rays by Compton scattering and short wavelength FELs. Another example for the use of photo-injectors is the production of a high charge in a short time, for wake- field acceleration, two-beam accelerators and high-power, long-wavelength FELs. There are other potential uses, such as the generation of polarized electrons, compact accelerators for industrial applications and more. Photoinjectors are in operation in many electron accelerator facilities and a large number of new guns are under construction. The purpose of this work is to present some trend setting recent results that have been obtained in some of these laboratories. In particular the subjects of high density in 6-D phase space, new diagnostic tools, photocathode advances and high-charge production will be discussed.
Date: October 1, 1996
Creator: Ben-Zvi, I.
Partner: UNT Libraries Government Documents Department

Correlation of magnetic dichroism in x-ray absorption and photoelectron emission using ultrathin magnetic alloy films

Description: We have begun a program to characterize magnetic alloy overlays using both magnetic x-ray circular dichroism (MXCD) and magnetic x-ray linear dichroism (MXLD). This will allow a direct comparison of MXCD-absorption and MXLD-photoelectron emission. First results from the Advanced Light Source will be presented.
Date: April 1, 1996
Creator: Tobin, J.G.; Goodman, K.W.; Mankey, G.J.; Willis, R.F.; Denlinger, J.D.; Rotenberg, E. et al.
Partner: UNT Libraries Government Documents Department

Bulk and surface electronic structure of actinide, rare earth, and transition metal elements and compounds

Description: This is the final report for a three-year, Laboratory Directed Research and Development (LDRD) project at the Los Alamos National Laboratory (LANL). This project sought to study of unusual magnetic and structural properties of rare earth, actinide, and transition metals through high-precision electronic structure calculations. Magnetic moment anisotropies in bulk and surface systems were studied, with emphasis on novel surfaces with unusual magnetic properties with possible applicability in magnetic recording. The structural stability, bonding properties, and elastic response of the actinides, as well as transition and rare earth elements and compounds, were also studied. The project sought to understand the unusual crystallographic and cohesive properties of the actinides and the importance of correlation to structural stability and the nature of the delocalization transition in these elements. Theoretical photoemission spectra, including surface effects, were calculated for rare earths and actinides.
Date: July 1, 1996
Creator: Wills, J.W. & Eriksson, O.
Partner: UNT Libraries Government Documents Department

Table top, pulsed, relativistic electron gun with GV/m gradient

Description: We present the design and performance characteristics of a compact high voltage pulser with 150 ps rise time, 0.2 to 2 ns adjustable flat top and up to 1 MV amplitude on a 80 Ohm load or up to 0.5 W on a 20 Ohm load, at 1 Hz repetition rate. Combination of a laser triggered SF{sub 6} and a liquid cap is used to form the fast rising pulse and maintain a low jitter between the laser, external trigger, and the high voltage pulsed output. The dark current and breakdown studies with this pulse applied between the electrodes of a diode indicate that fields up to 1 GV/m could be supported by stainless steel and copper cathodes without breaking down. The dark current from a conditioned cathode in a background pressure of 10{sup -7} Torr is below the detection limit of 0.5 mA of our system. Photoemission studies had been conducted with 300 kV applied between copper cathode and stainless steel anode separated by 2 mm. KrF laser of 5 eV photon energy and 20 ns FWHM was used to irradiate the cathode. In these preliminary measurements, 3 nC charge and corresponding quantum efficiency of 3.5 x 10{sup -4} have been obtained. Future plans include increasing the gradient to GV/m range, decreasing the laser pulse duration to ps and subps range and increasing the electron energy to a few MeV.
Date: October 1, 1996
Creator: Srinivasan-Rao, T. & Smedley, J.
Partner: UNT Libraries Government Documents Department

Photoelectron angular distributions beyond the dipole approximation

Description: This paper reports the measurement of nondipolar asymmetry parameters for the angular distributions of Ar 1s, Kr 2s, and Kr 2p photoemission within 2-3 keV above their respective thresholds. Pronounced asymmetries with respect to direction of photon propagation are present even at low electron kinetic energies. The energy dependence of the asymmetry is Z- and subshell-specific and causes the direction of preferred electron emission to change sign at least once in each case. Towards higher energies, the asymmetry increases and the photoelectron emission pattern is forward-skewed for all 3 cases. Measured asymmetry parameters are in good agreement with recent predictions from nonrelativistic calculations which include interference between electric-dipole and electric-quadrupole transition amplitudes in the photoabsorption process.
Date: August 1996
Creator: Kraessig, B.; Jung, M.; Gemmell, D. S.; Kanter, E. P.; LeBrun, T.; Southworth, S. H. et al.
Partner: UNT Libraries Government Documents Department

Wave packet dynamics of excited atomic electrons in intense laser fields

Description: Dynamics of muliphoton ionization in the tunneling (long wavelength, high intensity) regime is described. Photoemission by tunnel ionized atoms is dominated by the odd harmonics of the driving laser field. Excitation by ultra short ({similar_to} 10-20 fs) pulses produces hiogh harmonics with characteristics which will alow them to be compressed to given coherent sources of VUV and possibly XUV radiation with pulse lengths near or below 1 fs.
Date: August 1, 1996
Creator: Kulander, K.C. & Schafer, K.J.
Partner: UNT Libraries Government Documents Department

Angle-resolved photoemission extended fine structure study of chemisorbed c(2{times}2)P/Fe(100): Comparison with self-consistent- field X{alpha} scattered wave calculations

Description: Angle-resolved photoemission extended fine structure (ARPEFS) was used to determine the structure of c(2x2)P/Fe(100). P 1s core-level photoemission data were collected normal to the (100) surface and 45{degree} off-normal along the [011] direction at room temperature. Close analysis of the auto-regressive linear prediction based Fourier transform and multiple-scattering spherical-wave calculations indicate that the P atoms adsorb in the high-coordination fourfold hollow sites. The P atoms bond 1.02 {angstrom} above the first layer of Fe atoms and the Fe-P-Fe bond angle is 140.6{degree}. It was also determined that there is no expansion of the Fe surface. Self-consistent-field X{alpha} scattered wave calculations were performed for the c(2x2)P/Fe(100) and the c(2x2)S/Fe(100) systems. These independent results are in excellent agreement with this P/Fe structure and the S/Fe structure previously published, confirming the ARPEFS determination that the Fe{sub 1}-Fe{sub 2} interlayer spacing is contracted from the bulk value for S/Fe but not for P/Fe.
Date: February 1, 1996
Creator: Huff, W.R.A.; Chen, Y. & Zhang, X.S.
Partner: UNT Libraries Government Documents Department