In this Letter we report the first observation of low-energy positron (e*) diffraction (LEPD) from a solid surface, Cu(lll). 1 LEPD offers the possibility of becoming a quantitative tool for the study of surfaces to complement the wellestablished technique of low-energy electron diffraction (LEED)c The change in the sign of the The first observation of low-energy positron diffraction from a solid surface is reported. Slow (20-400-eV) monochromatic positron beams were focused onto a Cu(lll) surface and their elastically scattered distributions detected with a channel electron multiplier. Measurements of the scattered intensity versus angle as a function of incident energy show peaks at the predicted (01) and (02) diffraction angles. Profiles of intensity versus energy at fixed angles exhibit maxima corresponding to the primary Bragg peaks.
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.
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