Discrimination between domain wall and uniformly incommensurate structures by surface extendex x-ray absorption fine structure: Absorption of chlorine on Ag{100}

A well-established problem in surface structure determination is the experimental difficulty in distinguishing a uniformly incommensurate structure from a domain wall structure. Surface extended x-ray absorption fine structure (SEXAFS), as a local structural probe, should provide an unambiguous means of making the distinction. We have studied chlorine adsorbed on Ag{110} by SEXAFS at several coverages. The Cl-Ag nearest-neighbor distance, 2.56 ± 0.04 A, is found to be substantially shorter than on the {111} and {100} faces, and is independent of coverage. At heta = 0.75 an incommensurate phase is formed. A multishell analysis (up to 5.6 A) allowed a complete structural determination and has, for the first time, confirmed our expectation. The CI-Cl spacing unambiguously favors a domain wall structure. Structural models have been tested by laser simulation experiments, and by kinematic analysis of the low-energy electron diffraction data.