Acronyms used in X-ray Absorption Spectroscopy

A few words of explanation


Which terms are used is often a matter of preference. The general preference is to use either XAS or XAFS to refer to the entire spectrum, XANES to refer to the spectra near the edge (within 30eV or so), and EXAFS to refer to the extended, wiggly part.



The terms XANES and NEXAFS are completely interchangeable. In general, the soft x-ray people (say, Carbon through Sulfur) use the term NEXAFS, while the hard x-ray people tend to use the term XANES. In any case, they mean the same thing.


Following the recommendation by A. Kotani/F. de Groot (XAFS13, Stanford 2006), RIXS should be used for experiments where the emitted energy is not tuned to a fluorescence energy but to small energy transfers of a few eV. Measurements of fluorescence lines after resonant excitation should be referred to as RXES. RIXS and RXES coincide in valence band RXES where small energy transfers are measured at the fluorescence energies of valence band X-ray emission.


These terms all refer to making XAFS more surface sensitive. This can be done by detecting emitted electrons instead of x-rays, or by detecting x-rays with a sample placed at very shallow angle to the incident x-ray beam.

As electrons are emitted from only a very shallow depth (hundreds of Angstroms), detecting them naturally gives a surface-sensitive measurement.

For enhanced surface-sensitivity with x-rays, first some background: At very shallow angles, x-rays are totally externally reflected from a flat surface. Critical angles at which this happens are typically 0.1degree or so, depending on the density of the surface material. When total external reflection happens, the penetration depth of x-rays into the surface is much less than 1 micron. As will normal XAFS, XAFS-like fine-structure can be detected two ways: in the modulation of the reflected x-rays, and in fluoresced x-rays.

As far as I (MNewville) can tell, the usage in the literature is generally that REFLEXAFS measures the modulation in the reflected x-rays, while GIXAFS and GIXAS refer to measuring the fluoresced x-rays. Because of the energy dependence of the reflectivity critical angle, and the strong dependence on reflected intensity near the critical angle, a REFLEXAFS measurement can be more complicated. In addition, the intensity of the reflected signal will include other factors, and so can be more complicated to model. In contrast, a GIXAFS measurement may deliberately done at slightly larger angle than the critical angle to make the measurement less sensitive to the exact critical angle of the sample.

Acronyms (last edited 2009-10-09 19:51:21 by localhost)