Electron Pair Emission Research Articles

Plasmon-assisted electron-electron collisions at metallic surfaces

We present a theoretical treatment for the ejection of a secondary electron from a clean metallic surface induced by the impact of a fast primary electron. Assuming a direct scattering between the incident, primary electron and the electron in a metal, we calculate the electron-pair energy distributions at the surfaces of Al and Be. Different models for the screening of the electron-electron interaction are examined, and the footprints of the surface- and the bulk-plasmon modes are determined and analyzed. The formulated theoretical approach is compared with the available experimental data on the electron-pair emission from Al.

Spin-dependent two-electron emission from ferromagnetic Fe(001)

We present a joint experimental and theoretical study of correlated electron pair emission from a ferromagnetic Fe(001) surface induced by spin-polarized low-energy electrons. Spin-dependent angular and energy distributions of the emitted pairs have been measured and calculated. They are analyzed with the aid of the spin-, momentum-, symmetry-, and layer-resolved valence electron density of states, which we obtained by an ab initio density-functional theory calculation. The observed spectra are found to arise almost completely from only three surface-parallel atomic layers. Momentum distributions for parallel spins of the emitted electrons exhibit an exchange-correlation hole, which is larger than the correlation hole in the antiparallel spin case. By comparing experimental antiparallel-spin pair spectra with their theoretical counterparts we determine an effective screening strength of the Coulomb interaction in the surface region.

Surface state and resonance effects in electron-pair emission from Cu(111)

We report on an electron-pair emission study from a Cu(111) surface excited by electron impact with different primary electron energies. We identify in the energy spectra, features that can be directly related to the underlying band structure. This, in turn, proves an important assumption in the theoretical description of the pair-emission process, namely that an effective single-particle picture for the electronic structure is an adequate description. With this observation, it is possible to identify the orbital character of the participating valence state. The relative pair-emission intensity from the surface state and 3$d$ states is observed to vary dramatically with the primary energy. We observe particular conditions where the contribution from the Shockley surface state is strong. We propose a simplified model to explain this observation.

Spin-Resolved Mapping of Spin Contribution to Exchange-Correlation Holes

By means of spin-polarized electron coincidence spectroscopy we explore the fundamental issue of spin-resolved contributions to the exchange-correlation hole in many-electron systems. We present a joint experimental and theoretical study of correlated electron pair emission from a ferromagnetic Fe(001) surface induced by spin-polarized low-energy electrons. We demonstrate that the contribution to the exchange-correlation hole due to exchange is more extended than the contribution due to the screened Coulomb interaction.

Electron pair emission from a W(001) surface: photon versus electron excitation

The electron pair emission from a W(001) surface was studied using a coincidencetime-of-flight spectrometer. The aim of this study was to compare the pair emission uponelectron impact and upon photon absorption. The energy distributions are markedlydifferent for these two experiments. From this we conclude that the photon-stimulated pairemission carries a significant contribution from a double photoemission process, while theprocess of first creating a photoelectron, which in a subsequent collision leads to pairemission, is of less importance.

Sensing the electron–electron correlation in solids via double photoemission

AbstractWe discuss the electron pair emission from surfaces upon the absorption of a single photon, also called double photoemission (DPE). This experiment is particular sensitive to the electron–electron interaction, because for independent electrons the DPE intensity is zero. We outline the experimental development of this technique over the past decade. Going beyond the mere detection of pairs we advanced the instrumentation. Now we are able to measure the kinetic energies and emission angles of a wide angular acceptance. We will show how the available energy is distributed among the electrons and how the angular distributions look like. The latter enabled us to make contact to an important concept of modern solid state theory, namely the exchange–correlation hole. We demonstrate that the exchange–correlation hole manifests itself in a depletion zone of the coincidence intensity around the fixed emission direction of one electron. The experiments were performed at the synchrotron facilities BESSY I and BESSY II. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Electron pair emission from a Cu(111) surface upon photon absorption

We studied the electron pair emission from a Cu111 surface upon photon absorption. We found that the energy sharing depends on the angle between the trajectories of the two emitted electrons. The angular distribution of the coincidence intensity displays a zone of reduced intensity, if the emission direction of one electron is fixed. We are able to observe the full extension and shape of this depletion zone. It has an angular extension of 1.2 rad, which is independent of the electron energy. This depletion zone is a manifestation of the exchange-correlation hole. The experimental results are discussed in connection with a detailed theoretical description.

Mapping out electron–electron interactions in angular space

Many-body effects in solids are related to the correlation among electrons. This mutual interaction between the electrons can be probed by electron pair emission spectroscopy. We have investigated the electron pair emission from a LiF(100) surface upon excitation with low kinetic energy electrons. Our angular distributions clearly show that the emission direction of one electron is surrounded by a reduced intensity of the other electron. This depletion zone of electronic intensity is a manifestation of the exchange and correlation hole. We show that we are able to observe the full extension and shape of the depletion zone. It has an angular extension of ≈1.2 rad and is independent of the electron energy. Additionally, we discovered that the angle between the trajectories of the electrons has a profound effect on the two-dimensional energy distribution.

Correlation Effects in Two Electron Photoemission

Many-body effects in solids are ultimately related to the correlation among electrons, which can be probed by double photoelectron emission. We have investigated the electron pair emission from a Cu(111) surface upon photon absorption. We are able to observe for the first time the full extension and shape of a depletion zone around the fixed emission direction of one electron. It has an angular extension of approximately 1.2 rad, which is independent of the electron energy.

Selection rules in (e, 2e) spectroscopy from ferromagnetic surfaces

Electron pair emission from ferromagnetic crystalline surfaces, which occurs due to thecollision of an incident low-energy electron with a valence electron, is shown to be governedby spatial and spin selection rules. Valence electron states with spatial parts antisymmetricwith respect to the reaction plane are not observable for any spin configuration. If there isa spatial symmetry plane normal to the reaction plane, spin-polarized primary electronsselectively probe equal and opposite spin valence electrons. These analytical findings arefurther elaborated for ferromagnetic cubic (110) surfaces and illustrated by numericalresults for Fe(110).

Theory of electron-pair emission from random alloys

A theory is developed for the treatment of correlated electron-pair emission from alloys with substitutional disorder following the impact of fast electrons. The influence of disorder on the emitted, hot-electron states is treated using the virtual-crystal approximation. Numerical results for several metallic binary alloys are presented and analyzed revealing the interplay between disorder effects and scattering dynamics. On the basis of this work conclusions are drawn on the potential of utilizing the electron-pair spectroscopy for the study of electronic collisions in random alloys.

Visualizing spin-dependent electronic collisions In ferromagnets

This work demonstrates experimentally and theoretically that the coincident two-electron emission from a ferromagnetic surface, upon the impact of a polarized electron, carries detailed information on the spin-dependent electronic collisions in ferromagnets. The analysis of the calculated and the measured two-electron spectra reveals the potential of the electron-pair emission technique for the study of (a) surface magnetism and (b) spin-dependent electron scattering dynamics in ferromagnets.

Probing the Spin Polarization in Ferromagnets

The emission of correlated electrons from an itinerant ferromagnet following the impact of a polarized electron beam is analyzed in terms of irreducible tensorial parameters that can be measured. Under favorable conditions, specified in this work, these parameters are related to the spin polarization in the ferromagnet. The formal results are illustrated by numerical studies of the polarized electron pair emission from a Fe(110) surface, and a novel technique for the investigation of magnetic properties of ferromagnets is suggested.

Pair correlation in two-electron emission from surfaces

An electron pair emitted from a crystalline surface system is represented by a relativistic two-electron scattering state, which contains the pair correlation mediated by a screened Coulomb interaction. This state is obtained from the solutions of two one-electron Dirac equations with potentials, which incorporate the electron–electron interaction as a dynamical screening of the usual effective one-electron potentials. Numerical applications to the electron pair emission due to low-energy electron impact on a He atom and on a clean W(001) surface demonstrate that pair correlation effects can be quite strong and significantly improve the agreement of calculated results with experimental data.

Geometrical and dynamical aspects of the correlated electron pair emission from ordered materials

The scattering of N-body correlated systems from a non-overlapping multi-center periodic potential is formulated in terms of scattering path operators. An expansion of the total T-matrix in terms of scattering path operators is derived. Special attention is devoted to the separation of the geometrical arrangement of the scatterers from the internal correlation of the scattered compound. The role of internal correlation is illustrated in the case of the scattering of correlated electron pairs from a perfect crystal potential and the electron-pair excitation from core levels into the vacuum upon a photon absorption.

Electron pair emission in nuclear pion capture

We report the first observation of the rare pion capture mode in beryllium consisting in the emission of an electron-positron pair. Rate, angular distribution, and energy-spectrum are in quantitative agreement with the ones expected assuming internal pair-creation from single radiative capture.

A Search for Emission of Electron Pairs from Nuclei

A Search for Emission of Electron Pairs from Nuclei

Field Theory of Nuclear Interaction

The problem of explaining the recent results on nuclear interaction by means of a field theory is studied. Fermi's theory of the electron-neutrino field is used as a model which is sufficient to account for the symmetries of the problem, although it fails to explain the order of magnitude of the forces. The equality of forces between like and unlike particles is exactly accounted for by introducing interaction terms involving the emission of electron pairs or neutrino pairs. The interaction law may be stated very simply with the aid of an isotopic spin variable for light as well as for heavy particles. The ratio of force constants obtainable from the theory of mass defects may be accounted for in detail by a suitable choice of the light particle field. However, it is difficult to explain any law involving more than one potential function $J(r)$.