AVS 53rd International Symposium
    Surface Science Wednesday Sessions
       Session SS2-WeM

Paper SS2-WeM5
The Origin of Work function Changes Induced by Adsorbates on Metal Surfaces: A Theoretical Analysis

Wednesday, November 15, 2006, 9:20 am, Room 2004

Session: Electronic and Vibrational Excitations and Dynamics
Presenter: P.S. Bagus, University of North Texas
Authors: P.S. Bagus, University of North Texas
C. Wöll, Ruhr-Universität Bochum, Germany
Correspondent: Click to Email
Adsorbates may induce large shifts in the work function, @phi@, of a metal surface. A traditional explanation of work function changes has been in terms of charge transfer, CT, between the substrate and the adsorbate with CT from the adsorbate to the substrate leading to a decrease in @phi@ and to an increase in the interface dipole, as for alkali atom adsorbates.@footnote 1@ For covalently bonded CO/Cu, the CT interpretation of @DELTA@@phi@>0 suggests a Cu-CO interaction dominated by @sigma@ donation.@footnote 2@ However, weakly bound physisorbed adsorbates@footnote 3@ on metal surfaces also induce large @DELTA@@phi@ and this cannot easily be explained by CT. For physisorbed species, the Pauli exclusion principle, which requires that an electronic wavefunction is anti-symmetric, has been shown@footnote 3,4@ to make a large contribution to the @DELTA@@phi@. On Cu(111), we have studied the @DELTA@@phi@ induced by several adsorbates, Xe, C@sub 6@H@sub 12@, C@sub 6@H@sub 6@, CO, Cs, and I, representative of quite different types of bonding to the surface. The origins of the changes in the interface dipole are interpreted based on a decomposition into individual contributions from: (1) Pauli exclusion; (2) Polarization of the metal substrate; (3) Covalent bonding; and (4) Charge transfer. More than one contribution to @DELTA@@phi@ may be large. The theoretical approach used is based on ab initio electronic structure theory applied to models of an adsorbate on a cluster of atoms representing the surface.@footnote 4@ We provide a unified understanding that makes it possible to correctly relate @DELTA@@phi@ to the chemistry of the adsorbate-substrate bond. @FootnoteText@ @footnote 1@J. P. Muscat and I. P. Batra, Phys. Rev. B, 34, 2889 (1986).@footnote 2@D. Heskett et al., Phys. Rev. B, 32, 6222 (1985).@footnote 3@P. S. Bagus, V. Staemmler, and C. Wöll, Phys. Rev. Lett., 89, 096104 (2002).@footnote 4@P. S. Bagus, K. Hermann, and C. Wöll, J. Chem. Phys. 123, 184109 (2005).