Self-Consistent Calculation of Molecular Chemisorption on Metals

As a first step towards an understanding of the reactivity of metals and their ability to break chemical bonds we have performed self-consistent calculations on a model with H₂ standing upright on a jellium surface. Data for the adsorbate-induced electron structure and potential-energy curves with substrate parameters characteristic for Al, Mg and Na are presented. We extract concepts and trends that should be useful for other molecule-metal systems as well, e.g.,

(i) the downshift of the antibonding affinity level, in a symmetry-affected correlation with the effective electron potential of the clean surface,

(ii) the filling of the corresponding molecular resonance, localized primarily on the outer H atom,

(iii) the thereby weakened intramolecular bond, and

(iv) the natural separation of the molecular chemisorption energy into intra- and extramolecular terms, whose relative size determines the molecular adsorption to be dissociative, activated etc. The model calculations provide illustrations of different classes of molecular chemisorption systems and support the Lennard-Jones description of molecular adsorption, provided that the molecular potential-energy curve is due to chemical rather than van der Waals' forces.