Adsorption of Cs on H-precovered W(110) surfaces

Deposition of Cs on clean W(110) at room temperature produces a single layer with a compressed hcp structure and gives the characteristic work-function curve of alkali-metal atoms on metals which goes through a minimum value (${\phi }_{\min }$), and a subsequent maximum (${\phi }_{\max }$) at saturation Cs coverage. During Cs deposition on hydrogenated W(110) the Cs atoms form an independent overlayer. The presence of a H underlayer weakens the bond and increases the ordering of the Cs atoms. Cesium forms a p(2×2) structure and a compressed hcp structure on 0.5 monolayers (ML) of H, whereas on 1 ML of H, it forms an oblique (3×1) structure. Also, the preadsorption of H causes (i) an increase of the initial dipole moment, (ii) a lowering of ${\phi }_{\min }$, (iii) a shift of the ${\phi }_{\min }$ to smaller Cs coverage ${\Theta }_{\mathrm{Cs}}$, and (iv) an increase of ${\phi }_{\max }$. These are attributed to an increase of the Cs-W interlayer distance by the preadsorbed hydrogen, which restricts the electronic communication between Cs and the W substrate with a consequence an increase of metallization in the Cs overlayer. This explanation is consistent with results of theoretical calculations.