ANHARMONIC ANISOTROPIC VIBRATIONS OF THE S-NI BONDS IN C(2X2)S OVERLAYERS ON NI(100) AND NI(110) SURFACES

Temperature- and angular-dependent S K-edge SEXAFS spectra of c(2 x 2) S/Ni(100) and c(2 x 2)S/Ni(110) were measured and analyzed in order to investigate dynamical properties of the surface S-Ni bonds. In the ca se of c(2 x 2)S/Ni(100) the vibration between S and the first layer Ni has much higher frequency and smaller anharmonicity than that for the second layer Ni, while in c(2 x 2)S/Ni(110) the vibration between S a nd the first layer Ni exhibits lower frequency and larger anharmonicit y. Although it is commonly believed from diffraction studies that the vertical motion is much larger and more anharmonic than the lateral on e, the present study suggests that in terms of a relative motion or a local pair potential surface anisotropic vibration is strongly correla ted to the bond distance rather than the bond direction.