Acronyms and Terms for X-ray Absorption Spectroscopy

There is a range of acronyms and terms surrounding X-ray absorption spectroscopy and found in the literature. Here we hope to define and describe the most common acronyms and terms. Several of these have overlapping definitions and so usage varies.

Acronyms (and typical English pronunciations)

If no pronunciation is given, the acronym is typically spelled out.

  • XAS: X-ray absorption spectroscopy: The entire field and spectral range of an X-ray absorption spectra.

  • XAFS: X-ray absorption fine-structure spectroscopy (“EX-afs”, sometimes “Zafs”): The entire field and spectral range of an X-ray absorption spectra.

  • EXAFS: Extended X-ray absorption fine-structure spectroscopy (“EX-afs”): The spectral range well beyond (say, +30 eV) the absorption threshold. Especially the part of the spectra for which a simple picture of photo-electron scattering in which single- (or a small-number of multiple-) scatterings can be used to interpret the spectra.

  • XANES: X-ray absorption near-edge spectroscopy (“Zanes”, sometimes “EX anes”): The spectral range at and near (say, +/- 50 eV) the absorption threshold. This is especially sensitive to formal oxidation state and coordination chemistry. Used mostly to describe spectra with absorption edges energies > 2000 eV (“hard X-rays”).

  • NEXAFS: Near-Edge X-ray absorption fine-structure spectroscopy (“NEX afs”): The spectral range at and near (say, +/- 50 eV) the absorption threshold. This is especially sensitive to formal oxidation state and coordination chemistry. Used mostly to describe spectra with absorption edges energies < 2000 eV (“soft X-rays”).

  • XMCD: X-ray magnetic circular dichroism

  • XMLD: X-ray magnetic linear dichroism

  • XES: X-Ray Emission Spectroscopy: Spectroscopy of the detailed shape of X-ray emission or fluorescence lines.

  • RXES: Resonant X-Ray Emission Spectroscopy: Spectra measured by monitoring an X-ray fluorescence lines after resonant excitation, with incident X-ray energy at or near the absorption threshold.

  • RIXS: Resonant Inelastic X-Ray Scattering: Spectra measured with the emitted X-rays not tuned to a fluorescence energy but to small energy transfers of a few eV.

  • HERFD: High Energy Resolution Fluorescence Detection

  • TEY: Total Electron Yield

  • TFY: Total Fluorescence Yield

  • XEOL: X-ray excited optical luminescence

  • AXAFS: Atomic X-Ray Absorption Fine Structure

  • MXAFS: magnetic XAFS

  • SPEXAFS: Spin-selective EXAFS

  • SEXAFS: Surface EXAFS

  • GIXAFS: Grazing Incidence XAFS:

  • REFLEXAFS: reflectivity EXAFS

  • DAFS: diffraction anomalous fine structure

  • DANES: diffraction anomalous near-edge structure

Notes

SEXAFS, GIXAFS, GIXAS, or REFLEXAFS? 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.