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 (square-root 2 X square-root 2)R45-degrees structure with a stoichiometric barium-to-oxygen ratio, and a (2 square-root 2 X square-root 2)R45-degrees 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 (square-root 2 X square-root 2)R 45-degrees overlayer. The Ba-to-O distance in this overlayer is 3.20 +/- 0.05 angstrom and beta = 82-degrees +/- 5-degrees, where beta is the angle between the Ba-O internuclear axis and the surface normal. For the (2 square-root 2 X square-root 2)R 45-degrees 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 angstrom and beta = 75 +/- 4-degrees. Additional oxygen atoms lie outside the barium plane at a distance 2.03 +/- 0.05 angstrom and beta = 23 +/- 10-degrees from the nearest barium atoms.