Surface structure determination using X-ray standing waves

An X-ray standing wave (XSW) may be established in any crystalline solid by tuning either the energy or the angle of incidence of the X-rays through the Bragg diffraction condition of a set of planes within the bulk. As the Bragg condition is traversed, the X-ray standing wave so produced moves by half the layer spacing of the planes involved in the diffraction. Atoms immersed at different positions within the period of the standing wave will experience different intensities of electric field, and hence undergo differing relative X-ray absorption. By experimentally monitoring the absorption profile of surface atoms using the X-ray generated photoelectron or Auger electron intensities, the positions of these surface atoms can be determined relative to the standing wave, and hence relative to the substrate diffracting planes. In the normal incidence X-ray standing wave (NIXSW) technique the Bragg angle is fixed at 90degrees, allowing the method to be applied to samples with relatively poor mosaic structures, such as metal single crystals. Using two or more different sets of diffracting planes, a real space triangulation of the complete adatom position can be achieved. Three systems are discussed which illustrate the elemental and chemical sensitivity of NIXSW; silane reaction with Cu(111) where reactive intermediates, SiHx, formed at 140 K are shown to reside exclusively in the hexagonal close packed (hcp) type 3-fold hollows on the surface; chloroform (CHCl3) physisorbed on chemisorbed chlorine covered Cu(111), where chemical shifts in both the photoelectron (1s) and high energy Auger electron (KL2,L-3(2,2)) spectra of chlorine can be used to simultaneously determine the adsorption sites of the chlorine in both surface chemical states (chemisorbed atoms and physisorbed chloroform); the c(2x2) structure of mercury adsorbed on Cu(100) at room temperature, where the NIXSW technique shows that the mercury adlayer can translate rather freely across the substrate along f.110} surface furrows.