Implications of Fractional Occupation of Adsorbate Resonances on van der Waals Interactions at Surfaces

Abstract

Partial occupation n a of a resonant adsorbate valence state due to chemisorption leads to n a -dependent phase space for adsorbate dipole electronic transitions. This fundamentally influences the dynamic polarizability and related physics of an adsorption complex. We outline a unified interpretation of such different observations as the induced dipole moment of adsorbed rare-gas atoms, the cross section of He scattering from adsorbed CO, and the change in effective deexcitation energy of metastable atoms in Penning spectroscopy of adsorbates, based on the n a dependence of van der Waals forces at surfaces.PACS numbers: 73.20.HbThe concept of adsorbate-induced resonances has greatly augmented the understanding of the electronic structure of chemisorption systems. 1,2 The electronic occupation of such an adsorbate resonance depends on its position with respect to the substrate Fermi level. In the case when the resonance intersects with the Fermi level, then, in general, fractional or noninteger occupation n a of the localized resonant state | a> may be expected. Its occupied and unoccupied states can then be considered as a filled and empty part of a localized band, respectively. Such static fractional electronic occupation is often used as a criterion for the estimate of the ionic (heteropolar) versus neutral (homopolar) character of the chemisorption bond considered. Large charge transfer from (to) a localized adsorbate resonance signifies cationic (anionic) chemisorption, both being the case of ionic chemical bonding. On the other hand, small charge transfer between the adsorbate resonance and the substrate valence band of states is a characteristic of covalent or homopolar chemisorption. 1,2 In addition to this chemical aspect of the problem, the existence of a partially occupied resonance will also affect the dynamic adsorbate electronic properties, namely, all the real and virtual electronic transitions within the adsorbate and the entire adsorption complex. This, in turn, will greatly influence the physical properties of the adsorbate, such as the van der Waals (vdW) forces acting between the adsorbate and the substrate, 3,4 between the adsorbed particles themselves, and between adsorbate and gas-phase particles, 5 " 8 as well as all the electronic relaxation processes within the adsorbate. 9 Both the vdW forces and the relaxation properties play an important role in surface spectroscopies 5 " 12 involving particle collisions 5 " 8,10 ' 11 or the interaction of adsorbates with radiation. 8 " 12 In this Letter we describe a unified view of the physical origin of these various, apparently quite different surface phenomena and processes, which is based on a correlation between the fractional occupation of the resonance and the thereby modified dynamic polarizability of adsorbates.Electronic transitions in adsorption systems in their ground states are subject to usual restrictions emerging from the Pauli exclusion principle. The initial states (from which the transitions take place) have to be occupied and the ...