Interpretation of diffuse He-atom scattering from steps

With the recent improvements in dynamic range of He-surface scattering experiments, the measurement of diffuse scattered intensity from surface defects"step edges or point defects"has become a frequently executable experiment. There remain, however, certain features in the experimental data that, to date, have not been fully explained. A calculational method applicable to the scattering from step edges is developed here, firstly to calculate the basic oscillatory form of the intensities, and then to look at the previously unexplained finer structure. In particular, the paper attempts to reproduce some experimental data; that is, helium-atom scattering from a randomly stepped Pt(111) crystal, in a fixed 90°-angle geometry [see A. M. Lahee, J. R. Manson, J. P. Toennies, and Ch. Wöll, Phys. Rev. Lett. 57, 471 (1986)]. Even under a simple, hard-wall, eikonal approximation some of the previously unexplained features can be reproduced by the inclusion of a natural periodicity corrugation in the neighborhood of a step. This corrugation, with the periodicity of the lattice parameter, is allowed to decay away from the step. It is this decay length that is found to determine the characteristic width of the fine structure. The diffuse diffraction from a randomly stepped Pt(111), incidentally, now exhibits a certain degree of threefold symmetry. The enhanced corrugation amplitude in the neighborhood of a step is believed to be, of order at least, six times that observed on an unstepped Pt(111) surface. However, this enhancement factor is certainly very surface-orientation, and/or material, dependent.