X-ray standing waves at surfaces

Abstract

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 photoelectron 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 (ClCH2CH2F) 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, methyl thiolate on Au(111).

Structural characterisation of molecular conformation and the incorporation of adatoms in an on-surface Ullmann-type reaction

“…Molecular adsorption and conformation: chemically sensitive normal incidence X-ray standing wave (NIXSW) analysis. The identification of specific chemical states via XPS characterisation of the intermediate and final structures of the on-surface reaction facilitates our use of chemically sensitive NIXSW studies to perform structural characterisation 29,30 . NIXSW analysis based upon the specific chemical environments defined above (e.g., aromatic and organometallic carbon species) provides two structural parameters; a coherent fraction (C f ) and a coherent position (C p ).…”

X-ray Standing Wave Studies of Minerals and Mineral Surfaces: Principles and Applications

“…Since the BRXSW method is employed at soft X-ray energies (typically from E γ = 1 keV to 4 keV), it is not useful for in situ studies of the mineral-fluid interface. However, the BRXSW is a very useful probe for UHV surface science measurements of adsorbed molecules on metal single crystal surfaces (Jones et al 2002) and for site-specific valence-band photoemission studies developed by Woicik and coworkers (Woicik et al 2001;Kim et al 2002).…”

Thin film structural analysis using variable-period x-ray standing waves

“…Generally a standing wave is formed when two waves of the same wavelength and a given phase relationship cross. An Xray standing wave can be formed by X-ray diffraction 7,[12][13][14][15][16] where the incident and diffracted X-ray beams overlap. For Bragg diffraction, λ=2d hkl sinθ, where λ is the Xray wavelength, d hkl the spacing of the (hkl) set of planes and θ the angle of incidence and reflection.…”