C. Winkler

Direct and core-resonant double photoemission from Cu(001)

We have measured the correlated electron pair emission from a Cu(001) surface by both direct and core-resonant channels upon excitation with linearly polarized photons of energy far above the 3p threshold. As expected for a single-step process mediated by electron correlation in the initial and final states, the two electrons emitted by the direct channel continuously share the sum of the energy available to them. The core-resonant channel is often considered in terms of successive and independent steps of photoexcitation and Auger decay. However, electron pairs emitted by the core-resonant channel also share their energy continuously to jointly conserve the energy of the complete process. By detecting the electron pairs in parallel over a wide range of energy, evidence of the core-resonant double photoemission proceeding by a coherent single-step process is most strikingly manifested by a continuum of correlated electron pairs with a sum energy characteristic of the process but for which the individual electrons have arbitrary energies and cannot meaningfully be distinguished as a photoelectron or Auger electron.

Phase-locked MHz pulse selector for x-ray sources

Picosecond x-ray pulses are extracted with a phase-locked x-ray pulse selector at 1.25 MHz repetition rate from the pulse trains of the accelerator-driven multiuser x-ray source BESSY II preserving the peak brilliance at high pulse purity. The system consists of a specially designed in-vacuum chopper wheel rotating with ≈1  kHz angular frequency. The wheel is driven in an ultrahigh vacuum and is levitated on magnetic bearings being capable of withstanding high centrifugal forces. Pulses are picked by 1252 high-precision slits of 70 μm width on the outer rim of the wheel corresponding to a temporal opening window of the chopper of 70 ns. We demonstrate how the electronic phase stabilization of ±2  ns together with an arrival time jitter of the individual slits of the same order of magnitude allows us to pick short single bunch x-ray pulses out of a 200 ns ion clearing gap in a multibunch pulse train as emitted from a synchrotron facility at 1.25 MHz repetition rate with a pulse purity below the shot noise detection limit. The approach is applicable to any high-repetition pulsed radiation source, in particular in the x-ray spectral range up to 10 keV. The opening window in a real x-ray beamline, its stability, as well as the limits of mechanical pulse picking techniques in the MHz range are discussed.

Correlated positron–electron emission from surfaces

We studied positron–electron pair emission from a LiF(100) surface following excitation by a positron beam with a kinetic energy of 85 eV. We show for the first time that emission of time-correlated positron–electron pairs occurs.

Reflection-time-of-flight spectrometer for two-electron "e,2e… coincidence spectroscopy on surfaces

In this article, a novel time-of-flight spectrometer for two-electron-emission (e,2egamma,2e) correlation spectroscopy from surfaces at low electron energies is presented. The spectrometer consists of electron optics that collect emitted electrons over a solid angle of approximately 1 sr and focus them onto a multichannel plate using a reflection technique. The flight time of an electron with kinetic energy of Ekin approximately 25 eV is around 100 ns. The corresponding time- and energy resolution are typically approximately 1 ns and approximately 0.65 eV, respectively. The first (e,2e) data obtained with the present setup from a LiF film are presented.

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.

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 coincidence time-of-flight spectrometer. The aim of this study was to compare the pair emission upon electron impact and upon photon absorption. The energy distributions are markedly different for these two experiments. From this we conclude that the photon-stimulated pair emission carries a significant contribution from a double photoemission process, while the process of first creating a photoelectron, which in a subsequent collision leads to pair emission, is of less importance.

Correlated positron-electron emission from LiF (100)

Using two hemispherical energy analyzers with transfer lenses we have observed positron-electron pair emission from a surface upon low energy positron impact. An 85 eV positron beam from the NEPOMUC positron source was incident upon a LiF(100) surface and emitted electrons were detected in coincidence with the positrons. Energy analysis of the positron-electron pair confirms that electrons are emitted from the valence band upon positron impact. Electron-electron pairs emitted upon electron and positron impact are also discussed. The information obtainable from these experiments has the potential to provide important insights into electron-electron correlation in solids.

Two-particle emission from LiF(1 0 0) upon photon, electron and positron excitation

Electron pairs simultaneously emitted from a solid surface upon excitation with photons (double photoemission or (γ, 2e)) or electrons (e,2e) provide information about the electron correlation in the solid which is mediated by exchange and Coulomb interactions. Without the Pauli restriction, the correlation of a positron with many electrons in a solid is dominated by Coulomb interactions. We have therefore undertaken the first comparative study of electron-electron and positron-electron pair emission upon impact with low energy photons, electrons and positrons. The target surface chosen for this study was LiF(1 0 0), a wide band gap dielectric that allows for a simple kinematical interpretation of the energy distribution of emitted pairs. We present the first results and discuss insights into the mechanisms of pair emission upon positron, electron and photon impact.