.. _Acronyms:

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:

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

.. _xafs:

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

.. _exafs:

   * **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:

   * **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:

   * **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:

   * **XES**: *X-Ray Emission Spectroscopy*:  Spectroscopy of the detailed
     shape of X-ray emission or fluorescence lines.   See :ref:`related_xes`.

.. _rxes:

   * **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 :ref:`related_xes`.

.. _rixs:

   * **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 :ref:`related_rixs`.

.. _tfy:

   * **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:

   * **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:

   * **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 :ref:`related_xes`.

.. _tey:

   * **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:

   * **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:

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

.. _xmcd:

   * **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:

   * **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:

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

.. _spexafs:

   * **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.

.. _sexafs:

   * **SEXAFS**: *Surface EXAFS*  Surface-sensitive X-ray absorption
     spectra.  See :ref:`Surface Enhanced XAS`.

.. _gixafs:

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

.. _reflexafs:

   * **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 :ref:`Surface Enhanced XAS`.

.. _dafs:

   * **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 :ref:`related_dafs`.

.. _danes:

   * **DANES**: *diffraction anomalous near-edge structure*   The near-edge
     portion of a DAFS signal. See :ref:`related_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:

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

.. _fluorescence:

.. rubric:: 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 :math:`\mu` can be measured as
:math:`I_f/I_0` where :math:`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:

.. rubric::  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.