X-ray standing waves

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.

Batterman in the 1960s,[1] the availability of synchrotron light has stimulated the application of this interferometric technique to a wide range of problems in surface science.

X-ray reflectivity from a mirror surface at small incidence angles may also be used to generate long-period XSWs.

[4] The spatial modulation of the XSW field, described by the dynamical theory of X-ray diffraction, undergoes a pronounced change when the sample is scanned through the Bragg condition.

[5] Depending on the position of the atoms within this wave field, the measured element-specific absorption of X-rays varies in a characteristic way.

Therefore, measurement of the absorption (via X-ray fluorescence or photoelectron yield) can reveal the position of the atoms relative to the reflecting planes.

The crystal is rocked through a Bragg diffraction condition, and the reflectivity and XSW yield are simultaneously measured.

XRF detection enables in situ measurements of interfaces between a surface and gas or liquid environments, since hard X-rays can penetrate these media.

While XRF gives an element-specific XSW yield, it is not sensitive to the chemical state of the absorbing atom.

Chemical state sensitivity is achieved using photoelectron detection, which requires ultra-high vacuum instrumentation.

Crystal defects such as mosaicity can substantially broaden the measured reflectivity, which obscures the modulations in the XSW yield needed to locate the absorbing atom.