Anti-bonding states

The anti-bonding state lies about the same amount above the level of the separated atoms, or +4.7 eV. Thus the energies of the valence electrons on the H-atoms split into increased and deceased values in the H2 molecule. This is shown schematically in Figure 3.7 and introduces the next topic, the variation of the energies with the separation distance. 

The concept of correlation enters via the inclusion of asymptotic base states. Translated into the particle model, one type of asymptotic state is given by a doubly occupancy of an anti-bonding state the fragments show a repulsive... 

Fermi-level of the metal, and when the metal has d-states around the Fermi level, as in the transition metals there will be a strong covalent interaction between the anti-bonding states and the metal d-states. 

To understand these variations it is instructive to look at the variations in the electronic structure of adsorbed on the different metals, see Figure 4.2. As shown in Chapter 2, bonding of an atom like to a transition metal results in the formation of bonding and anti-bonding states below and above the metal d bands. It can be seen how the anti-bonding states for on Ru are less filled than on Pd and Ag,... 

Figure 4.2. The density of states projected onto the d states of the surface atoms for the surfaces considered in Figure 4.1 (grey). Also shown (black) is the oxygen 2px projected density of states for adsorbed on the same surfaces. The formation of bonding and anti-bonding states below and above the metal d states is clearly seen. Adapted from Ref. [4].

When an adsorbate is chemisorbed on the Pt surface, the overlap between the orbitals of Pt and the adsorbate leads to the presence of empty anti-bonding states (AS). The AS of... 

This paper will show that an Pt-H anti-bonding state shape resonance is present for both strongly and weakly adsorbed hydrogen. Both types of hydrogen are thus atomically... 

Figure 5 The shaperesonance of the Pt-H anti-bonding state for the weakly (solid line) and strongly (broken-dotted line) adsorbed H in (A) model 1 (simultaneous adsorption) and (B) model 2 (rearrangement of Pt) with the corresponding fits (dotted line).

The lower energy of the anti-bonding state of the weakly adsorbed H compared to that of the strongly adsorbed H (-0.3 eV) shows that the weakly adsorbed H indeed has a weaker Pt-H bond than the strongly adsorbed H, since a lower energy of the anti-bonding state is the result of a weaker Pt-H bond (Figure 8). 

Previous work by our and other groups has shown that the Pt L3 XANES is sensitive to the adsorption of H. This sensitivity was interpreted by our group as being caused by the creation of a chemical bond between Pt and H30. The Pt-H anti-bonding state (AS) above the Fermi level was thought to produce a shaperesonance arising from the interference between the resonantly and nonresonantly scattered photoelectron waves. 

D C. Koningsberger, M.K. Oudenhuijzen, J.H. Bitter and D.E. Ramaker, Study of geometrical and electronic effects induced by hydrogen chemisorption on supported Pt particles. Analysis of Pt-H EXAFS and Pt-H anti-bonding state shape resonances , Top. Catal., 10 (2000), 167-177. 

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