Valley-orbit interaction

Fig. 6.17. Enlargement of the spectral regions of the 2p i and 3p i lines of Mg+ showing their splitting (236 and 74 peV, respectively) by valley-orbit and central-cell interactions. The split components are indexed l and h in Table 6.18 [249]. Copyright 1994 by the American Physical Society...

Earlier we discussed the increased 2ir orbital population due to interaction with the d-atomic orbitals from the step atoms. This, taken together with the lowered surface dipole, enhances the probability for CO dissociation on steps. On nickel, steps have been shown to be more active than the planar surface, in contrast with the high-workfunction metals Pt and Ir. On the latter metals CO prefers adsorp tion on the edge atoms of the steps rather than on the valley atoms, where the CO 2x orbitals would get a favorable interaction with the step-edge d orbitals (see Fig.(3.17)). Whereas this interaction is favorable, the interaction with the It CO orbitals that are doubly occupied is repulsive with d-valence electron bands of high electron occupation. For a low-workfunction metal such as Ni this repulsion may be overcome by the 2t interaction. On high-wofkfunction metals with less backdona-tion the overall effect may be that CO molecules experience a repulsive interaction... 

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