X-ray standing waves

From Wikipedia, the free encyclopedia

1 The X-ray standing wave technique
2 Basic Principles
3 Selected Applications
4 See also
5 External links
6 References

[edit] The X-ray standing wave technique

The X-ray standing wave (XSW) technique provides element specific information on surface structures and adsorbate sites by combining a diffraction based approach with X-ray spectroscopic measurements. Essentially an interferometric method, this technique uses the interference field produced by the superposition of an incoming and coherently scattered wave, as a probe to measure distances similar to an Ångstrøm ruler. Pioneered by B. Batterman in the 1960s the availability of brilliant and tunable synchrotron light has stimulated the application of this technique to a wide range of problems in surface science.

[edit] Basic Principles

A standing wave field can be produced by Bragg diffraction of monochromatic X-rays from a nearly perfect single crystal. By interference the incident and reflected beams create the standing wave which pentrates the crystal and extends out of the surface. The dynamical theory of X-ray diffraction gives a complete description of the intensity and phase of the XSW field. By altering the reflection angle or the photon energy the beam is scanned through the Bragg condition and the phase of the interference field changes by $π$ . As a consequence the nodal planes of the XSW field shift by half a lattice constant.

Usually photoelectrons excited by the XSW field or X-ray fluorescence from adsorbate or dopant atom are measured to determine their positions. Depending on the site of the emitting atoms relative to the Bragg planes the XSW yield exhibits a characteristic shape. Based on the fundamental standing wave equation

$Y_{p} = 1 + R + 2C \sqrt{R} f_H \cos (\nu - 2\pi P_H )$ ,

which describes the variation of the XSW yield, very precise structural information regarding the site of the emitting atoms is obtained. Generally, a high degree of crystal perfection is necessary to observe the XSW signal.