  Alkali metals are known as efficient promoters in heterogeneous catalysis but
the nature of this phenomenon is as yet not understood. Our first principles
calculations reveal a huge increase and delocalization of the isoelectronic
reactivity index of Cu and Pd surfaces on alkali adsorption. We trace this
phenomenon to an unusual feature in the surface potential formed by the
adsorbate and propose it as a driving force for the "promotion effect". Our
results also provide clues to some of the unusual optical properties of quantum
wells formed when alkali adsorb on Cu surfaces.
