| The embedded cluster treatment of chemisorption has been extended in this thesis and applied to the studies of chemisorption of molecules on a 62 atom cluster model of Ni(111) surface.;Dissociated H and CH$sb{rm x}$ (x = 1,3) fragments are found to bind strongly to the Ni(111) surface at threefold and bridge sites. The bond strength is CH $>$ CH$sb2$ $>$ H $>$ CH$sb3$, with adsorption energies of 3.1, 2.9, 2.7 and 1.7 eV and Ni-C and Ni-H equilibrium distances of 2.04, 2.04, 1.87 and 2.25 A at threefold sites. However, the interaction between CH$sb4$ and the Ni(111) surface is repulsive for all distances and orientations considered.;CH is calculated to be perpendicular to the surface; CH$sb2$ is in a plane nearly perpendicular to the surface with a symmetric orientation of hydrogens; and for CH$sb3$ the hydrogens are in a plane parallel to the surface with a nearly tetrahedral geometry.;The calculations show that the reaction of CH(ads) + H(ads) = CH$sb2$(ads) on Ni(111) surface is 1.45 exothermic; CH$sb2$(ads) + H(ads) = CH$sb3$(ads) is 0.76 eV exothermic; and the dissociation of CH$sb4$ to CH$sb3$(ads) + H(ads) is 0.1 eV endothermic.;Molecular H$sb2$O adsorbed on Ni(111) surface is found to prefer an atop atom site, with an adsorption energy of 12 kcal/mol and a Ni-O equilibrium distance of 2.06 A. The equilibrium geometry of H$sb2$O is calculated to lie in a plane inclined by about 25$spcirc$ from the surface normal. The OH radical binds strongly to the Ni(111) at both threefold and bridge sites with adsorption energies of 87 kcal/mol and Ni-O bond lengths from 2.02 to 2.08 A. The dissociation of H$sb2$O into OH(ads) + H(ads) is 52 kcal/mol exothermic. A high energy barrier is found at the initial stage of dissociation.;Molecular NH$sb3$ chemisorbed on Ni(111) surface at an atop atom site and a fcc threefold site as well as a bridge site are of comparable stability, with adsorption energies of 19, 18 and 17 kcal/mol and Ni-N equilibrium distances of 2.12, 2.78 and 2.74 A, respectively. The lone pair 3a$sb1$ orbital of NH$sb3$ strongly interacts with the Ni 3d. These interactions and a strong electrostatic interaction due to the NH$sb3$ dipole characterize the bonding to the surface. |
