Hills or hollows? — An ambiguity in the determination of surface corrugation

The diffraction intensities from a corrugated hard wall are shown to be the same for a given corrugation function ζ(x, y) and ?ζ(?x, ?y), provided that coupling to evanescent waves is negligible. To illustrate the consequences of this ambiguity for both surface structure and surface bonding, the corrugation of the adsorbate system Ni(110) + H(1 × 2) determined from Hediffraction measurements is considered. Possible conditions under which the ambiguity could be resolved are discussed.