XSW study of oxygen/alkali metal/Si(1 1 1) interfaces

The X-ray standing wave technique has been applied to investigate the localization of oxygen adsorbed on clean Si(1 1 1)7×7 and Si(1 1 1) partly covered by alkali metal (Na, K) atoms. To this end we scanned over the Si(1 1 1) Bragg reflection excited with 3.3 keV photons while detecting the O 1s core level. O 1s photoelectrons with ∼2.7 keV kinetic energy where detected with a resolution high enough to resolve chemically shifted components in the O 1s core level separated by ∼1 eV. This allows to perform a chemical-state-resolved X-ray standing-wave analysis. The possibility of detecting low Z elements opens a new field to the use of the X-ray standing-wave technique.