VIBRATIONAL AND ANHARMONIC PROPERTIES OF THE SURFACE S-NI BONDS IN C(2X2)S NI(100) AND C(2X2)S/NI(110) FROM THE SEXAFS POINT-OF-VIEW/

Temperature- and angular-dependent S K-edge SEXAFS (surface-extended X -ray absorption fine structure) spectra of c(2 x 2)S/Ni(100) and c(2 x 2)S/Ni(110), where S locates at the hollow site of the Ni surfaces, w ere measured and analyzed in order to investigate the static surface s tructure including surface relaxation, anisotropic mean square relativ e displacements and surface local thermal expansion. By analyzing the S-Ni coordinations for the first and second layer Ni, we observed noti ceable thermal vibration and expansion of the surface S-Ni bonds. Sign ificant differences between the two systems are also found in the vert ical S-Ni bonds; in the case of c(2 x 2)S/Ni(100) the vibration betwee n S and the second layer Ni has a much lower frequency and larger anha rmonicity than that for the first layer Ni because there is no chemica l bond with the second layer, while in c(2 x 2)S/Ni(110) the vibration between S and the second layer Ni exhibits higher frequency and small er anharmonicity possibly due to a shorter bond length. Although it is commonly believed from diffraction studies that the vertical motion i s much larger and more anharmonic than the lateral one, the present st udy suggests that in terms of a relative motion or a local pair potent ial surface anisotropic vibration is strongly correlated to the bond d istance rather than to the bond direction.