ANISOTROPIC ANHARMONIC VIBRATIONS OF THE SURFACE S-NI AND CL-NI BONDSIN C(2X2)S AND CL NI(100) SYSTEMS STUDIED BY MOLECULAR-DYNAMICS/

The vibrational anisotropy and anharmonicity of the surface adsorbate- substrate bonds in c(2 x 2)S and Cl/Ni(100) systems were investigated by means of classical molecular dynamics calculations. The obtained re sults were compared to those experimentally given by surface extended X-ray absorption fine structure and high-resolution electron energy lo ss spectroscopies. Good agreement between calculated and experimental results indicates high reliability of the present method. The calculat ions allow us to distinguish the component of motions parallel to the surface from the normal one. In both systems, the mean-square displace ments of the adsorbates were found to be noticeably greater in the nor mal direction than in the lateral one. On the other hand, it was revea led that the mean-square relative displacement concerning the S-Ni pai r is enhanced in the vertical direction, while that of the Cl-Ni one s hows less directional dependence. Such a difference can simply be unde rstood from the local surface geometries.