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. See also: https://www.iucr.org/resources/commissions/xafs/xafs-related-definitions-for-the-iucr-dictionary, and resources at https://en.wikipedia.org/wiki/X-ray_absorption_spectroscopy

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”).

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

  • 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. See X-Ray Emission Spectroscopy.

  • 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. See Resonant Inelastic X-ray scattering (RIXS).

  • TFY: Total Fluorescence Yield X-ray absorption spectra measured in fluorescence mode, generally meaning that there is little or no energy-discrimination to select a particular fluorescence line from the absorbing core-level.

  • PFY: Partial Fluorescence Yield X-ray absorption spectra measured in fluorescence mode, generally meaning that there some energy-discrimination to select a particular fluorescence line from the absorbing core-level.

  • HERFD: High Energy Resolution Fluorescence Detection X-ray absorption spectra measured in fluorescence mode with very high resolution on the energy selection of the fluorescence line of interest - especially when using a crystal-analyzer to select an energy bandwith comparable to the natural energy width of the core-level. See X-Ray Emission Spectroscopy.

  • TEY: Total Electron Yield X-ray absorption spectra measured in emission mode by measuring the intensity of emitted electrons by the sample, typically without energy resolution.

  • PEY: Partial Electron Yield X-ray absorption spectra measured in emission mode by measuring the intensity of emitted electrons by the sample, typically with some energy resolution.

  • XEOL: X-ray excited optical luminescence X-ray absorption spectra measured in emission mode using optical luminescence.

  • XMCD: X-ray magnetic circular dichroism The difference of X-ray absorption spectra measured with right- and left- circularly polarized light for a sample in a magnetic field.

  • XMLD: X-ray magnetic linear dichroism The difference of X-ray absorption spectra measured with horizontally- and vertically- linear polarized light for a sample in a magnetic field.

  • MXAFS: magnetic XAFS XAFS measured in XMCD- or XMLD-like difference mode.

  • SPEXAFS: Spin-selective EXAFS X-ray absorption spectra measured using a spin-sensitive probe, such as difference of spin-up and spin-down X-ray emission spectra.

  • GIXAFS: Grazing Incidence XAFS X-ray absorption spectra measured in fluorescence mode near or slightly above the critical angle of a samples surface. See Surface Enhanced XAS.

  • REFLEXAFS: reflectivity EXAFS X-ray absorption spectra measured using the X-rays reflected from a surface at or below the critical angle of a samples surface. See Surface Enhanced XAS.

  • DAFS: diffraction anomalous fine structure X-ray spectroscopy measured by monitoring the intensity of a particular X-ray diffraction reflection. This will be a mixture of the real and imaginary components of the resonant X-ray scattering factors. See Diffraction Anomalous Fine Structure (DAFS).

Terms for Measurement Modes

There are a few different common measurement modes for X-ray absorption and emission spectroscopy. Many of the acronyms above are really referring to these different modes, so we include a brief discussion here.

Transmission or Absorption Mode XAS

This mode measures the X-ray absorption coefficient from the fraction of X-rays transmitted through a sample. That is, the X-ray intensity before the sample (\(I_0\)) and after - and so transmitted through - a sample (\(I_t\)) are measured and the value \(-\log(I_t/I_0)\) is used as a value that is proportional to the X-ray absorption coefficient \(\mu\) from the Beer-Lambert Law, \(I_t = I_0 \exp(-x\mu)\) for a sample of thickness \(x\).. This mode of measurement is most suitable for samples containing high concentrations (say 10% by weight or more) of the absorbing element.

Fluorescence or Emission Mode XAS

This mode measures the X-ray absorption coefficient from a secondary process such as X-ray fluorescence of Auger electron emission that is due to the filling of the empty core-electron level that was created by the X-ray absorption process. With some caveats and conditions, the measured emmission signal will be directly proportional to the absorption coefficient, so the \(\mu\) can be measured as \(I_f/I_0\) where \(I_f\) is the measured intensity of the fluorescence or emission. This mode of measurement is generally most suitable for samples where the absorbing element is in low concentrations.

Surface Enhanced XAS

As X-rays are generally highly penetrating of matter (especially in contrast to charged particles such as electrons), X-ray absorption is generally a bulk-sensitive technique. A few variations can be used to make XAS more surface-sensitive. First, measuring emitted electrons instead of transmission or X-ray fluorescence will greatly enhance the contribution from the top several tens to hundred Angstroms since Auger electrons generated deeper in the sample are very unlikely be able to escape to the surface.

In addition, the sample can be placed at a very shallow angle to the sample so that the penetrating X-rays stay relatively close to the surface. This requires a large, flat sample. In addition, at shallow enough angles (typically below 0.5 degree, but depending on sample density and X-ray energy), X-rays will be totally reflected from a material - “total external reflection”. When total external reflection happens, the penetration depth of x-rays will be hundreds of Angstroms. (Note: under these conditions, one can also set up an X-ray standing wave and use or modulate the fluorescence signal from particular atomic layers at the surface).

With the sample at or below the critical angle, XAS can be measured in two ways: in the modulations of the X-rays reflected by the sample (REFLEXAFS) or in the fluoresced X-rays from the absorbing element in the sample (GIXAS or GIXAFS).

The surface-reflected signal will be complicated by including both absorption and refraction effects, and by the fact that the critical angle varies (relatively slowly) with the X-ray energy. On the other hand, a GIXAS 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.