Search

A method by which negative electron affinity GaAs photocathodes can be made to recover their photoyield following decay is reported. The source of decay can be either an oxidizer in the background gas or gas introduced through deliberate dosing. Conventional methods employ the use of the alkali Cs as the photoyield recovery agent. This work describes an alkali application-free method, similar to electron stimulated desorption, for photoyield recovery.

Amorphous silicon and amorphous silicon germanium photoemitters grown via radio frequency plasma enhanced chemical vapor deposition have been investigated to determine their usefulness as photoinjector electron sources. Characterized properties include activation process, wavelength dependent photoyield, germanium induced photoyield shift, background gas and ion sensitivities, and average transverse emission energy. While overall lower in yield than the GaAs photoemitter, many of their properties are comparable. The amorphous silicon photoemitter robustness upon gas and ion exposure is superior to that of GaAs. The combined properties of amorphous silicon germanium photoemitters make them ideal candidates for low cost photoinjector sources or as protective photoemitting layers on more sensitive photogenerating materials.

Negative electron affinity GaAs photoemitters require ultrahigh vacuum conditions to achieve the best performance and to maintain the quantum yield. This limits their utility in applications where the background pressure is high. The authors have developed an activation procedure by which the reactivity to CO2, a principal cause of yield decay, is greatly reduced. The use of a second alkali in the activation process is responsible for the increased immunity of the activated surface. The best immunity was obtained by using a combination of Cs and Li without any loss in near band gap yield.

An anisotropy in the quantum efficiency (QE) has been observed in photoemission from strained GaAs photocathodes excited by linearly polarized light. The wavelength dependence of the anisotropy is closely correlated with that of the electron-spin polarization. Based on a theoretical analysis, we show that the QE anisotropy is caused by an in-plane strain anisotropy arising from anisotropic strain relaxation. The QE anisotropy calculated from the in-plane strain anisotropy measured with X-ray diffraction and the measured QE anisotropy are in good agreement.

Positron annihilation induced Auger electron spectroscopy (PAES), electron induced Auger electron spectroscopy (EAES), and low‐energy electron diffraction have been used to study the temperature dependent composition of vapor‐deposited of Rh on Ag(100). Earlier work using AES, ion scattering spectroscopy, and TDS has shown that a Ag layer diffuses to the Rh surface upon annealing to form a structure in which Rh is sandwiched between a Ag cap layer and the Ag substrate. In this work, the top layer selectivity of PAES was utilized to study the diffusion of Ag to the surface as Rh films deposited at 173 K were heated to 573 K. Analysis of the PAES spectra indicates that Rh remains in the top layer during the ∼3 h required to take PAES data when the sample is maintained at the 173 K temperature of deposition. There is a clear indication that significant migration of Ag to the surface takes place by 373 K. The Ag content of the top layer increases to approximately 100% above 473 K.

A modified Rice University‐type compact Mott spin polarimeter operating at 20 kV is adapted to a large commerical hemispherical electron energy analyzer. Normal energy analyzer functions are preserved via a retractable channeltron in the polarimeter acceleration column. In the spin‐detection mode, the polarimeter permits analysis of two orthogonal transverse spin‐polarization components. Electron trajectory analysis is used to optimize polarimeter lens column voltages in both normal and spin‐detection modes. Performance levels are established by experiments and significantly improved spin‐detection efficiency is shown to be accessible by changes in the polarimeter collection solid angle.

Chemisorbed {ital p}(1{times}1) oxygen on ultrathin epitaxial Fe films grown on W(001) are studied using spin- and angle-resolved photoemission. In agreement with theoretical predictions, single-monolayer (ML) Fe films on W(001) are found to be nonmagnetic; 2-ML-thick Fe films are magnetic, and chemisorbed {ital p}(1{times}1) oxygen does not destroy the magnetism. Even- and odd-symmetry oxygen 2{ital p}-derived bands are measured along the {bar {Gamma}}-{ital {bar X}} and {bar {Gamma}}-{ital {bar M}} directions of the two-dimensional Brillouin zone. Oxygen-derived features in the photoemission spectra exhibit magnetic exchange splitting as well as spin-dependent intrinsic linewidths that are governed by lifetime effects and initial-state mixing with Fe bands. Oxygen 2{ital p}-band narrowing resulting from the expanded Fe thin-film lattice constant is observed. The results are compared with relevant calculations and corresponding experimental studies of {ital p}(1{times}1) oxygen on bulk Fe(001) surfaces.

RF photoinjectors for next generation spin‐polarized electron accelerators require photo‐cathodes capable of surviving RF gun operation. Free electron laser photoinjectors can benefit from more robust visible light excited photoemitters. A negative electron affinity gallium arsenide activation recipe has been found that diminishes its background gas susceptibility without any loss of near bandgap photoyield. The highest degree of immunity to carbon dioxide exposure was achieved with a combination of cesium and lithium. Activated amorphous silicon photocathodes evince advantageous properties for high current photoinjectors including low cost, substrate flexibility, visible light excitation and greatly reduced gas reactivity compared to gallium arsenide.

Mesure de la variation avec la temperature, des courbes des distributions des energies des bandes de valence des films ultraminces de Fe/W(100), a l'aide de la photoemission a angle et a spin resolus. Les structures majoritaires et minoritaires au point Γ sont stationnaires lorsque l'on eleve la temperature; un elargissement apparent des pics est cause par l'emergence de pics «extraordinaires». La comparaison avec un calcul du cluster (amas) suggere que de l'ordre ferromagnetique a courte distance existe au-dessus de Tc dans le film d'epaisseur egale a 1,5 monocouche

Spin-polarized electron-energy-loss spectra have been obtained from Cu(100) and Mo(110) surfaces by use of a polarized primary beam coupled with energy-resolved spin analysis of the scattered electrons. The data reveal strong evidence for inelastic spin-exchange scattering. In particular, a prominent polarization-loss feature evident in the Mo(110) data correlates with the joint density of states available for electron-hole pair excitation. Exchange scattering is observed to decrease rapidly with increasing primary beam energy.

The GaAsP/GaAs superlattice (SL) structure has been widely recognized as the most efficient spin polarized electron source with 90% maximum polarization and more than 1% quantum efficiency. The main spin depolarization mechanisms in these structures are: interband absorption smearing due to band-edge fluctuations, hole scattering between the heavy hole (HH) and light hole (LH) states that causes a broadening of the LH band, spin precession due to an effective magnetic field generated by the lack of crystal inversion symmetry and spin orbit coupling.

Polarized electron beams will play a crucial role in maximizing the physics potential for future e/sup +//e/sup -/ linear colliders. We review the SLC polarized electron source (PES), present a design for a conventional PES for the Next Linear Collider (NLC), and discuss the physics issues of a polarized RF gun.

Previous magneto‐optic Kerr effect studies of ultrathin epitaxial Fe films grown on stepped W(001) surfaces yielded evidence of in‐plane uniaxial magnetic anisotropy with magnetization perpendicular to the steps. We report spin‐polarized secondary electron emission spectroscopy studies of the same system that confirms this novel micromagnetic phenomena, and provides a more detailed characterization of the zero‐field in‐plane spin configuration as a function of initial applied field direction.

The doping concentration dependence of the charge limit phenomenon is studied using a set of thin unstrained GaAs. The p-type doping concentration ranges from 5×1018 cm−3 to 5×1019 cm−3. The surface photovoltage effect on photoemission is found to diminish rapidly with increasing doping concentration. The doping concentration effect on electron polarization is studied using high surface doped strained GaAs. The charge enhancement is explored using a direct, lateral surface charge sink in the form of a metallic grid overlaid on the surface.

Normalized rms emittances well below 10{sup {minus}6}m (with thermal emittance ignored) are now predicted for a 1-nC, 10-ps, flattop beam using an S-band rf gun. The expected thermal emittance of a Cu cathode excited at 263 nm is shown to be {approximately}0.3 x 10{sup {minus}6}m, which is potentially a serious limit on the overall minimum emittance. For GaAs, the photoelectron energy parallel to the emitting surface is now known as a function of the perpendicular energy. By adjusting the vacuum level for the semiconductor, it appears that the thermal emittance can be reduced (compared to Cu) by a factor of 2-even more if the cathode is cooled. The prospects for operating an rf gun with a III-V semiconductor photocathode such as GaAs are summarized.

The electronic and magnetic properties of thin (one to three monolayer) epitaxial Fe films grown on W(100) are studied using spin‐polarized angle‐resolved photoemission. Low‐energy electron‐diffraction studies of a number of samples confirm excellent epitaxy for monolayer films and suggest that two‐ and three‐layer films begin to form strain‐relieving dislocations. Submonolayer films manifest no in‐plane remanent spin polarization at T=115 K, but effectively quench W(100) surface states and surface resonances. Monolayer and thicker films yield spin‐polarized photoelectrons under remanent magnetic conditions. Analysis of spin‐polarized electron energy distribution curves (EDCs) corresponding to even‐symmetry initial states along the Γ‐M direction of the two‐dimensional Brillouin zone yields a flat majority‐spin band at 0.95 eV and an exchange split pair of bands (ΔEexΔEex= 2.15 ± 0.2 eV at Γ, 1.2±0.2 eV at M) which exhibit significant dispersion. No significant changes in the binding energies of spin‐u...

The successful operation of a GaAs‐based polarized electron source utilizing a DC high voltage gun for the SLC program at SLAC has raised the prospects for a similar source for next generation linear colliders (NGLC). A major challenge in meeting the NGLC requirements is to produce ≳1012 electrons per macrobunch from the cathode. The physics issues that are involved in limiting charge extraction from a GaAs‐type cathode and the prospects of realizing a NGLC polarized electron source based on an SLC‐type gun will be discussed.

Positron Annihilation induced Auger Electron Spectroscopy (PAES), Electron induced Auger Electron Spectroscopy (EAES), and LEED have been used to study the sandwich‐like structure resulting from vapor‐deposition of Rh on Ag(100). Schmitz et al. found using AES, ISS, and TDS that the Ag layer diffuses to the Rh surface upon annealing to form a structure which Rh is sandwiched between a Ag cap layer and the Ag substrate. In this work, the top layer selectivity of PAES was utilized to study the mechanism of formation of the sandwich structure. The shapes and peak positions of PAES spectra for as‐deposited (173K) and annealed (573K) Rh films confirm that Ag diffuses to Rh surface upon annealing. The temperature at which Ag starts to diffuse to the surface was directly monitored by PAES as the films deposited at 173K were heated to 573K.

Previous polarized electron experiments and recent ab initio calculations suggest that the surface layer magnetic moments of Gd(0001) are antiferromagnetically coupled to the bulk magnetic moments. Spin-polarized photoemission data are presented which show that the spin polarization of the magnetic surface state and the surface 4f states of Gd(0001) are coupled ferromagnetically to the bulk magnetic moment.

We present the results of a spin-polarized photoemission experiment that yields unambiguous evidence of layer-dependent core-level magnetic exchange splitting in ultrathin films. Analysis of the layer dependence of 3p-level spin polarization also establishes the existence of bulklike local magnetic order at 1.5 monolayer thickness

Cette etude de l'effet Kerr est realisee pour tester des recentes previsions concernant le ferromagnetisme 4d. Pour des temperatures allant jusqu'a 40 K, on n'observe aucun ordre ferromagnetique. On presente des explications possibles pour l'absence d'arrangement de spin a longue distance

Emission microscopes have been used to image devices used for cold cathodes, in situ. The use of emission microscopy in the evaluation of these materials and devices is described.

A simple and elegant technique is outlined in which a specially designed crucible may be used for noble metal evaporation in a pendant‐drop type electron beam evaporator with virtually no modifications.

Experimentally-determined exchange and spin-orbit induced scattering asymmetries A ex and A so for spin-polarized electrons diffracted from a magnetized Ni(111) surface are reported together with the results of fully relativistic dynamical LEED calculations. Comparison between theory and experiment shows generally good agreement and suggests that use of a model scattering potential incorporating an energy-dependent exchange correlation and localized absorption inside the muffin-tin spheres is to be preferred. Such comparisons also indicate that the surface is ferromagnetic with a top-layer magnetization comparable to that of the bulk.

The energy and angle dependence of the spin polarization of secondary electrons ejected from a Cu(100) surface by a polarized incident electron beam of energy 14, 50, and 100 eV has been investigated. At energies approaching the incident beam energy the secondary-electron polarization is comparable to that of the incident beam. The polarization decreases with decreasing secondary-electron energy, however, and at the lowest energies approaches zero. The data are interpreted in terms of the fractional contributions of rediffused primary and true secondary electrons to the total secondary-electron current.