Very low energy electron reflection at Cu(001) surfaces

Measurements of the coefficient of elastic reflection of very low energy electrons at Cu(001), Cu(001) (2 × 4)45°O and Cu(001)c(2 × 2)N surfaces are reported. The measurements refer to normally-incident electrons with kinetic energies E in the range 0.5–22 eV. The elastic reflection coefficient R_el was determined from separate observations of the total reflection coefficient and of the energy distribution of reflected electrons. For Cu(001) R_el is 0.55 at E = 0.5 eV and drops monotonically to 0.03 with increasing E, the maximum slope being at E = 3 eV. Theoretical calculations of R_el are reported. The reflection amplitude of the substrate crystal was parameterized using existing results of accurate band structure calculations, and the surface scattering matrix was evaluated for assumed surface scattering potentials. It is shown that to fit the observed R_el it is necessary to take account of both the image potential and the extension of the imaginary part of the crystal scattering potential into vacuum. From the fit, the range of the imaginary potential is 1.0 Å. For Cu(001) (2 × 4)45°O and Cu(001)c(2 × 2)N the values of R_el at E = 0.5 eV were 0.35 and 0.15, respectively. The effect of adsorption in reducing R_el is especially marked for E < 2 eV. Adsorption of either O or N results in an additional peak in R_el near E = 12 eV.