Surface geometry of BaO on W(100): A surface-extended x-ray-absorption fine-structure study

A surface-extended x-ray-absorption fine-structure study of ordered monolayers of coadsorbed barium and oxygen on a single-crystal W(100) surface is described. A ({}2 ×{} {}2 )R45^∘{} structure with a stoichiometric barium-to-oxygen ratio, and a (2 {}2 ×{} {}2 )R45^∘{} structure with a nearly 1:2 barium-to-oxygen atomic ratio both form on W(100). The surface-extended x-ray-absorption fine-structure results indicate that all the barium and oxygen atoms are nearly coplanar in the ({}2 ×{} {}2 )R45^∘{} overlayer. The Ba-to-O distance in this overlayer is 3.20 ±{}0.05 {} and {β}=82^∘{}±{}5^∘{}, where {β} is the angle between the Ba-O internuclear axis and the surface normal. For the (2 {}2 ×{} {}2 )R45^∘{} overlayer, there are two types of oxygen sites. Oxygen atoms nearly coplanar with the barium atoms are also present in these films with a Ba-to-O distance of 3.27±{}0.05 {} and {β}=75±{}4^∘{}. Additional oxygen atoms lie outside the barium plane at a distance 2.03±{}0.05 {} and {β}=23±{}10^∘{} from the nearest barium atoms.