Polarized Electron Emission Research Articles

Ejected-electron-energy and angular dependence of fully differential ionization cross sections in medium-velocity proton collisions with He and H2

We have measured fully momentum-analyzed recoiling target ions and scattered projectiles, produced in ionization of He and ${\mathrm{H}}_{2}$ by 75 keV proton impact, in coincidence. The momentum of the ejected electrons was deduced from momentum conservation. From the data we extracted fully differential ionization cross sections as a function of the polar electron emission angle (for fixed electron energies) and as a function of the electron energy (for fixed electron emission angles). Comparison between experiment and various distorted wave calculations confirms that under kinematic conditions where the post-collision interaction plays an important role, the few-body dynamics underlying the ionization process are still poorly understood.

Electron capture to the continuum manifestation in fully differential cross sections for ion impact single ionization

We present theoretical calculations of single ionization of He atoms by protons and multiply charged ions. The kinematical conditions are deliberately chosen in such a way that the ejected electron velocity matches the projectile impact velocity. The computed fully differential cross sections (FDCS) in the scattering plane using the continuum-distorted wave-eikonal initial state show a distinct peaked structure for a polar electron emission angle θk = 0°. This element is absent when a first order theory is employed. Consequently, we can argue that this peak is a clear manifestation of a three-body effect, not observed before in FDCS. We discuss a possible interpretation of this new feature.

Circular Dichroism in Photoionization ofH2

Circular dichroism is a consequence of chirality. However, nonchiral molecules can also exhibit it when the measurement itself introduces chirality, e.g., when measuring molecular-frame photoelectron angular distributions. The few such experiments performed on homonuclear diatomic molecules show that, as expected, circular dichroism vanishes when the molecular-frame photoelectron angular distributions are integrated over the polar electron emission angle. Here we show that this is not the case in resonant dissociative ionization of H2 for photons of 30-35 eV, which is the consequence of the delayed ionization from molecular doubly excited states into ionic states of different inversion symmetry.

Optimized photocathode for spin-polarized electron sources

Photocathode for highly polarized electron emission has been developed, fabricated, and studied. The photocathode is based on short-period strained AlInGaAs∕AlGaAs superlattice grown by molecular beam epitaxy. Deformation of AlInGaAs quantum well results in 87meV energy splitting between heavy hole and light hole minibands. Electron emission from the developed photocathode demonstrates maximal polarization of 92% with quantum efficiency of 0.85% at room temperature.

Strain-compensated AlInGaAs–GaAsP superlattices for highly polarized electron emission

Spin-polarized electron emission from superlattice photocathodes developed with strain compensation is investigated. An opposite strain in the quantum well and barrier layers is accomplished using an InAlGaAs∕GaAsP superlattice structure. The measured values of maximum polarization and quantum yield for the structure with a 0.18μm thick working layer are excellent results for a strained superlattice photocathode structure, demonstrating the high potential of strain compensation for future photocathode applications. An analysis of the photoemission spectra is used to estimate the parameters responsible for the polarization losses.

Systematic study of polarized electron emission from strained GaAs∕GaAsP superlattice photocathodes

Spin-polarized electron photoemission has been studied for GaAs∕GaAs1−xPx strained superlattice cathodes grown by gas-source molecular beam epitaxy. The superlattice structural parameters are systematically varied to optimize the photoemission characteristics. The heavy-hole and light-hole transitions are reproducibly observed in quantum efficiency spectra, enabling direct measurement of the band energies and the energy splitting. Electron-spin polarization as high as 86% with over 1% quantum efficiency has been observed.

Optical spin orientation in strained superlattices

Optical orientation in the strained semiconductor superlattices (SL) is investigated theoretically. The dependence of the features in spin-polarization spectra on the structure parameters is clarified. The value of polarization in the first polarization maximum in the SL structures is shown to grow with the splitting between the hh and lh states of the valence band, the joint strain and confinement effects on the hh1-lh1 splitting being strongly influenced by the tunneling in the barriers. In strained structures with high barriers for the holes initial polarization can exceed 95%. Calculated polarization spectra are close to the experimental spectra of polarized electron emission.

Spin polarization spectra of strained superlattices

Optical spin orientation in the strained short-period semiconductor superlattices is investigated theoretically. Strong features in the spin polarization dependence on the electron energy are identified with the Van-Hove singularities in absorption spectra. The maximum polarization values found to be in the range of 70–94% are more sensitive to the absorption edge smearing than in the case of strained layer structures. Calculated polarization spectra are close to the experimental spectra of polarized electron emission.

Spin-polarized electron transport and emission from strained superlattices

Polarized electron emission from a series of new strained short-period AlInGaAs/AlGaAs superlattices (SLs) is investigated. The In layer content was chosen to give minimal conduction band offset with large strain splitting of the V-band. Simultaneous changing of Al content in both SL layers provides variation of the structure band gap. We demonstrate that tuning of the SL to the excitation energy can be achieved without loss of the electron polarization. The polarization of up to 84% was measured at room temperature.

Enhancement of binary encounter electron emission in collisions of highly charged ions with helium gas targets

The energy and angular distribution of δ-electrons produced in collisions of swift heavy ions with helium gas targets are presented. Double differential electron emission cross sections d2σ/dEedΩe for different projectiles have been measured as a function of the electron emission energy Ee and the polar electron emission angle ϑe. For systematics the cross sections were investigated for different projectiles from neon up to bismuth at ionic charge states between q=7+ and 24+. The specific projectile energy was kept constant within 10%, i.e. 5.4⩽EP⩽6.0 MeV/u for the different projectiles. The investigations are focussed on the emission cross sections of the Binary Encounter (BE) electrons. The BE electron emission cross section depends strongly on the ionic q as well as on the nuclear charge state ZP. At small emission angles ϑe the electrons are most enhanced but converging towards the q2-scaling at larger angles. An appropriate parameter was found to be the ratio of the ionic charge and the nuclear charge q/ZP. The observation of forward peaked, enhanced electron emission is of great importance for the understanding of radiation damages in dense targets caused by heavy ion collisions.

Electronic band structure of zinc blende

The electronic bulk and surface band structure of cubic zinc sulphide (``zinc blende'') has been studied by angle-resolved photoelectron spectroscopy using synchrotron radiation. The $s\ensuremath{-}p$ derived bands along the $\ensuremath{\Gamma}\ensuremath{-}K\ensuremath{-}X$ high-symmetry direction have been determined, and the region of the Zn $3d$ line has been examined. Spectra at fixed photon energy and variation of polar electron emission angle were used to determine the dispersion of the surface states along the $\overline{\ensuremath{\Gamma}}\ensuremath{-}\overline{X}$ within the surface Brillouin zone. The experimental data for bulk and surface bands are compared with results of a recent density-functional calculation, which includes the interaction between the $s\ensuremath{-}p$ and the cation d-derived states.

Binary encounter electron emission in collisions of highly charged ions with helium gas targets

The energy and angular distribution of δ-electrons produced in collisions of bare argon projectiles with helium gas targets are reported. Double differential electron emission cross sections, d 2 σ dE e dΩ e , have been measured as a function of the electron emission energy, E e, and the polar electron emission angle, V e. The major interest was directed upon the angular dependence of the integrated single differential emission cross sections of the binary encounter (BE) electrons, dσ(Θ e ) dΩ e . The cross sections are compared with electron argon scattering data (calculated and tabulated in Phys. Rev. A 15 (1977) 2173 [1]) as well as experimental data derived from light ion impact on helium (ionic charge q≤9+). The theoretical results are calculated with the impulse approximation (IA) model developed by Lee et al. (Phys. Rev. A 41 (1990) 4816 [2]). Even for higher ionic charge states the measured BE electron emission cross sections are well described by the IA model and are found to scale quadratically with the ionic charge state, q 2, and the projectile energy, E p 2.

Surface hysteresis curves of Fe(110) and Fe(100) crystals in ultrahigh vacuum — evidence of adsorbate influences

We have investigated hysteresis curves of silicon-stabilized Fe(100) and Fe(110) crystals using Polarized Secondary Electron Emission (PSEE) for surface magnetization processes and Magneto-Optical Kerr Effect (MOKE) for bulk hysteresis curves. In particular, the influence of adsorbates (oxygen and sulphur) and surface roughness on the magnetization reversal has been studied. For surface hysteresis curves we found adsorbate-induced reductions of coercive force of up to 50%; the bulk hysteresis curves showed significantly smaller deviations. Similar changes were caused by surface disorder on an atomic scale induced by ion bombardment. The results agree qualitatively with a previous experiment on Ni(110).

Angular‐resolved X‐ray photoelectron spectra and electronic structure of SnS2

AbstractValence band spectra of SnS2 are measured in dependence on the polar electron emission angle. Moreover, the valence band spectra of Sp‐like electrons are calculated on the basis of two different densities of states. The possibilities but also the limits of the ARXPS with reference to a detailed comparison of measurements and calculations are shown.

The adsorption of thiophene on a Cu(111) surface

The interaction of thiophene with a Cu(111) surface, at room temperature, is investigated by angle-resolved photoemission. Adsorbate induced features appear only after relatively large exposures, ca 10−2 torr s and appear to correspond to associatively adsorbed thiophene. From the energies of the adsorbate induced features and their dependence on photon incidence angle and polar electron emission angle, it is suggested that thiophene adsorbs on this face of copper with the aromatic ring parallel to the metal surface.