Orbital interactions and chemical reactivity of metal particles and metal surfaces

A review is presented with 101 refs. on chem. bonding to metal surfaces and small metal particles demonstrating the power of symmetry concepts to predict changes in chem. bonding. Ab-initio calcns. of chemisorption to small particles, as well as semiempirical extended Hueckel calcns. applied to the study of the reactivity of metal slabs are reviewed. On small metal particles, classical notions of electron promotion and hybridization are found to apply. The surroundings of a metal atom (ligands in complexes, other metal atoms at surfaces), affect bonding and reactivity through the prehybridization they induce. A factor specific for large particles and surfaces is the required localization of electrons on the atoms involved in the metal surface bond. At the surface, the bond energy is found to relate to the grou8p orbital local d. of states at the Fermi level. The use of this concept is extensively discussed and illustrated for chemisorption of CO and dissocn. of NO on metal surfaces. A discussion is given of the current decompn. schemes of bond energies and related concepts (exchange (Pauli)-repulsion, polarization, charge transfer). The role of non-orthogonality of fragment orbitals and of kinetic and potential energy for Pauli repulsion and (orbital) polarization is analyzed. Numerous examples are discussed to demonstrate the impact of those concepts on chem. bonding theor

Theory of Thermal Desorption of Light Noble Gases from Metals: Aplication to Ne-Cu(111)

A quantum theory of thermal desorption of light noble gases, at low temperatures, in the framework of the distorted-wave Born approximation, is presented. Such a theory has been applied to the calculation of the angle-resolved thermal desorption flux, and time-of-flight spectrum for Ne-Cu(111) using a microscopic description of the phonons of Cu(111), and a realistic gas-solid potential. We show that desorption is driven by long-wavelength phonons, but the precise angular dependence results from the temperature-dependent interplay of statistical and dynamical factors. The angular distribution is narrower than cosine at 4 K, broader than cosine at 12 K, and distinctly non-cosine for temperatures greater than 20 K with the maximum away from the surface normal. He desorbing from graphite shows similar features