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 $c(2\mathrm{}\times 2)$ 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 Å, 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. The main effect of the Cs overlayer on these states is their energetic stabilization; this effect is most pronounced for the contamination-sensitive ${\overline{\Gamma }}_1$ surface state just below the Fermi energy, which is lowered in energy by 1 eV due to hybridization with Cs $s$ states.