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, were measured and analyzed in order to investigate the static surface structure including surface relaxation, anisotropic mean square relative displacements and surface local thermal expansion. By analyzing the S-Ni coordinations for the first and second layer Ni, we observed noticeable thermal vibration and expansion of the surface S-Ni bonds. Significant differences between the two systems are also found in the vertical S-Ni bonds; in the case of c(2 x 2)S/Ni(100) the vibration between S and the second layer Ni has a much lower frequency and larger anharmonicity than that for the first layer Ni because there is no chemical 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 smaller anharmonicity possibly due to a shorter bond length. Although it is commonly believed from diffraction studies that the vertical motion is much larger and more anharmonic than the lateral one, the present study suggests that in terms of a relative motion or a local pair potential surface anisotropic vibration is strongly correlated to the bond distance rather than to the bond direction.