SURFACE-STRUCTURES OF S/NI(111) IN (ROOT-3X-ROOT-3)R30-DEGREES AND (5-ROOT-3X2) PHASES STUDIED BY SURFACE EXTENDED X-RAY-ABSORPTION FINE-STRUCTURE SPECTROSCOPY

The local surface structures of S/Ni(111) in the (root 3 x root 3)R30 degrees and (5 root 3 x 2) phases have been investigated by means of polarization-dependent sulfur K-edge surface EXAFS. In the (root 3 x root 3)R30 degrees phase, sulfur adatoms are found to occupy threefold hollow sites with a S-Ni distance of 2.13 Angstrom and an inclination angle omega of the S-Ni bonds at 44 degrees from the surface plane. In contrast, in the (5 root 3 X 2) phase, it is determined that the S-Ni bond is longer, 2.18 A, more inclined, omega=31 degrees, and that the coordination number is not 3 but 4. These results strongly support a picture involving reconstruction of the top nickel layer to form a rectangular structure. Consideration of several models proposed for the (5 root X 2) phase leads to one which is compatible with both the present results and results recently reported using STM.