HOLOGRAPHY OF CLEAN AND SULFUR-COVERED NI (111) USING MULTIPLE WAVE-NUMBER PHOTOELECTRON DIFFRACTION PATTERNS/

Angular distributions of Ni 3p(3/2) photoelectrons for clean Ni(111) a nd of S 2p(3/2) photoelectrons for the p(2 X 2)S and (5 root 3 X 2)S s tructures on Ni(111) were measured at seven different kinetic energies ranging from 239 to 417 eV for Ni 3p(3/2) and from 130 to 322.5 eV fo r S 2p(3/2). From the photoelectron diffraction patterns, holographic reconstructions were performed using (1) the single wave number recons truction method and (2) the multiple wave number phased sum method. Fo r the clean surface, neighboring atoms in the plane of the emitter and some atoms in the second and third plane above the emitter can be ide ntified; the holographic reconstruction in the first plane above the e mitter is however dominated by intense artifacts. For Ni(111)-p(2 X 2) S, the three-fold hollow fee adsorption site of sulphur can be unequiv ocally identified with a distance to the first Ni layer of about 1.6 A ngstrom. Holographic reconstructions for Ni(111)-(5 root 3 x 2)S allow to exclude the three-fold hollow position of sulphur and disagree wit h a proposed mixing of different non-symmetric adsorption sites. The d istance of the sulphur atoms from the top Ni layer is determined to 1. 1-1.2 A. The reconstructions are consistent with a quasi-(100) reconst ruction of the Ni(111) crystal surface. For all systems the potential of using only an azimuthal sector instead of the full hemispherical ho lograms for the reconstruction is discussed.