Adsorption and surface structural chemistry of Na and Na + O 2 on Ag(110)

Abstract

The adsorption of Na and the coadsorption of Na and O 2 on Ag(110) have been studied by LEED, thermal desorption, and Auger spectroscopy. For Na coverages in the regime 0 < θ Na < 2 the Na desorption spectra show a single peak (β) corresponding to a desorption energy of ∼195 kJ mol −1, and at θ Na ∼ 2 a (1 × 2) LEED pattern appears. At still higher coverages (2 < θ Na < 5), a (1 × 3) surface phase is formed, and a new peak (α) appears in the desorption spectra; this is identified with Na desorption from an essentially Na surface. The desorption energy of αNa (∼174 kJ mol −1) indicates that Na adatoms beyond the first chemisorbed layer are significantly influenced by the presence of the Ag substrate. The initial sticking probability of O 2 on Na-dosed Ag(110) is enormously enhanced over the clean surface value, being of the order of unity, and O 2 chemisorption ultimately leads to a (4 × 1) surface structure. The presence either subsurface Na alone, or of both Na and O below the surface, causes substantial changes in surface behaviour. In the former case, submonolayer doses of Na lead to the appearance of a (1 × 2) structure; and in the latter case, Na + O 2 coadsorption results in a c(4 × 2) structure. Auger spectroscopy indicates that the Ag(110)-c(4 × 2) Na O phase forms with a constant stoichiometry which is independent of the initial Na dose. The Na:O ratio in this adlayer is believed to be of the order of unity. The structures of the various ordered phases, the nature of the AgNa bonding, and the interatomic spacing between the alkali adatoms on Ag(110) are discussed.