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

We report here the first measurements of low-energy ($40\le E\le 280$ eV) positron-diffraction (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, ${\lambda }_{\mathrm{ee}}$, vary with incident beam energy $E$ approximately as ${\lambda }_{\mathrm{ee}}\sim E^{\frac{1}{2}}$ over the energy range considered, $40\le E\le 280$ 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.