Adsorption geometry of OH adsorbed atFcenters on a NaCl(100) surface

The orientation of molecular ${\mathrm{H}}_2$O in a bilayer on defect-free NaCl(100) and of chemisorbed OH adsorbed at F centers on the (100) surface of NaCl was investigated using angle-resolved O K edge near-edge x-ray-absorption fine-structure spectroscopy (NEXAFS). High-quality single crystalline thin films of NaCl(100) of about four monolayers thickness were grown on Ge(100). These films were bombarded with 250 eV electrons so that F and F′ centers of a maximum density of about 10% of a monolayer were created. Whereas on the defect-free surface only molecular water is adsorbed, dissociation occurs at the F centers and ${\mathrm{OH}}^{\mathrm{-}}$ is bound. NEXAFS of molecular water shows almost no angular dependence even in the monolayer. This finding is consistent with formation of a hydrogen bonded bilayer of water suggested earlier. The OH species formed at the F centers is found to be inclined with respect to the surface normal by 39°±4°. An adsorption model is proposed. © 1996 The American Physical Society.