Electronic potential of a chemisorption interface

Chemisorption of atoms and molecules controls many interfacial phenomena such as charge transport and catalysis. The question of how the intrinsic properties of the interacting materials define the electronic structure of their interface remains one of the most important, yet intractable problems in surface physics. Through two-photon photoemission spectroscopy we determine a common binding energy of similar to 1.8-2.0 eV with respect to the vacuum for the unoccupied resonance of the us valence electron of alkali atoms (Li-Cs) chemisorbed at low coverage (less than 0.1 monolayer) on noble metal [Cu(111) and Ag(111)] surfaces. We present a theoretical model based on the semiempirical potentials of the adsorbates and the substrates, their principal mode of interaction through the Coulomb interaction, and the ab initio adsorption structures. Our analysis reveals that atomic size and ionization potential independent interfacial electronic structure is a consequence of the Coulomb interaction among the ns electron, the alkali-atom ionic core, and the induced image charge in the substrate. We expect the same interactions to define the effective electronic potentials for a broad range of molecule/metal interfaces.