Low-energy electron diffraction intensities from the clean nickel (001) surface

Abstract

Experimental absolute intensity curves, I h,k , of the 00, 11 and 20 diffraction beams from a nickel (001) surface have been obtained at normal incidence for incident electron energies 0–175 eV. Results for the 00 beam are given for energies to 600 eV at 3° angle of incidence. A comparison with the corresponding copper results shows that the nickel intensities in general are higher, which at energies above 200 eV can be accounted for by the different Debye-Waller factors. At the lowest energies it seems necessary to assume that the atomic scattering strength is larger for nickel than for copper. The structure in the intensity curves from the two materials shows expected but not trivial close correspondence. Lattice spacings normal to the surface, determined from integer order peaks in I 00, were found to be 3.51 ± 0.02 Å and 3.62 ± 0.02 Å for nickel and copper respectively. These figures agree well with the accepted bulk values but they reflect the spacings in the surface region, as the electron penetration is small due to strong absorption. A small change in the spacing between the two outermost layers may not, however, be revealed, as the contribution from the surface layer to I 00 is greatly reduced by the excess vibrational amplitude of the surface atoms along the surface normal. Special attention is given to the nickel intensities in the energy region 0–175 eV where strong multiple scattering gives rise to a rich structure with intense noninteger order peaks and asymmetrical peaks. In spite of the strong interaction the scattering events associated with the peaks can be inferred from the free electron band structure with remarkable success. Thus are the noninteger order peaks 1 1 2 , 2 1 2 , 2 7 10 , 3 1 6 , 3 1 3 and 3 1 2 in I 00 identified. The deviations in energy the free electron values of the integer order peaks in I 00, I 11 and I 20 are found to be considerably larger than Pendry's calculated inner potential for nickel but show a similar energy dependence. The damping rate, due to inelastic electron-electron collisions, was inferred from the peak widths and used to estimate “effective” back-scattering matrix elements in a two-beam approximation. For the integer order peaks 2 and 3 the elements are 1 eV and 2 eV respectively, somewhat larger than expected from Pendry's calculation for the nickel (111) surface.