Abstract

This chapter demonstrates how X-ray and electron spectroscopies can be used together with Density Functional Theory (DFT) calculations to obtain an understanding of the local electronic structure and chemical bonding of adsorbates on metal surfaces. It attempts to use molecular orbital theory and relate the chemical bond formation to perturbations of the orbital structure of the free molecule. The electronic structure of the surface chemical bond is discussed in depth in this study for a number of example systems taken from the five categories of bonding types: atomic radical, diatomics with unsaturated π-systems (Blyholder model), unsaturated hydrocarbons (Dewar-Chatt-Duncanson model), lone pair interactions, and saturated hydrocarbons (physisorption). It illustrates examples of the simplest case where the fragment is an atom, such as N or O. The unpaired radical interacts with the metal d-states in the metal to form bonding and antibonding states. In the case where the interaction with the d-states leads to weak or repulsive interaction, there could still be some bonding involving sp-electrons. Ionic contributions can also be important, in particular for cases where the adsorbate has a high- or low-electronegativity with respect to the metal, as is the case for oxygen, halogen, and alkali atom adsorption. In molecular adsorbate systems, with no unpaired electrons, but with an unsaturated π-electron system it can be possible to obtain a bond-prepared radical state by mixing π and π∗ orbitals.

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