All-electron local-density theory of alkali-metal bonding on transition-metal surfaces: Cs on W(001)

A theoretical study of the nature and the mechanism of the bonding of an alkali metal (Cs) on a transition-metal surface (W(001)) in the high-coverage limit is presented in order to understand and explain the lowering of the work function and to elucidate the role of W surface states and surface resonance states in the adsorption process. The analysis is based on all-electron local-density--functional results obtained with our self-consistent full-potential linearized augmented-plane-wave method for thin films for (1) a five-layer slab of W, (2) an unsupported Cs monolayer, and (3) Cs in a c(2 x 2) structure on both sides of the five-layer W slab for three different Cs-W separations. We find that Cs forms a polarized-metallic rather than ionic overlayer: The Cs valence electrons originating from the atomic 6s states are polarized toward the W surface leading to an increase of electronic charge in the Cs/W interface region and a depletion of electronic charge on the vacuum side of the overlayer. In addition, the semicore Cs 5p electrons are markedly counterpolarized. The net result of these multiple surface dipoles is a lowering of the work function upon cesiation from 4.77 eV (clean five-layer W slab) to 2.77, 2.55, and 2.28 eV, corresponding to heights of the Cs atoms above the W surface of 2.60, 2.75, and 2.90 A, respectively. The Cs-induced changes in the charge density are essentially localized outside the surface W atoms. The W d surface states and surface resonance states which are so characteristic of the W(001) surface are found to persist on the cesiated W(001) surface.