- X-ray standing waves
= The X-ray standing wave technique =
The
X-ray standing wave (XSW) technique can be used to study the structure of surfaces and interfaces with high spatial resolution and chemical selectivity. Pioneered by B.W. Batterman in the 1960s the availability ofsynchrotron light has stimulated the application of this interferometric technique to a wide range of problems in surface science.Basic Principles
An X-ray interference field created by
Bragg reflection provides the length scale against which atomic distances can be measured. The spatial modulation of this field - as described by thedynamical theory of X-ray diffraction - undergoes a pronounced change when the sample is scanned through the Bragg condition. Due to a relative phase variation between the incoming and the reflected beam the nodal planes of the XSW field shift by half a lattice constant.Depending on the position of the atoms within this wave field the element specific absorption of X-rays varies in a characteristic way. Therefore, measurement of the photo yield - via
X-ray fluorescence orphotoelectron spectroscopy - can reveal the position of the atoms relative to the lattice planes.For a quantitative analysis the normalized photo yield Y_p is described by
Y_{p}(Omega) = 1 + R + 2C sqrt{R} f_H cos ( u - 2pi P_H ),where R is the reflectivity and u is the relative phase of the interfering beams. The characteristic shape of Y_p can be used to derive precise structural information about the surface atoms via the two parameters f_H (coherent fraction) and P_H (coherent position). Since the emitting atoms are located in the near field, XSW measurements do "not" suffer from the ubiquitous
phase problem of X-ray crystallography.Selected Applications
which require
ultra-high vacuum conditions
*Physisorption andchemisorption studies
* Diffusion of dopants in crystals
*Superlattice s andQuasi-crystal characterizationwhich do not require
ultra-high vacuum conditions
*Langmuir-Blodgett films
*Self-assembled monolayers
* Buried interfacesSee also
*
List of surface analysis methods External links
* [http://www.esrf.eu ESRF ] The European Synchrotron Radiation Facility
* [http://www.aps.anl.gov/ APS ] The Advanced Photon SourceReferences
J. Als-Nielsen & D. McMorrow, "Elements of Modern X-ray Physics", John Wiley & Sons, Ltd (2000)
B. W. Batterman & H. Cole, "Dynamical Diffraction of X Rays by Perfect Crystals", Rev. Mod. Phys. Vol. 36 681 (1964)
Wikimedia Foundation. 2010.