Low-energy positron and electron diffraction from Cu(100) and Cu(111)

Abstract

We report here the first measurements of low-energy (40(E(280 eV) positrondiffraction (LEPD) intensity profiles from a Cu(100) surface as well as a normalized version of previously reported LEPD intensity measurements from a Cu(111) surface. Also reported are low-energy electron-diffraction (LEED) intensity measurements for Cu(100) made at the same angles of incidence as used in taking the positron data. Calculations based on unrelaxed surface atomic geometries for Cu(100) and Cu(111) yield quantitative descriptions of these data. The model is also shown to predict correctly the results already reported in the literature for Cu(100) and Cu(111) as well as the new Cu(100) measurements reported here.The measurements suggest that the inelastic-collision mean free paths for positrons, A. ", vary with incident beam energy E approximately as A, "-E' over the energy range considered, 40 &E (280 eV, whereas those for electrons vary less rapidly with increasing beam energy. Finally, with the use of the model calculations as a guide, the consequences of the wide angular spread of positron beams relative to electron beams are examined and the potential of positron diffraction as a technique for surface-structure determination is estimated.

Observation of differences between low-energy electron- and positron-diffraction structural determinations of the cleavage faces of CdSe

“…7 The interaction of the incident electron or positron with the solid is described by complex phase shifts describing its scattering from the individual vibrating Cd or Se species, whereas its propagation in between these species is simulated by a uniform complex potential. A detailed account of the mathematical model of this interaction is given by Weiss et al 5 The rigid-lattice phase shifts are obtained from a muffin-tin potential constructed from self-consistent solutions to the Dirac equation as described by Ford, Duke, and Paton 13 for electrons and adapted for positrons as indicated by Weiss et al 5 An important aspect of this adaptation is the absence of an exchange interaction for positrons, leading to positronsolid phase shifts which are electrostatic in origin. The effects of positron-core-electron correlation have also been demonstrated to have a negligible effect on diffracted positron intensities.…”

Positron and Positronium Momentum Spectroscopy in Solids: An Overview

“…4 Slow positrons are also elastically scattered, leading to low energy e+ diffraction (LEPD) experiments. 67 The experimental implementation of the "brightness enhancement" proposed by Mills s has resulted in a high brightness positron microbeam at Brandeis that is being used to develop a brightness-enhanced "positron reemission microscope" . 68 The extension of the ACAR technique to the study of surfaces had to wait for the development of ultra-high intensity slow e+ beams.…”

Energy Relations of Positron-Electron Pairs Emitted from Surfaces

“…This is clearly different from our experimental geometry where the particles have momentum components along the backward direction. These two scenarios can be reconciled if we recall the existence of LEPD [1][2][3]. It is the reflected beam which acts as the primary beam for the positronelectron scattering.…”

“…This does not apply to the case of distinguishable particles, e.g., a positron and an electron which have different charges. Despite the process of annihilation, a sizeable fraction of low primary positrons hitting a surface are elastically backscattered [1][2][3]. This effect is termed low energy positron diffraction (LEPD) and reveals the surface structure in a similar way to low energy electron diffraction (LEED) [4,5].…”