X-ray standing waves at surfaces

Adatoms immersed in an x-ray standing wave at a surface betray their position within the wave by the way they absorb the x-rays; feebly when positioned at the nodes, strongly when positioned at the antinodes. The elemental (and chemical) identity of the adatoms are easily monitored using the binding energies of the photoclectron or Auger electron emissions, while the intensities of these emissions provide the information needed to determine the atomic positions relative to the crystalline substrate which formed the standing wave. By using normal incidence Bragg diffraction to generate the standing wave, the technique is applicable to the rather imperfect crystalline samples and standard manipulators used in most surface science studies. Examples of structural studies from a range of systems will be drawn from recent work carried out at the SRS in Daresbury to illustrate the strengths, and weaknesses, of this structural technique. Specifically, the structure of reactive intermediates (SiHx) formed by chemical reaction of silane on Cu(111); the structure of a physisorbed molecule (CICH2CH2F) on Cu(111); an example of how chemically shifted Auger peaks may be useful for chemical shift XSW (chloroform on a chlorinated copper surface), and a system which presents many difficulties when studied by this technique, inethyl thiolate on Au(111). (Some figures in this article are in colour only in the electronic version).